Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels
SEISMIC RESPONSE OF WOOD SHEARWALLS WITH OVERSIZED ORIENTED STRAND BOARD PANELS
by
JENNIFER PATRICIA DURHAM
BA.Sc, The University of British Columbia, 1995
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF APPLIED SCIENCE
in
THE FACULTY OF GRADUATE STUDIES
Department of Civil Engineering
We accept this thesis as conforming to the reauired standard ,
THE UNIVERSITY OF BRITISH COLUMBIA
October, 1998
© Jennifer Patricia Durham, 1998 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.
Department of C'V't- £/u6-( >Q&fa£WC-
The University of British Columbia Vancouver, Canada
Date HCXJfcH66/^ ^ (^9g
DE-6 (2/88) Abstract
This thesis reports on an experimental study on the earthquake/seismic resistance of wood based shearwalls sheathed with oversized oriented strand board panels. This work extends from a previous study on walls subjected to quasi-static cyclic loading regimes by investigating the dynamic behaviour of this new wall system on a shake table. Monotonic, cyclic quasi-static and dynamic loading tests that included an applied dead load were performed on 2.44 m x 2.44 m shearwalls with standard (1.22 m x 2.44 m) and oversize
(2.44 m x 2.44 m) oriented strand board panels. Measured and calculated properties for the 14 test walls are presented, which include the following: strength, stiffness and ductility values; energy dissipation values; and failure modes. This information was used to draw conclusions on the influence of panel size and panel-to-frame nail connection spacing on the behaviour of the shearwalls.
Shearwalls constructed with single oversized panels (Type B) had an increase in capacity of 26% over regular walls (Type A) as measured by the maximum load reached in monotonic tests. Shearwalls constructed with single oversized panels and reduced nail spacing around the panel edges (Type C) had an increase in capacity of 104% over regular panel walls. The maximum loads measured in cyclic tests and the maximum base shears calculated in dynamic tests were in excellent agreement with the monotonic peak loads.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels ii Abstract iii
In dynamic testing, the Type C walls were not significantly damaged when subjected to
the same ground motion that brought the Types A and B walls to failure. The wall with
the oversized panel and reduced nail spacing was subsequently failed when subjected to
the same ground motion scaled to a higher peak ground acceleration. Single oversized panel walls with reduced nail spacing dissipated roughly twice as much energy compared
to the other two wall types whether tested cyclically or dynamically.
Nail withdrawal was the dominant failure mode in all test types. A newly developed, relatively short cyclic test protocol was successful in producing the failure modes
compatible with the failure modes observed in dynamic tests.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Table of Contents
Abstract ii
Table of Contents iv
List of Tables vi
List of Figures vii
Acknowledgements x
CHAPTER 1 Introduction 1
Objectives 6 Methodology 6 Organization 7
CHAPTER 2 Background 8
Function and Construction ofWood-based Shearwalls 8 Review of Shearwall Research 10 Monotonic and Static-Cyclic Testing 12 Procedures 12 Geometry 14 Precursory Work 15 Dynamic Testing 17 Connections 20 Modelling 22 Monotonic Models 23 Dynamic Models 27 Oriented Strand Board (OSB) 32
CHAPTER 3 Methodology 34
Analytical Studies 34
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels iv Table of Contents v
Analytical Program 35 Procedures 38 Static Tests 41 Setup 41 Procedures 43 Dynamic Tests 47 Setup 47 Procedures 49
CHAPTER 4 Analyses 55
Modified Program and Verification 55
CHAPTERS Test Results and Discussion 61
Static Tests 61 Results 61 Discussion 68 Dynamic Tests 72 Results 72 Analysis 80 Discussion 99 Summary 110
CHAPTER 6 Conclusions and Recommendations 112
Bibliography 116
Appendix A Dynamic Test Frame Schematics .... 123
Appendix B Analyses 125
Experimental Planning Studies 125 Sensitivity Studies 131 Comparison of Experiment Results and Analysis Results ...134 Summary 139
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels List of Tables
Table 3.1 Nail model parameters from SWAP 37 Table 3.2 Nail parameters for 2.67 mm dia., 50 mm long spiral nails 39 Table 3.3 List of tests with wall descriptions 54 Table 4.1 Modified SWAP results compared with original SWAP example results 56 Table 4.2 Analyses replicating experimental test by Dolan (1989) 58 Table 5.1 Results from static tests 62 Table 5.2 Summary: Analysis of static testing results 69 Table 5.3 Dynamic test results 75 Table 5.4 Summary: Analysis of dynamic testing results. 101 Table 5.5 Deviation of calculated dynamic peak shear from measured cyclic peak shear for each wall type. 102 Table 5.6 Energy dissipation for walls tested cyclically and dynamically 110 Table B.l Characteristics of analysis input accelerograms. 126 Table B.2 Analyses for planning experimental testing program 127 Table B.3 Analyses for completing sensitivity studies 134 Table B.4 Comparison between analytical and experimental first natural frequencies 135
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels vi List of Figures
Figure 2.1 Load-slip relationship for connectors in Foschi's model (1977) described by Equation 2.1 23 Figure 2.2 Shearwall model by Tuomi and McCutcheon (1978) 24 Figure 2.3 Shearwall model by Easly et al. (1982) 25 Figure 2.4 Shearwall model by Gupta and Kuo (1985) 26 Figure 2.5 Typical pinching of hysteresis loops in timber structures (highlighted) 27 Figure 3.1 Frame and panel displacement assumptions in SWAP 35 Figure 3.2 Nail model parameters from SWAP 37 Figure 3.3 East-West accelerogram from Joshua Tree Station - 1992 Landers California Earthquake 40 Figure 3.4 Static test setup schematic for 7.3 m wall (He, 1997) and photograph of 2.44 m wall in the setup 42 Figure 3.5 Cyclic testing protocol 45 Figure 3.6 Standard hold down attached to test wall 47 Figure 3.7 Schematic and photograph of dynamic test setup. 50 Figure 3.8 Close-ups of dead load pulley system 51 Figure 4.1 Drift results for Dolan (1989) wall input analyzed with modified SWAP 58 Figure 4.2 Comparison of drift time histories from a previous experiment (Latendresse and Ventura, 1995; Durham et al., 1996) and from the modified version of SWAP 59 Figure 5.1 Uplift during Test 1 63 Figure 5.2 Nails pulling out of the frame and pulling through the sheathing in Test 2 64 Figure 5.3 Test 4. a) Separation of sheathing and frame prior to test, b) Failure and relative movement at blocking. 65 Figure 5.4 Test 6 a) Bottom of studs bending after peak load was reached, b) Split stud at bottom corner 67 Figure 5.5 Failure along blocking in Test 8. Top panels show more damage 67
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels vii List of Figures viii
Figure 5.6 Load vs. drift curves for monotonic tests 71 Figure 5.7 Load vs. drift curves for cyclic tests compared to monotonic tests 73 Figure 5.8 Deviation of cable force from approximately 5.56 kN 74 Figure 5.9 First lateral natural frequency for wall with dead load in Test 11 76 Figure 5.10 Test 10b: a) Nail fatigue failures, b) Nails pull- through failures 77 Figure 5.11 Tests 10a, 10b, 1 Or: a) Damaged stud, b) Poor quality damaged stud, c) Minimal crushing at hold down connection 78 Figure 5.12 Crushing of panels in Test 14 80 Figure 5.13 Frame model for force calculations 81 Figure 5.14 Free body diagram of inertial mass K 84 Figure 5.15 Free body diagram of inertial mass platform 85 Figure 5.16 Free body diagram of vertical supports for inertial mass 87 Figure 5.17 Free body diagrams to determine the shear force transferred from the load transfer beam of the frame to the wall 90 Figure 5.18 Free body diagram of the shake table platform. Resultant forces in the link supports under the platform must be in the direction of the links at all times 91 Figure 5.19 Verification of earthquake input and measurements for Test 10a 98 Figure 5.20 Drift time histories for tests with 0.35 g peak ground acceleration 105 Figure 5.21 Acceleration time histories for tests with 0.35 g peak ground acceleration 106 Figure 5.22 Drift time histories for tests with 0.52 g peak ground acceleration 107 Figure 5.23 Acceleration time histories for tests with 0.52 g peak ground acceleration 107 Figure 5.24 Hysteresis loop for Test 11 (Type A), assuming 1% damping 108 Figure A.l Front view schematic of dynamic testing frame (measurements in mm) 123
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels List of Figures
Figure A.2 Side view schematic of dynamic testing frame (measurements in mm) 124 Figure B. 1 Drift time histories for Type A and Type C walls with different lengths 128 Figure B.2 Acceleration time histories for Type A and Type C walls with different lengths 129 Figure B.3 Changes in slope occur at the peak of each hysteresis loop (highlighted) 130 Figure B.4 Drift time histories showing the effect of varying the damping ratio in analyses of Type C walls 132 Figure B.5 Comparison between analytical and experimental monotonic behaviour 136 Figure B.6 Drift and acceleration response comparisons of analytical and experimental results for a Type C wall 138
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Acknowledgments
"He who walks with the wise grows wise..."
Proverbs 13:20, New International Version
I would like to express my sincere thanks to all those who have provided help, advice and
encouragement to me during the completion of this thesis. These include but are not
limited to Dr. Prion, Dr. Lam, Henry He, Paul Symons, Howard Nicol, Vincent
Latendresse, Dominik Sieber, Dr. Foschi, Dominique Janssens of the Structural Board
Association, my family, Anand and my friends.
"O Lord my God, I will give you thanks forever."
Psalm 30:12, New International Version
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panelsx CHAPTER 1 Introduction
Wood based shear walls are used extensively to resist lateral loads in wood frame construction for residential and commercial buildings. A shearwall is an efficient structural system consisting of frame members that resist vertical and transverse lateral loads as columns or beam columns, panels that resist in-plane lateral loads in shear, and fasteners that connect all the elements. The close spacing between the fasteners, connecting the panels and the vertical framing members (or studs), creates a redundant system where loads can be shared and the strength reducing effect of defects is minimized.
Thus, the structure is relatively tolerant of weak or weakened components.
Experience has also shown that wood based shearwall systems have excellent performance under lateral seismic loading. Yasumura (1996) reports that buildings constructed with shear walls according to the North American Frame Construction
Method were able to resist high ground accelerations and significant liquefaction during
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels1 Introduction 2
the 1995 Kobe earthquake in Japan. Yet, changes in typical construction practises, new materials, new shearwall configurations, new shearwall applications in commercial and industrial buildings, and increasing quality and code demands challenge the confidence that designers can place in their experience with wood-based shearwall systems.
Continuing interest in studying these systems is necessary to meet these new challenges, maintain the advantages of these systems in residential construction and promote wood- based shearwalls for other applications.
There have been many experimental and analytical studies focussing on the behaviour of shearwalls. Monotonic tests on full size shearwalls have been used to determine the ultimate load carrying capacities of varying wall systems. TECO (1980, 1981) and
Atherton (1982, 1983) focussed on racking behaviour and ultimate strengths of waferboard sheathing and diaphragms. Atherton also examined particleboard diaphragms.
Tissel and Elliot (1977), COFI (1979), and Adams (1987), researched plywood shearwalls and diaphragms. Such studies have largely formed the basis for code design tables in
North America. There have also been tests on full scale buildings (Sugiyama et al., 1988 and Boughton, 1988). All of the above were monotonic push-over tests on different sized walls made with full-size components.
The behaviour of a system under reversed loading, however, cannot be determined with monotonic testing only. Understanding the hysteretic behaviour is vital for understanding the dynamic response of structural components and systems. Thus, several researchers
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Introduction 3 have completed reversed cyclic tests on full scale walls, including Rose (1995) and
Karacabeyli (1995). It is necessary, however, to agree on a standard test protocol, so that meaningful comparisons of test results can be carried out. Foliente and Zacher (1994) have addressed this issue and provide a detailed review of timber structure testing that highlights the need for international cyclic testing standards or guidelines.
Although cyclic testing yields information that is useful for modelling and to understand the dynamic behaviour of shearwalls, the picture is not complete without observing their dynamic behaviour through experiment. For example, Stewart (1987) tested shearwalls with plywood sheathing under cyclic quasi-static, sinusoidal and arbitrary dynamic loading conditions. He varied nail spacing, plywood thickness and hold down details.
Dolan (1989) conducted monotonic, cyclic racking tests, as well as free vibration tests, sine wave and frequency sweep tests, and arbitrary dynamic loading tests on shearwalls with both nailed plywood and nailed waferboard sheathing. He found that the frame to sheathing nail joint governed the overall wall behaviour and that increased nail density increased stiffness. Dolan reported that the use of actual earthquake records is important to investigate the effects of earthquake parameters such as frequency content and duration of strong motion. He did not feel that this type of testing was sufficient for evaluating the seismic performance of a wall. The variability and uncertainty of earthquake loadings precluded making judgements based on earthquake testing alone. The fact that timber structures develop a memory (their behaviour under future loading is dependent on past loading) further complicated this issue. Even when subjected to a single earthquake event,
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Introduction 4 the behaviour of a timber structure near the end of the event was dependent on the specific nature of the earthquake at the beginning of the event.
Models of wood based shearwalls have been developed by several researchers who have used different methods to predict the response of timber shearwalls to one or more loading type. When models or programs are developed to predict dynamic behaviour, the empirically based hysteretic behaviour (load-slip relationship) of the components is commonly described mathematically. The body of research points out the importance of connection behaviour in modelling, so all models must make assumptions to define this behaviour. The behaviour of the connectors is typically determined from obvious test results.
Once the load-slip behaviour has been defined, calculations are performed to obtain the wall's response. Calculation methods used for models have included energy conservation methods (Tuomi and McCutcheon, 1978), equilibrium principles (Easly et al, 1982), variational principles (Filiatrault, 1989, 1990), strain energy principles (Gupta and Kuo,
1985) and finite element methods (Foschi, 1977; Dolan, 1989). All models cited above were developed for monotonic loading, except Filiatrault's and Dolan's, which were developed for dynamic loading. Both of these adapted Foschi's load-slip assumptions into their dynamic models who modelled the load slip curves with power curves.
Most of the previous work was done with plywood panels. Since the 1970's, however, oriented strand board (OSB) has been used increasingly as an alternative to plywood.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Introduction 5
Researchers such as Rose (1995), Karacabeyli (1995) and White and Dolan (1995), have studied OSB in shearwall systems and compared their performance to shearwalls constructed with other materials. The overall mechanical properties of OSB have been studied by Lau and Lam (1989). More recently, OSB mills have acquired the capacity to produce panels of up to 3.3 m x 7.2 m in size. Normally, these sheets are cut up into standard size 1.2 x 2.4 m panels. To investigate the potential of using full size panels, a study by He (1997) investigated the monotonic and static cyclic behaviour of 2.4 x 7.3 m shearwalls constructed with single sheets of oriented strand board.
Taking advantage of the increased continuity of large panels proved beneficial. Walls constructed with single sheets were markedly stronger and suffer, especially when nails that would be at the edges of smaller panels were relocated to the perimeter of the large panels. Though significant increases in stiffness and strength were achieved, peak loads were reached with much smaller overall displacement of the walls. This raised questions about the behaviour of single panel walls under earthquake type loading. The study presented here sought to determine if similar benefits could be achieved under dynamic loading conditions.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Introduction 6
1.1 Objectives
The objectives of this study were as follows:
1. To prepare an experimental testing program for timber shearwalls with regular and
oversized panels.
2. To experimentally determine the behaviour of full scale timber shearwalls with regular
and oversized panels under monotonic and cyclic loading.
3. To experimentally investigate the dynamic performance of timber shearwalls sheathed
with regular and oversized panels.
4. To experimentally investigate the effects on dynamic behaviour of perimeter (at panel
edges) nail spacing on timber shearwalls sheathed with a single oversized oriented
strand board panel.
1.2 Methodology
The objectives of this study were achieved by first choosing wall configurations for the experimental testing program. An attempt was made to use model results from an existing nonlinear dynamic shearwall analysis program to assist in preparing the experimental program. Several walls of each chosen wall type were constructed. At least one monotonic test and one cyclic static test were performed on each type of wall. Two of each wall type of were tested dynamically. The natural frequency of each type of wall was determined through vibration testing using a calibrated impact hammer. Experimental results were then analyzed and compared.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Introduction
1.3 Organization
This thesis is organized into the following chapters:
Chapter 1: Introduces the thesis.
Chapter 2: Gives background information relevant to the study.
Chapter 3: Describes the procedures and set ups for analytical and
experimental testing.
Chapter 4: Details the computer analysis.
Chapter 5: Reports experimental results.
Chapter 6: Draws conclusions and offers recommendations.
Appendix A: Provides detailed schematics of the dynamic test setup.
Appendix B: Contains additional information on the computer analyses.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels CHAPTER 2 Background
2.1 Function and Construction of Wood-based Shearwalls
Shearwalls resist lateral loads such as those imposed on structures by winds or earthquakes. Shearwalls also form part of the vertical load path in structures. When they are subjected to out of plane loads as well as in plane loads, these shearwalls act as diaphragms and the vertical load resisting components can be considered as beam columns. Although these load combinations can cause buckling and can significantly reduce the load capacity of the walls, it is unlikely that earthquake loading occurs simultaneously with other loads. Therefore, the combined type of load demand is not considered in this study.
Wood-based shearwalls are composed of frame members, sheathing and fasteners. The frame members are typically 38 mm x 89 mm or 38 mm x 140 mm timber studs and plates. The timber studs are typically spaced approximately 400 mm or 600 mm apart.
Since the frame must carry the vertical loads, two or three studs are typically nailed together at the perimeter of the wall. Diagonal braces may be used within the frame to
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels8 Background 9
increase its in-plane stiffness. The stiffness of the frame is, however, small compared to the stiffness of the sheathing. Sheathing for load bearing walls generally consists of 1.2 x
2.4 m panels of plywood or oriented strand board. Other walls may be sheathed with gypsum board or drywall. Plywood or oriented strand board panels are connected to the frame with nails and sometimes staples.
When a wall is subjected to lateral in-plane loads, the fasteners work with the rigid sheathing panels and the relatively flexible frame to carry the load. The deformation of the walls from a rectangular shape into a parallelogram is known as "racking". The fasteners are the most important components determining the shearwall performance, influencing both stiffness and strength. According to Karacabeyli (1995), the stiffness of tested walls was most influenced by the nail load-slip relationship between sheathing and wood frame members and then by the shear deformation of sheathing panel. Also important is the way in which the panel to frame fasteners interact with the sheathing. It is therefore worthwhile to investigate the influence of fastenings and sheathing on shearwalls, and to explore improvements that can be made with these components.
Wood frame walls are highly redundant systems; incorporating closely spaced members and fasteners. Such a system is forgiving of weak components, but complex to understand, model and analyse. One reason is the nonlinear load-slip characteristic of the fasteners. Their ability to redistribute loads and to dissipate energy as they deform are factors that contribute to their suitability to resist earthquake ground motion. Historically,
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 10 however, the confidence placed in timber shearwalls often comes from experience from small residential buildings. In recent years wood frame construction has been applied in buildings of up to 5 storeys high and of irregular shape. Since it is very expensive to test large scale models of such buildings, analytical tools are essential to gain insight into the response of such buildings under earthquake loading. The investigation and modelling of the behaviour of timber shearwalls under earthquake ground motion forms an important component of such analytical tools. It is therefore of great importance to develop dynamic analysis techniques that can model the response of individual walls for inclusion in system analysis programs.
2.2 Review of Shearwall Research
He (1997) presented a detailed review of the history of shearwalls in the Literature
Review section of his thesis. Much of the information within this section was revisits the same material. Carney (1975) wrote an extensive bibliography on works related to wood and plywood diaphragms. It shows that forms of dynamic tests on timber walls were conducted as early as 1930 (Jacobson). Some early research was done to investigate damage to wood construction in earthquakes (Wailes and Horner, 1933). Researchers later became interested in understanding wood mechanics and properties and in creating design and manufacturing standards. Until the 1940's light-frame structures typically did not use
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 11 panel products, rather, diagonal bracing was incorporated for shear resistance. This type of structure remained the standard even after the use of sheathing became popular.
Standards accepted panel sheathing in place of diagonal bracing by the 1950's.
Through the 1950's and 1960's, many experimental studies were conducted to determine the influence of different parameters on shearwall behaviour. Parameters of interest included sheathing type, sheathing orientation, fastener type, fastener spacing and diaphragm geometry. Analysis methods were simple and reflected a limited understanding of the static and dynamic behaviour of the walls. The information from tests on full scale shearwalls and houses combined with experience, was used to create design guidelines. The advent of computers provided researchers with a means to study shearwalls analytically.
Today, most low rise North American residential buildings are constructed using wood- based shearwalls. The use of wood based shearwalls in commercial and industrial applications is also increasing. Modern design codes are being used for the design of shearwalls with new shearwalls configurations carrying potentially greater loads. These structures have more openings, multiple levels, irregular floor plans, heavier roofs and inter-storey floors. Greater understanding of expected earthquakes has led to changes in building codes requiring lateral load resisting systems to withstand higher base shears.
Newer shearwall products such as oriented strand board are gaining ever increasing shares
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 12
of the market. These changes lead to the need to improve the basic understanding of the behaviour of timber shearwalls following experimental and/or analytical approaches.
2.3 Monotonic and Static-Cyclic Testing
2.3.1 Procedures
The most commonly used testing protocols are those published by the American Society of Testing Materials Standards ASTM E72-77 (ASTM, 1991a) and ASTM E564-76
(ASTM, 1991b), which are monotonic push-over testing procedures for full size wood- based shearwalls. Results from tests performed by these procedures were used to determine racking resistance values and to measure the static shear capacity of the walls.
These methods were preceded by methods developed by Whittemore and Stang (1938) and a US FHA Standard (Federal Housing Administration, 1949).
Several methods have subsequently been developed to perform quasi-static cyclic tests on structures and structural components such as connections and fasteners. The term "quasi- static" implies that loads are being applied at a rate slow enough that material strain rates do not affect the test results (Leon and Deierlein, 1996). Advantages of these tests over true dynamic tests are that they are easier to conduct, especially with full size specimens, their results are more easily explained, and detailed observations of failure mechanisms are possible throughout the test.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 13
For earthquake applications, monotonic "push-over" tests do not provide sufficient information and cyclic tests are the preferred method of evaluating specimens. In general, tests that have been performed with the same cyclic protocol can be easily compared, though this is not always the case. At the time this thesis was prepared, international experts in the field generally agreed that the displacement history for a given type of cyclic test should depend on results from a monotonic push-over test. These results include parameters such as maximum load, yield point, ductility and stiffness. To date there is no agreement on the cyclic displacement history's amplitude, frequency, number of cycles and loading rate. Furthermore, there is no recognised standard for determining the important yield point parameter from the monotonic results. This yield point can vary significantly in wood based shearwalls since wood structures do not exhibit a definite yield plateau. Calculations for the ductility ratio also vary because of inconsistencies in the definitions of the calculation parameters.
Karacabeyli (1995), in a report on monotonic and cyclic displacement schedules imposed on 4.88 m. long walls, made suggestions for a uniform testing standard. He called for an internationally accepted cyclic displacement schedule and definition of yield point. Lam et. al (1996) showed that long test protocols may be unsuitable for some specimens, generating unrealistic failure modes that significantly influenced maximum load capacities. They suggested a much shorter protocol that more realistically reflected the demand and failure modes during an earthquake.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 14
During cyclic testing, under repeated cycles of constant displacement amplitude, a significant drop in load usually occurs in the second cycle compared to the first cycle.
After the third cycle the load typically does not deteriorate further which is considered as the stabilized lateral resistance. A "stabilized curve", which can be constructed by joining all the peaks from each third cycle, is often used for design purposes.
2.3.2 Geometry
Several geometric factors affect the performance of wood based shearwalls. Capacity and stiffness are approximately proportional to the length of the wall. A wall sheathed on two sides has approximately double the capacity of a wall sheathed on one side, given that the same sheathing material is used on both sides. If the sheathing is different, then the static capacity is roughly the sum of two individual walls, each sheathed on one side with one of the sheathing materials. The stabilizing force of imposed vertical loads helps to increase the shear capacity by reducing the over-turning effect, but this influence diminishes as wall length increases. In the same vein, longer walls do not need as much hold down force to enforce pure racking as do short walls, which overturn easily. The overturning effect can significantly influence load distribution patterns, and thus the wall performance. This must be considered, especially in multi-storey buildings with relatively narrow shearwalls.
Nail spacing around the panel perimeter has a marked effect on wall performance, although the capacity of a wall is not proportional to the number of nails.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 15
Openings understandably reduce the capacity and stiffness of walls. They are also likely to induce significant localized failures. Stud spacing was found to affect capacity, not stiffness (De Klerk, 1985). Walls with dense nailing patterns require higher strength hold- down details (Bier, 1994). Soltis and Patton-Mallory (1986) report from tests on small scale walls that plywood and gypsum walls have strength capacities proportional to their wall length. Consideration of these many, varied and variable parameters makes simplified modelling of shearwalls very difficult and necessitates more complex analysis methods that are based on fundamental component properties.
2.3.3 Precursory Work
He's research (1995) is an important precursor to this work. He experimentally and analytically investigated shearwall systems with nonstandard large dimension oriented strand board panels. A test facility was built to carry out the monotonic and static-cyclic tests on 2.4 x 7.3 m walls. The effects of panel size, panel-frame nail connector type, panel-frame connector spacing, and cyclic testing protocol on shearwall behaviour were considered.
The resulting database showed that walls constructed with single oversize panels had significantly greater stiffness and lateral load carrying capacity than walls constructed with standard panels. The strength and stiffness were further increased by decreasing the nail spacing at the edge of the oversize panels. Failure modes in monotonic tests included panel-frame connectors primarily withdrawing from the frame, but also pulling through
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 16
the panel. The main failure mode in cyclic tests was low cycle nail fatigue, which could be ascribed to the long testing protocol. Yung (1991) also notes that in static cyclic tests, failure modes included sheathing to frame nails pulling through the sheathing, pulling out of the framing or shearing off in a fatigue type failure.
The low cycle nail fatigue failures led to the investigation of different cyclic protocols, as nail fatigue was not known to be a significant failure mode in dynamic tests. Testing was displacement controlled and walls constructed with standard panels appeared to dissipate more energy under cyclic loading, as they reached greater displacements for the same engendered load.
In addition to full-size testing, tests were also performed on small connection specimens to determine the load-slip characteristics of the panel-frame connectors. This is discussed in more detail below. A nonlinear finite element analysis program was used to model the monotonically tested walls. The modelled behaviour of the walls showed good agreement with the initial behaviour of tested walls, prior to nail failure. Refinements were suggested to achieve better modelling of the walls as they undergo nail failures.
The research described here is an extension to He's work, moving from static to dynamic behaviour. Dynamic testing and modelling has shown that shearwalls constructed with nonstandard, large dimension oriented strand board panels also exhibit better performance in earthquakes. For the purposes of comparison, and for compatibility between test programs, monotonic and static cyclic tests for this work were performed to the same
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 17
specifications as those of He. The stiffnesses and lateral load capacities from these tests have given indications of the relative response frequencies of dynamically tested walls and also the relative level of earthquake acceleration that would result in failure. Also, hysteresis loops from static-cyclic testing are used in preliminary analyses to predict the dynamic behaviour of structures. Dolan (1989), Filiatrault and Foschi (1991), Yung
(1991) and ATC (1995) all preceded dynamic testing programs with monotonic and cyclic tests.
2.4 Dynamic Testing
Several dynamic testing programs have been carried out at the University of British
Columbia (UBC). Dolan (1989) developed a testing frame to allow testing of 2.44 m x
2.44 m shearwalls on the UBC Earthquake laboratory 3.3 x 3.3 m shake table. The frame was hinged to allow lateral movement in the direction of table motion and was braced to prevent out of plane movement. It was designed to support an inertial mass of up to 4,500 kg. The inertia mass which therefore would not act as vertical dead load on the test specimen. The shearwall was then attached to the table along the bottom plate and to the loading frame along the top plate. Hold downs were incorporated into the design. Dolan performed several dynamic shake table tests along with static and cyclic tests mentioned above. All of the tests were carried out on the shake table with the walls supported by the frame. His dynamic test regime included free vibration tests, tests with a sine sweep input and tests where the ground motion was a recorded earthquake. The tests were used to
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 18
verify computer models discussed below. The primary earthquake inputs originated from the East-West component of the 1953 Kern County Earthquake with maximum accelerations scaled to 0.18 g and 0.3 g. He also used the 1971 San Fernando, California earthquake.
These tests showed little difference in behaviour between walls sheathed with plywood and walls sheathed with waferboard. The differences were only apparent when loads nearing ultimate capacity were applied. Dense nailing produced stiffer walls which engendered higher inertia loads when subjected to the same earthquakes. The peak acceleration, frequency content and duration of the simulated earthquake were shown to be important factors with regards to wall damage. The full scale earthquake tests were not able to push the walls to ultimate behaviour. It was difficult to glean concrete information about the wall force, hysteresis, stiffness and strength of the walls from the earthquake tests. The failure modes would not have been readily apparent as ultimate behaviour was not reached.
In Yung's tests (1991), which were designed to determine the effect of specially designed friction dampers, two earthquakes with different frequency components were used. These were the 1940 El Centro Station, Imperial Valley, California record, North-South component, with its seismic energy concentrated mainly in the high to mid frequency range; and the 1977 Bucharest, Romania earthquake, North-South component, with its seismic energy mainly in the low frequency range. There were a few sheared nails in the
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 19
dynamic tests which occurred with stiffer walls or walls subjected to greater accelerations.
In general, a structure with a high frequency will have a greater response when subjected to a high frequency excitation, while a structure with a low frequency would have a greater response when subjected to a low frequency excitation. Yung's tests were also carried out on the UBC shake table using Dolan's test frame.
In 1991, Foschi and Filiatrault used the same set up to test walls where the sheathing was attached with nails and adhesive. Under moderate excitation levels, the stiffer adhesive walls attracted higher loads but showed less damage. Although the stiffer walls suffered less damage under moderate load level, the glued connections failed in a brittle, catastrophic manner at relatively high load level. It should also be pointed out that the natural frequency of the adhesive walls differed significantly from that of the non- adhesive walls, resulting in different levels of to be excitation by different earthquakes.
In He's tests (1997), walls constructed with oversized panels were also stiffer than those constructed with standard sized panels. Since their connections were formed only with nails that have ductile behaviours, however, their response under earthquake loading was expected to be ductile and non-catastrophic. This typically results in a lengthening of the structure's period which shifts it out of the high energy frequency ranges.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 20
2.5 Connections
Dolan (1989) reports that early studies of wood based shearwalls focused mainly on nailed connections. Current understanding of the importance of the sheathing to framing connections to shearwall behaviour validates the work of Westman and McAdoo (1969),
Wilkinson and Laatsch (1970) and Senft and Suddarth (1971). These studies examined the initial stiffness and ultimate load capacity of various fasteners under static uni• directional withdrawal or shear testing mode. The earlier research did not consider nonlinear effects. Dolan reports that later tests focused on complete connections but still reported only static uni-directional load capacities. Later still, Soltis and Mtenga (1985) imposed dynamic cyclic loading onto nailed joints and did not find a difference in load capacity, whether the joint was tested statically or dynamically. Others looked at the damping and stiffness of nailed joints, reporting equivalent viscous damping ratios in the range of 10% to 40%. Foschi (1982) investigated the load-slip characteristics of connections formed with nailed waferboard and Douglas-fir members. He fitted an exponential equation to the load deflection curve to describe the behaviour of the connector and used the information in a finite element model for full size shearwalls.
Several newer shearwall models including Dolan (1989) and Filiatrault (1989, 1990), are based on Foschi's earlier work. Dolan also performed nail tests as part of his detailed study, and used the results in his shearwall models.
To complement He's tests of long walls with OSB sheathing (1997), Sieber et al. (1997) conducted a study on the performance of nails. These tests were geared towards finding
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 21
nonlinear and hysteretic nail characteristics for use in the monotonic modelling portions of
He's work and the dynamic modelling performed as part of this thesis.
Sieber et al. (1997) constructed test specimens were constructed with common nails and
spiral nails of nominal length 50 mm, 2x4 dimensional lumber (38 x 89 mm SPF) and 9.5 mm thick Performance Rated W24, CSA-0325.0-M88, OSB sheathing. There were about
150 specimens, divided into different test groups. Tests involved displacement controlled monotonic tension shear loads. Nail type, lumber grain to force direction, nail grouping, nail spacing and conditioning time between assembly and testing were all investigated.
Conditioning proved especially important when plain shanked common nails were used.
The load carrying capacity and ability of the connections to dissipate energy did not vary with the type of connection, but the common nails achieved higher ductility values due to their higher initial stiffnesses. A group effect was not observed between connections with three nails compared to connections with one nail. The different angle of force to lumber
grain direction also did not show significantly different results.
Nail bending tests showed that while the spiral nails had higher bending strengths, the weaker common nails still had bending strengths averaging 900 MPa. Also, the bending yield moment and stiffness of spiral nails was smaller, as their shank diameter was
smaller.
Several cyclic testing procedures were used. As noted earlier, many regimens result in
nail fatigue failures in full wall tests, although this is not a dominant failure mode in true
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 22
earthquake loading. The CEN short test (Comite Europeen de Normalisation, 1995) and the FCC (Karacabeyli, 1995) regimes were used in the connection tests, as was a new regime. Of these, the FCC showed the most nail fatigue failures. Connections with spiral nails failed in both panel pull-through and lumber withdrawal (pull-out). Common nails tended slightly toward panel pull-through. Overdriven nails were found to significantly weaken connections.
2.6 Modelling
Because of the relatively high cost of experimental tests and the limitations of test equipment to do full scale tests, comprehensive studies of shearwall buildings ultimately have to rely on analytical models. Several researchers have therefore created models of varying sophistication to simulate the behaviour of wood-based shearwalls in buildings.
With verified models, a reduced amount of calibration testing would be necessary to achieve confidence in designs, because such models would potentially incorporate the effect of the most important parameters in the prediction of shearwall behaviour. The basis for most models are assumptions regarding the properties and deformations of the shearwall components. Modelling of dynamic behaviour necessitates additional assumptions about cyclic loading characteristics of the components and also the nature of the earthquake.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 23
2.6.1 Monotonic Models
Foschi's analysis technique for wood diaphragms (1977) was one of the first using the finite element method. His nonlinear finite element model of a shearwall incorporated elements for the panel, the frame, the frame to frame connectors and the panel to frame connectors. It has been successful in reproducing test results for walls tested in uni• directional loading. The investigation revealed that the nonlinearity of the wall is primarily due to the nonlinear behaviour of the panel to frame connectors. In the model, the connectors were idealized as 3-degree-of-freedom (frame to frame) and 2-degree-of- freedom (panel to frame) spring elements. The nonlinear load-slip relationship (Figure
2.1) of these spring elements is expressed by the equation:
P= (m0 + WjAXl - exp(-kA/m0)) Equation 2.1
Figure 2.1 Load-slip relationship for connectors in Foschi's model (1977) described by Equation 2.1.
p , X Tan1 nij m,. /A Tan-1 k A
The sheathing was modelled with linear elastic orthotropic plane stress elements, and the frame members with linear elastic beam elements. Neither the softening characteristics of the connectors nor their hysteretic behaviour were considered by this model.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 24
Figure 2.2 Shearwall model by Tuomi and McCutcheon (1978).
Frame •4 P
Small arrows denote fastener displacements
Sheathing Panel
P
The analytical procedure by Tuomi and McCutcheon (1978) predicts the racking strength using the principle of energy conservation, the lateral resistance of individual fasteners, and the assumed deformation of individual fasteners. They assumed that under lateral loading, the sheathing remains rectangular, while the frame distorts as a parallelogram
(Figure 2.2). These assumptions were the basis for further derivations of the frame to panel connectors deformations. The load-deflection behaviour for a single nail was assumed linear. Nail tests were performed to find appropriate nail parameters. The interior nails were assumed to deflect in the same manner as the exterior nails. The wall, as modelled, therefore had a high dependence on the nailing pattern. Full scale and small scale tests were also included for comparison. The test results contradicted assumptions such as the four corner nails distort along the sheathing diagonals and racking strength per unit length of a panel is almost constant (Robertson, 1980). The authors found that the method was not suitable for stiffness calculations.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 25
Figure 2.3 Shearwall model by Easly et al. (1982).
Frame Small arrows denote fastener forces
Easly et al. (1982) also created a shearwall model by assuming the deformation characteristics for the sheathing, frame and connectors. In their model, the frame and sheathing deform as parallelograms. The bottom and top of the frame remain horizontal
(Figure 2.3). Formulae are based on assumed nail displacement and load direction according to observation, and load-slip curves from nail joint tests. Since these assumptions define the relative displacements of the nails, nail forces can be calculated using force and moment equilibrium. The model does not appropriately handle nonlinear load-slip relationships for nails, because it assumes that the vertical component of the nail force for a given nail is proportional to its distance from the center line of the panel. This is true only in the event of vertical slip or when the load-slip relationship is linear.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 26
Figure 2.4 Shearwall model by Gupta and Kuo (1985).
A similar model was developed by Gupta and Kuo (1985). The frame is assumed to deform into a distorted parallelogram and its deformation is defined by a shear angle. The sides of the frame are assumed to deform sinusoidally and the amount of this deformation is defined by the amplitude of a sine wave. The top and bottom remain horizontal (Figure
2.4). The sheathing may also deform as a parallelogram where the top and bottom edges do not remain horizontal. This deformation is defined by the angle between the panel's sides and the frame's sides, and the angle the panel's top and bottom edges make with the frame's top and bottom members. These two angles are independent of one another. The nail deformations follow from the frame and sheathing deformations. Therefore simplifying assumptions, that the corner nails deform along the sheathing diagonal
(Tuomi and McCutcheon) and that the vertical load is proportional to the distance from the centerline (Easly et al.), cannot be used. The model formulas are derived using the strain energy of the sheathing in shear, the studs in bending and the nails in deformation The authors reported that the model achieved good agreement with experimental tests. They
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 27
also found that the load-slip relationships of the nails dominated the shearwall performance.To accurately predict the dynamic response of shearwalls, the non-linear cyclic behaviour of connections need to be incorporated in the models.
2.6.2 Dynamic Models
Successful dynamic models can only be achieved with accurate modelling of the hysteretic behaviour of the wall and its components. Timber structures generally exhibit so-called "pinched hysteresis loops" (Figure 2.5) because of the gap that forms around the nail when the wood fibres are crushed. The next time the structure displaces, the load carrying capacity is greatly reduced when connectors move through the gap until contact with the wood is reinstated. When moving through the gap, the load carrying capacity is derived mainly from the bending resistance of fasteners and friction in the connections.
This is a highly nonlinear process.
Figure 2.5 Typical pinching of hysteresis loops in timber structures (highlighted).
Some models approximate the hysteresis loops with several straight line segments. For example, Karacabeyli's (1995), experimental results were successfully matched to
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 28
analytical results with a hysteresis model consisting of straight lines, which was an extension of previous work by Yasumura (1991). Other hysteresis models that are constructed from straight line segments include the elastic-plastic model, the bilinear model, the modified Clough model, the Q-hysteresis model, the Takeda model, and the slip model (Foliente, 1994). DRAIN 2D (Prakash et al, 1993), for example, is a multipurpose dynamic analysis program that has been used successfully for modelling shearwalls. Its hysteresis loops are also constructed with straight line segments developed for timber structures by Ceccotti and Vignoli (1989). It has been shown (Frenette, 1997), however, that the hysteresis model must reflect the behaviour of the structure in the displacement range experienced during an earthquake to yield accurate results. This is typically a limitation of straight line models that assume constant stiffness values for the hysteresis loops throughout the entire displacement range.
Dolan (1989) created a computer program for the dynamic analysis of shearwalls with the aim of creating a generally applicable model. Since it was too complex for widespread use in design, its main purpose was as a tool for benchmarking further analysis programs.
If it could be proven to be dependable, then simplified models could be checked against it.
Once verified, these simplified models would be more suitable for design applications.
Dolan expanded Foschi's 1977 connector model to incorporate a linear softening component following the ultimate load of the connector. He also incorporated a hysteresis model consisting of curves that oscillate within the monotonic envelope curve (Chapter 3,
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 29
Figure 3.2). This hysteresis model was subsequently adapted by Filiatrault (1989,1990) who constructed a finite element model of a shearwall with a general two-dimensional beam element to model the framing, a bi-linear corner connector element to model the frame to frame connectors, and an orthotropic plate element to model the sheathing. The sheathing connector element was composed of a non-linear, three-dimensional spring element modelling the panel to frame connection and a bi-linear sheathing bearing connector modelling bearing effects between adjacent sheathing panels. The bearing elements prevented the panels from overlapping. The connection between the sheathing and framing was modelled as three independent, non-correlated, non-linear spring connectors with exponential load-deflection curves following Foschi's exponential model.
Out of plane deflection of the panel was also allowed.
Time step integration methods are necessary when solving nonlinear dynamic problems.
Dolan used the constant acceleration method with tangent stiffnesses. His computer program reduced the time increment in a step if the error became too large. Viscous damping was assumed to be proportional to the mass of the structure. Models were verified by comparing to other models and to the full size shear wall tests carried out in the same study. The program predicted the in-plane behaviour of the walls well. It did not accurately predict the out of plane motion of the walls, but testing showed this behaviour to be insignificant for 9.5 mm thick sheathing and up. Suggested simplifications to the model included the elimination of the out-of plane component, restricting bearing
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 30
elements to the nodes only, modelling the sheathing as rigid plates, and modelling the frame as rigid beams with pinned ends.
Filiatrault incorporated some of Dolan's simplifications into his computer analysis program, also developed in 1989. His program, Shear Wall Analysis Program (SWAP) was used in this study for preliminary analysis of walls to be tested. It predicts the static or dynamic linear or nonlinear response of timber shearwall subassemblies when subjected to static or dynamic lateral loading. It also predicts the free vibration natural frequencies of walls. In 1991, Filiatrault and Foschi used it for that purpose.
The model treats the shearwall as a perimeter wood stud frame with rectangular sheathing panels that can have different sizes. These are attached to the framing with frame to panel connectors. The vertical studs are assumed to be rigid and pin ended. The sheathing is assumed to be thick enough that out of plane deformations can be neglected. The frame distorts into a parallelogram when loaded laterally, while the panels experience in plane shear deformations coupled with rigid body translations and rotations. The degrees of freedom used in the model are explained in more detail in Chapter 3.
The static and dynamic equilibrium equations are obtained through a displacement based variational formulation on the total strain energy of the system and the work done by the applied external force. The model of the nail behaviour is derived from Foschi's 1977 functional load-slip relationship and Dolan's additions to that relationship. The nonlinear responses are determined through Newmark-Beta constant average acceleration time-step
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 31
integration. A convergence criterion is used to control the iteration. For nonlinear static analyses, it is a displacement based model. For nonlinear dynamic analysis, it is energy based. A lumped seismic mass is modelled at the top of the frame while the mass of each panel is concentrated at its center. Hence, the global mass matrix is diagonal. It is assumed that the viscous damping in the wall can be represented as mass-dependent, so an equivalent viscous damping constant is used.
The model was verified against Dolan's test results. Good matches were reported, although the wall stiffnesses predicted by SWAP were higher than the test results. The crushing of the sill plate by the compression cord of the frame was cited as a possible explanation. This mechanism is not considered by SWAP and could reduce wall stiffness.
A comparison of predicted and measured fundamental frequencies indicated that SWAP predictions have good agreement with free vibration tests. Predicted mode shapes demonstrate a highly uncouple first mode, leading to the conclusion that shearwalls could be effectively modelled as single degree of freedom (SDOF) systems. Reasonably good agreement was observed between nonlinear dynamic predictions and dynamic test results with respect to amplitude and phase. Differences were attributed to using average nail parameters in the analysis.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 32
2.7 Oriented Strand Board (OSB)
Oriented strand board (OSB) is an engineered wood product. It is made from aspen or poplar wafers and was designed to be equivalent to plywood in its major mechanical properties even though it is composed of thin wooden strands approximately 102 m long and 25 mm wide. These are dried and then coated with resin and wax before being oriented in cross directional layers. The bond between the strands is achieved with water resistant adhesive, high temperature and high pressure. A benefit of this product is that it can use wood from small diameter trees and underutilized species resulting in economic and environmental benefits.
Another advantage of the OSB product is that it can be manufactured in a range of dimensions, from 0.9 x 1.8 m to 3.3 x 7.3 m. Often, it is cut into or manufactured as 1.2 x
2.4 m standard size panels. In the tests by He (1997), walls constructed with single, large size sheets of OSB were shown to be significantly stronger and stiffer than those constructed with standard size panels. Continuous large size panels may also behave better in earthquakes if stiffer walls do suffer less damage. These walls would still depend on nail deformation for ductility and energy dissipation. Thus, taking the nails that would be at the edges of standard size panels and distributing them to the edges of a large size panel, may provide much higher stiffness to prevent damage and add to durability, while ensuring that failure, when it does occur, is inelastic and non-catastrophic.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Background 33
OSB panels must meet certain manufacturing and/or performance requirements in
Canada. One such performance standard is Canadian Standards Association CSA-0325, which incorporates requirements for stiffness and strength, depending on the intended use of the product. The requirements do allow flexibility in manufacturing. In 1994, the OSB
output was 9.6 million m3, or half of the plywood output in North America (Wood
Technology, 1996). It is expected that 82% of the structural panel production in Canada in
2000 will be of OSB. The corresponding figure for the US is 44%.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels CHAPTER 3 Methodology
This chapter describes the methodology employed to complete the tests on which this thesis is based. The first section describes the analytical program used and procedures followed during analytical studies. The second and third sections describe the setup and procedures for the static tests and dynamic tests, respectively.
3.1 Analytical Studies
A series of analyses were performed using the Shear Wall Analysis Program (SWAP).
They considered shearwalls under static and dynamic loading. The analysis results were used to plan the static and dynamic testing programs.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels 34 Methodology 35
3.1.1 Analytical Program
The Shear Wall Analysis Program (SWAP) is described briefly in Chapter 2. Detailed information can be obtained from Filiatrault (1990). His paper describes the model development based on variational energy formulations of the stiffness and mass matrices, the incorporation of the equivalent viscous damping ratio and the constant average acceleration Newmark-Beta time integration method.
Figure 3.1 Frame and panel displacement assumptions in SWAP.
The degrees of freedom con• sidered in the Shear Wall Analysis Program (SWAP) model are as follows:
Uj, Vj and 6j - the absolute rigid body translations and rotations of the center of mass of each sheathing panel, i.
usi - the symmetric shear deformation at the top and bottom of each sheathing panel, i.
uf - the absolute lateral displacement at the top of the frame. x
Each wall was modelled as panel elements and nails. The panel elements can translate horizontally, rotate rigidly, and deform in shear (Figure 3.1). The input file for each model required only the shear modulus and density of each panel in addition to the information describing its geometry. The frame was defined only by the locations of the
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 36
nail lines. Assumptions about the frame were that the studs and plates behave as rigid pin ended members, with no vertical displacement allowed (Figure 3.1). These assumptions about the frame movement serve only to define the slip demanded from each nail by comparing the frame position and sheathing position at the nail location. A full set of nail parameters had to be specified for each line of nails in the model's data file. The parameters used to define each nail's behaviour are described in Figure 3.2 and Table 3.1.
SWAP is capable of performing the following types of analyses: static linear, linear free vibration, static nonlinear, linear dynamic and nonlinear dynamic. For the dynamic analyses, the data file requires the inertia mass at the top of the wall. The model does not include dead load for any type of analysis, whereas all experimental tests were carried out with dead load. Other input parameters are as follows:
• the equivalent viscous damping coefficient for mass proportional damping
• the time step increment of the earthquake data
• the time interval for results reporting
• the number of time integration steps
• the energy balance tolerance for each step
• the number of time-acceleration pairs that describe the dynamic excitation
• the peak ground acceleration to which the excitation is scaled.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 37
Table 3.1 Nail model parameters from SWAP.
Symbol Unit Description
Ko kN/mm Initial tangent slip modulus of initial curve K, kN/mm Modulus of asymptote drawn at ultimate load of initial curve
K2 kN/mm Modulus of assumed linear drop off portion of the load-slip curve
K3 kN/mm Modulus where hysteresis loops intercept the load axis
Po kN Load value where the asymptote drawn at ultimate load intercepts the load axis
Pi kN Load value where the hysteresis loops intercept the load axis
^max mm Slip value corresponding to ultimate load
Figure 3.2 Nail model parameters from SWAP.
A P
A Pi " max —-— ^ A
Po 1 1
Initial Curve: P= -(P0 + K1|A|)(1 - exp(-K0|A|/P0))
Some modifications to the original program were made. Changes were made to the format with which data was read from input files and reported in output files. Some array sizes
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 38
were changed to accommodate the needs of the study. The permissible number of nail lines per wall was increased from 4 to 15. The number of time-acceleration pairs was increased from 3000 to 10000, as was the number of time integration steps. These changes were verified by comparing output from the new program to the sample output reported in the SWAP user's manual (Filiatrault, 1989) and also to results reported in
Filiatrualt's paper (1990).
3.1.2 Procedures
Once the modified program was verified, several analyses were to be performed to study different types and lengths of walls in preparation for the experimental testing program on the shake table. It was desirable to predict whether the difference in dynamic response between a wall constructed with a single, oversize OSB panel and a wall constructed with multiple, standard size panels could be detected in shake table tests. There was concern that longer walls would have to be tested to demonstrate the benefits of oversized panels.
Since the existing dynamic test setup could only accommodate 2.44 m long walls, a new dynamic test setup accommodating longer walls would have been required if the program predicted that the difference between single and multiple panel 2.44 m walls was not sufficiently apparent. Furthermore, exterior nail spacing was found to have a significant effect on the walls tested by He (1997), therefore several analysis runs were planned to predict the effect of this parameter.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 39
Another set of analyses were designed to analyze the sensitivity of the program to input parameters, such as the iteration time step size, the panel shear modulus, the damping ratio, the earthquake input and the nail parameters.
The information used to construct input files for the models is as follows:
• Wall length = 2.44 m, 3.66 m or 4.88 m
• Spacing of nails in the field of a sheathing panel (interior) = 300 mm
• Spacing of nails at the edges of a sheathing panel (exterior) = 150 mm or 75 mm
• Stud spacing = 400 mm
• Damping ratio = 1%, as per Filiatrault, 1989, 1990 (also 2%, 5% and 10%)
• Time step integration size (At) = 0.01 s (also 0.005, 0.02 and 0.05 s)
• Shear modulus for OSB = 1.5 kN/mm2 (also 0.3, 0.75, 3.0 and 7.5 kN/mm2)
Table 3.2 Nail parameters for 2.67 mm dia., 50 mm long spiral nails.
Parameter Perpendicular to grain value Parallel to grain value KQ (kN/mm) 0.561 0.579
K, (kN/mm) 0.034 0.013
K2 (kN/mm) 0.044 0.018
3 K3 (kN/mm) 0.080 0.080
Po(kN) 0.751 0.926
P,(kN) 0.141a -0.141
Amax (mm) 12.5 13.0
"Cyclic values are only available from parallel to grain nail tests.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 40
The nail parameters were taken from Sieber et.al., (1997). Unless otherwise stated, the values for nails perpendicular to grain were used. These values and the parallel to grain values appear in Table 3.2. They are for 2.67 mm diameter, 50 mm long spiral nails.
Figure 3.3 East-West accelerogram from Joshua Tree Station - 1992 Landers California Earthquake.
Event: Landers California Earthquake Date: June 28,1992 Station Location: Joshua Tree, California Historical Magnitude: 7.3 Accelerogram Peak Acceleration: 0.29 g
I BlLflAflJVAl^ilMA AllAflArMrtl 1 f 'r r i-iinl- - nrtf.nr ,-,,r,r M. ,, mm -0.1 1 -0.2 -0.3
0 10 20 30 40 50 60 70 80 Time (s)
The selection of a suitable earthquake input motion was an important step to assure that relevant results would be obtained from the shake table tests. It was important to cause sufficient damage to the walls to observe nonlinear behaviour, ultimate behaviour and failure modes. Based on results from several analyses and previous experience, the East-
West accelerogram recorded at Joshua Tree Station, during the Landers California
Earthquake (1992) was used as input motion. It was shown to cause severe damage in walls in previous tests of shearwalls (Latendresse and Ventura, 1995). The walls in the
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 41
1995 study were significantly excited by the Joshua earthquake because the large excitations in this record endure for approximately 30 seconds and include two major acceleration peaks (Figure 3.3). Figure 3.3 also reports quantitative characteristics of the ground motion. The work by Latendresse and Ventura (1995) reported that the spectral accelerations for the Joshua earthquake indicate that this ground motion has dominant periods in the range of the natural periods measured for 2.44 m long shearwalls.
3.2 Static Tests
This section describes the setup used for static testing. It then relates the procedures followed during the tests. A summary of all static tests is also provided.
3.2.1 Setup
The test setup that was built for He's tests (He, 1997) was used in this study (Figure 3.4).
The assembly was capable of monotonically and cyclically testing shearwalls of maximum size 2.4 x 7.3 m. The shearwalls were mounted vertically between a base beam and a load distribution beam. Each beam was a W150 x 22 steel I section. The base beam was 7.6 m long, while there were load distribution beams of different lengths were used to accommodate walls of different lengths. The shearwalls were bolted to each beam with up to 18 12.7 mm diameter class 5 steel bolts at a spacing of approximately 400 mm (bolts were positioned between studs). The base beam was also bolted to the laboratory floor, which prevented translation of the bottom of the shearwall. The purpose of the load
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 42
distribution beam was to evenly distribute in-plane lateral shear loads and a vertical dead load to the wall. Lateral support frames at each end of the wall prevented out-of-plane displacement. Figure 3.4 shows a photograph of a 2.44 m wall in the test assembly.
Figure 3.4 Static test setup schematic for 7.3 m wall (He, 1997) and photograph of 2.44 m wall in the setup.
Load Lateral load Out-of-plane support distribution actuator frames (2)
Vertical load application (constant)
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 43
A 222 kN servo-controlled double-acting hydraulic actuator with an in-line load cell mounted to the reaction frame in line with the top of the wall was used to apply the lateral load. Pairs of hydraulic jacks were used to pull on both sides of the wall, simulating dead loads. They were attached to the floor and mounted to cross beams that were bolted to the load distribution beam. Each jack had a load capacity of 17.24 kN.
3.2.2 Procedures
The lateral load was applied through the displacement controlled actuator at a rate of 0.13 mm/s for the monotonic tests. The cyclic tests were carried out at a constant velocity of
0.4 mm/s, which meant declining frequency with increasing displacement amplitude. The constant vertical load of 9.12 kN/m was applied through pressure controlled actuators prior to lateral loading. This was meant to simulate the weight of a second floor of a two storey building. The load is assumed to be carried by four walls (He, 1997). It is equivalent to a gravity load of 5.0 kPa over a 7.3 x 7.3 m area. Information from each test was gathered by a 386/25 personal computer data acquisition system with 16 AID channels and LabTech Notebook data acquisition software. Data were sampled at 1 Hz during monotonic tests and at 10 Hz during cyclic tests. The following data were recorded:
• the load and the displacement of the actuator
• the displacement of the top of the wall
• the diagonal shear displacement of the sheathing
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 44
• the uplift displacement at the bottom corner of the wall
• the vertical separation between the sheathing and top plate at one corner.
Figure 3.5 describes the cyclic testing protocol. For each type of wall, one monotonic test was done to establish the virgin envelope curve. The results were used to determine the
lateral displacement when the loads reached 50% and 80% of its recorded maximum load, respectively. These values were used to define the displacements in the testing protocol.
Cyclic test results could be also fitted to hysteresis models in programs such as DRAIN
2D (Prakash et al., 1993), which are commonly used in the dynamic analysis of full
structures.
During cyclic testing, each wall was first subjected to three cycles at the displacement
corresponding to 50% of its maximum load in the monotonic test. Next it was subjected to
three cycles at a displacement corresponding to 80% of its maximum load in the
monotonic test. Then it experienced one or three cycles at the displacement corresponding
to 50% of maximum load. Finally, the walls were monotonically loaded to failure. This
unusual cyclic testing schedule was developed in response to findings from He (1997) and
Sieber et. al. (1997). These showed that schedules with more cycles promoted fatigue
failures in the nails, which was not expected to be dominant in walls subjected to actual
dynamic earthquake excitations.
The multiple cycles at each displacement level are used to observe the load degradation
between subsequent cycles. Returning to lesser displacement cycles revealed the extent of
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 45
the degradation of the wall's stiffness. Only one wall was subjected to three cycles when returning to the lesser displacement level. This one test revealed that degradation was not significant in this cycle group, so multiple cycles were not used at this stage in the remaining tests.
The static tests that were carried out are summarized in Table 3.3 at the end of this chapter and the wall types considered during testing are defined in the table. They are as follows:
• Type A - standard panels, standard exterior nail spacing (150 mm).
• TypeB- single panel, standard exterior nail spacing (150 mm).
• Type C - single panel, reduced exterior nail spacing (75 mm).
Only one Type B wall with vertically oriented sheathing was tested because its monotonic behaviour was not sufficiently different from the monotonic behaviour of the Type B wall with horizontally oriented sheathing to merit further tests.
Figure 3.5 Cyclic testing protocol.
I Time •
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 46
The test walls were constructed in the University of British Columbia Structures Lab, using No. 2 and better Spruce Pine Fir 38 x 89 mm dimensional lumber as framing members and 9.5 mm thick Performance Rated W24 Oriented Strand Board (CSA-
0325.0-M88) as sheathing panels. The frame members were primarily connected by 76 mm coil-fed common nails using a Bostitch-N80CB pneumatic nail gun. A few of these connections were hand driven. The sheathing members were nailed with gun driven 50 mm spiral nails.
All the walls were 2.44 m long. Interior nails were always spaced at approximately 300 mm, whereas the nail spacing at panel edges (exterior) varied according to wall type.
Studs were spaced at 400 mm and end studs of each wall were double members, as were the top plates. Type A walls were blocked continuously, with each block being end-nailed to a stud on one side and toe-nailed on the other side. The long axis of the oriented strand board (OSB) was parallel to the length of the wall in all but one type B wall. This resulted in a predominantly horizontal orientation of the strands. In addition to the dead load imposed by the testing apparatus, MGA HD8 hold downs were used at the bottom corners of the walls. The hold downs, which are required on walls in real structures, prevented uplift, ensuring that the racking strength of the walls was being tested (Figure 3.6).
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 47
Figure 3.6 Standard hold down attached to test wall.
3.3 Dynamic Tests
3.3.1 Setup
Dynamic tests were carried out on the shake table in the Earthquake Engineering Research
Laboratory at the University of British Columbia. The shake table, 3 m x 3 m in plan
dimension, is an aluminum cellular structure that can support a payload capacity of 156
kN. Its motions are driven by five hydraulic actuators, one horizontal and four vertical.
Each actuator has a displacement range of +1-1.6 cm. In this configuration the table can
move in one direction horizontally and also vertically. Only the horizontal direction was
considered in this study, so the vertical actuators were replaced with pin ended vertical
supports. The actuator used to produce the horizontal motions can generate up to 156 kN
of force.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 48
A specially constructed steel shearwall test frame, built by Dolan (1989), was used to test
2.44 x 2.44 m walls. It was built to carry an inertia mass of 4545 kg. It was designed to
allow the application of horizontal loads to the wall specimen, without providing any
lateral resistance. The inertia mass approximately represents the upper two storeys of a
three-storey North American-style residential building. The frame is braced in the out of plane direction. A photograph and schematic of the frame and shake table appear in
Figure 3.7. For more detailed schematics of the frame, consult Appendix A. A vertical
load was applied to the wall by a single cable strung through a system of pulleys and
attached to a pressure controlled actuator that was mounted to the shake table (Figure 3.8).
The wall was attached to the base beam with bolts and hold downs similar to the static test
setup. The top of the wall was bolted to a spreader beam that was connected to the frame
at each end with a pinned link.
Adjustments had to be made to the base beam to ensure that the top of the wall lined up
horizontally with the attachment points on the frame. This was important because the
horizontal load was transferred to the wall with the pinned links at each end of the
spreader beam. These links permitted the wall a small amount of vertical motion. In the
initial position, however, the links had to be horizontal, otherwise a significant vertical
load component would be introduced to the wall.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 49
Data from tests were stored digitally and checked for integrity immediately after running each test. All data acquisition channels were conditioned with variable gain buffers and variable cut-off filters. Acquisition software provided results in ASCII format data files.
3.3.2 Procedures
Although the inertial load of 4545 kg represented the mass of two stories, the applied vertical load on each wall was kept consistent with that applied in the static tests completed in this study and by He (1997). As stated above, this load was approximately
9.12 kN/m.
All the tests were carried out using the Joshua Tree earthquake described in section 3.1.2.
Recall that this excitation had a peak acceleration of 0.29 g. Most of the tests used the original record scaled to a maximum acceleration of approximately 0.35 g. This is the same amplification used in previous tests of 2.44 m long plywood walls (Latendresse and
Ventura, 1995) and was found to represent an earthquake of medium intensity. Other tests were performed with the maximum acceleration of the earthquake scaled to approximately
0.52 g. The complete list of tests is given in Table 3.3.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 50
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 51
Figure 3.8 Close-ups of dead load pulley system.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 52
In addition to videotaping, the following time history data were recorded during testing:
• the displacement of the shake table
• the displacement of the top of the wall
• the acceleration of the shake table
• the acceleration at the top of the wall
• the acceleration at the top of the frame
• the vertical uplift of the west end stud
• the vertical uplift of the east end stud
• the tension in the cable used to apply the dead load
• the base shear as estimated by the change in load transducer in the actuator.
The first tests revealed the need to measure uplift. Table displacement and acceleration of the table were measured to quantify the simulated motion. These data are used to check that the simulated excitation and the recorded excitation are in good agreement, as electronic and mechanical limitations of the earthquake simulator prevent them from matching perfectly. The weight of the frame specimen, capacity of the actuator hydraulic pump, friction in the system and maximum allowable shake table displacement contribute to difference between simulated and recorded displacements. More importantly, these table displacement and acceleration readings ensure that the movement of the table is consistent between tests.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology S3
Upper bound values for the free vibration frequencies of most walls were determined by
hammer tests. For more information on hammer tests, see Ewins (1992) and Villemure
(1995). Free vibration values were measured with and without dead load. Chapter 5 provides a more detailed description of individual tests and the values obtained during
those tests.
Walls for dynamic testing were constructed in the same manner as for static tests (detailed
in section 3.3.2). Descriptions of all of the test walls are given Table 3.3. Note that the table makes a division between static and dynamic tests.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Methodology 54
Table 3.3 List of tests with wall descriptions.
Test Wall Number Sheathing number Type3 of Panels Size of Panels Orientation Loading Static Tests 1 C 1 2.44 x 2.44 m Horizontal Monotonic 2 B 1 2.44 x 2.44 m Horizontal Monotonic 3 C 1 2.44 x 2.44 m Horizontal Monotonic 4 A 3 1.22 x 1.22 m Vertical Monotonic 1.22 x 1.22 m 1.22 x 2.44 m
5 B 1 2.44 x 2.44 m Horizontal Monotonic 6 C 1 2.44 x 2.44 m Horizontal Cyclic 7 B 1 2.44 x 2.44 m Horizontal Cyclic 8 A 3 1.22 x 1.22 m Horizontal Cyclic 1.22 x 1.22 m 1.22 x 2.44 m
9 C Demonstration only. No data recorded. Cyclic Dynamic Tests 10 C 1 2.44 x 2.44 m Horizontal Earthquake 11 A 3 1.22 x 1.22 m Horizontal Earthquake and Impact 1.22 x 1.22 m 1.22 x 2.44 m
12 B 1 2.44 x 2.44 m Horizontal Earthquake and Impact 13 B 1 2.44 x 2.44 m Horizontal Earthquake and Impact 14 A 3 1.22 x 1.22 m Horizontal Earthquake and Impact 1.22 x 1.22 m 1.22 x 2.44 m 15 C 1 2.44 x 2.44 m Horizontal Earthquake and Impact
Wall Types: Nail spacing at panel edges Type B for Type A,B: 150 mm. and Type A Type C Nail spacing at panel edges for Type C: 75 mm.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels CHAPTER 4 Analyses
Non-linear time step analyses using a modified version of Shear Wall Analysis Program
(SWAP) formed a part of this study. Chapter 3 describes the program modifications to vector sizes and input/output formatting. This section reports on verification of the modified program.
4.1 Modified Program and Verification
Program verification was achieved through several methods. First, comparisons were made with example files that were provided in digital format with the user manual
(Filiatrault, 1989). Provided were input files for a 2.4 m wall sheathed with two 9 mm thick vertically oriented plywood panels. The files, created using imperial units, included input and output files for each type of analysis. In nominal metric measurements the wall had 38 x 89 mm spruce-pine-fir studs spaced at 600 mm and 8d (63.5 mm long)
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels 55 Analyses 56
galvanized common nails. Nails were spaced at 100 mm on the panel exterior edges and
150 mm along the intermediate studs. The panelling had a shear modulus of approximately 600 MPa and the nail parameters were as given by Filiatrault (1989).
Table 4.1 Modified SWAP results compared with original SWAP example results.
Free vibration analysis: Free vibration analysis: approximate Program version natural frequencies (Hz) mode shapes Original 1 3.37009 (Filiatrault, 1989) OJ 2 280.779 ! 3 342.065
Modified 1 3.37009
2 280.779 ! 3 342.065
Program version Nonlinear static analysis: Nonlinear dynamic analysis: Max. top Peak load (kN) of wall acceleration (mm/s2) Original 37.8 4783.53
Modified 37.8 4783.53
In all types of analyses, (free vibration, linear static, nonlinear static, linear dynamic and nonlinear dynamic), the output data from the modified program matched that provided by
Filiatrault. Some of the results are presented in Table 4.1. Note that the first natural
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Analyses 57
frequency is highly dominant, and that it corresponds to a mode shape involving rotation of the panels and shear deformation of the frame.
Second, a data file was prepared to represent one of the walls tested by Dolan (1989).
Filiatrault used this wall, which had the same configuration as that in the example files, for verification of the original program. Filiatrault published free vibration, nonlinear static and nonlinear dynamic results obtained from these data files and compared them to actual test results. His results are reported in Table 4.2, along with results from the modified version of SWAP. These results should be the same as the results from the example files described above. They differ slightly because when Filiatrault used metric values to describe Dolan's walls, they were not exact conversions from the corresponding imperial values.
In the nonlinear dynamic analysis, the input excitation was the E-W component of the Taft recording of the 1952 Kern County earthquake with a maximum acceleration of 0.18g.
Filiatrault published the first 15 seconds of the relative displacement time history of the top of the wall (drift) generated by SWAP as compared to the experimental results. The results (Figure 4.1), obtained from the modified SWAP program were compared visually to the published results and were found to be in good agreement.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Analyses 58
Table 4.2 Analyses replicating experimental test by Dolan (1989)
Analysis or Program Frequency analysis: natural frequency (Hz) and Peak load experiments version approximate mode shapes (kN)
Mode 1 Mode 2 Mode 3
Analysis Original3 3.21 191.31 357.78 31.4 CO 1 Analysis Modified 3.21 191.31 357.78 31.8 1 1 Experiments3 3.4 - - 33.6 (Average values)
a Reported by Filiatrault, 1989
Figure 4.1 Drift results for Dolan (1989) wall input analyzed with modified SWAP.
0 5 10 15
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Analyses 59
Lastly, a nonlinear analysis was performed with an input file designed to represent a wall from previous tests (Latendresse and Ventura, 1995; Durham et al, 1996). The wall had similar configuration to those used in the above verifications, except the stud spacing was approximately 450 mm. The results of the analysis are compared to actual results in
Figure 4.2. It is clear from the figure that the match is poor, possibly because the equivalent viscous damping ratio of 1% was assumed by referencing Filiatrault's work
(1989, 1990). Filiatrault used this value in his analyses because it produced the best match with test results. The effect of varying this parameter is explored in sensitivity analyses
(Appendix B).
Figure 4.2 Comparison of drift time histories from a previous experiment (Latendresse and Ventura, 1995; Durham et al, 1996) and from the modified version of SWAP.
30 hi i—lr~ 20 A cxpenrneni 10 [4 /ka
10 •ii i.iiii IU u 0 -10 -20 -30 I.I, 10 20 30 40 50 60
Time (s)
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Analyses 60
Although the modified SWAP program was able to effectively reproduce Dolan's test results, its dependability in predicting other test results is called into question in the Figure
4.2 comparison. In Figure 4.2, the analysis shows significantly higher frequencies in its drift time history response when compared to the drift time history response recorded in the experiment. The analytical drift values are also much lower than the experimental values. Not shown is a comparison of acceleration time history response. The analytical acceleration response had a higher frequency and magnitude compared to the experimental acceleration response.
Further analyses, including the aforementioned sensitivity analyses, showed SWAP to be unreliable. These are described in more detail in Appendix B. Caution should be used whenever the behaviour of structures or structural components is modelled. With SWAP, however, this caution should be extreme.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels CHAPTER 5 Test Results and Discussion
The experimental results for the various tests and the discussion thereof are presented in this chapter. First, the static test results and discussion are treated, followed by the dynamic results and discussion. Finally, a summary presents the main issues and findings from static and dynamic testing.
5.1 Static Tests
5.1.1 Results
Results from monotonic and cyclic quasi-static tests are presented in this section.
Information on the testing methodology, including the displacement controlled loading regimes can be reviewed in Chapter 3. Table 5.1 lists,the wall schematics and gives the
maximum load, Pmax, and the top of wall displacement (drift) that corresponds to the peak load, A„.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels61 Test Results and Discussion 62
Table 5.1 Results from static tests.
Test Number Wall Type3 Loading A„ (mm) Number of nails 1 cb Monotonic 23.13 42.85 173 2 B Monotonic 21.94 54.63 109 3 C Monotonic 35.40 60.85 173 4 A Monotonic 17.38 57.49 157
5 Bc Monotonic 19.44 53.78 109 6 C Cyclic 38.01 53.07 173 7 B Cyclic 21.72 51.24 109 8 A Cyclic 20.42 78.58 157
Wall Types: Nail spacing at panel edges Type B for Type A,B: 150 mm. and Type A Type C Nail spacing at panel edges for Type C: 75 mm.
This wall did not have any hold downs c This wall had vertically oriented sheathing
During all of the tests, the applied dead load fluctuated between 21.63 kN and 22.66 kN.
Recall that the target value was 22.25 kN (9.12 kN/m). As discussed in Chapter 3, uplift, diagonal strain and sheathing to frame separation at a top corner were measured. The maxima and minima for these values depended on the displacement at which each test was stopped, because during push over, uplift increases with displacement. The uplift did not exceed approximately 20 mm in walls tested with hold downs. The diagonal displacement did not exceed 2 mm in tests of Type B and Type C (single panel) walls. Type A walls
(with multiple panels) experienced diagonal strains a magnitude greater. Sheathing to framing separation did not exceed 15 mm, and was smallest in Test 1 where there was no
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 63
hold down. The Type A walls had slightly less sheathing to frame separation at the top corner than the Types B and C walls.
Monotonic loading. Test 1 was performed on a Type C wall. No hold downs were used because the dead load was expected to prevent significant uplift and overturning. Large uplifts were observed, however, and many of the studs began pulling out of the bottom plate (Figure 5.1). Also, the measured peak load of 23.13 kN was significantly lower than expected compared to previous tests (He, 1997; Durham, 1996). All further tests were carried out with hold downs. A standard type hold down that was used in all static and dynamic tests appears in Figure 3.6.
Figure 5.1 Uplift during Test 1.
~"~"J * 1 M 4 1 •?
J>L. \ ji
1
Test 2 was performed on a Type B wall. The peak load for the wall was measured as 21.94 kN. Nails pulling through the sheathing at the bottom corners dominated the failure.
Nails pulling out of the frame and then pulling through the sheathing were observed in the
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 64
top comers as well (Figure 5.2). The uplift was minimal compared to Test 1. See the
photograph of the bottom corner in Figure 5.2a.
Figure 5.2 Nails pulling out of the frame and pulling through the sheathing in Test 2.
a) Bottom Corner b) Top Corner
Another Type C wall was loaded in Test 3. Small and temporary drops in load were observed in conjunction with audible cracking noises, although specific failures that could be attributed to the cracking noises were not observed. The studs remained straight and deflected in the manner of pin-ended members before the wall reached its peak load of approximately 35 kN. After this point, the studs experienced significant bending. The bending tended to concentrate in the bottom half of the studs.
A warp in the frame of the Type A specimen used in Test 4 caused local separation between its sheathing and framing (Figure 5.3a). It also appeared that many of the nail heads had been damaged, perhaps from improper loading of the nail gun. Nevertheless, pull-through of nails was not excessive compared to pull-out of nails, suggesting that the damaged nail heads did not critically affect the test results. There appeared to be less
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 65
uplift of studs with this regular multiple panel wall than was observed with the previous
single panel walls. The nailing at the blocking was poor. The nails tended to be near the
edge of the blocks, which induced wood splitting of the blocking and nail pull-out. Most
of the relative panel-to-panel and panel-to-frame motion occurred at the blocking. The
failure also initiated along the blocking (Figure 5.3b). The measured peak load was 17.38
kN.
Figure 5.3 Test 4. a) Separation of sheathing and frame prior to test, b) Failure and relative movement at blocking.
There were nailing faults in the Test 5 specimen. Some nails at the top protruded between
the double plates. Like the other walls, pull-out and pull-through of nails were the
dominant failure modes of this wall, which was a Type B with vertically oriented
sheathing. All the other walls were sheathed with the principal flake orientation of the
panel parallel to the horizontal direction. Its peak load was approximately 89% of the
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 66
peak load for the Type B wall with horizontally oriented sheathing. Vertically oriented walls were not included in the rest of the static and dynamic testing programs.
Cyclic loading. The first cyclic loading regime was performed on a Type C wall in Test 6.
Nails began to pull through the sheathing by the second cycle in the first set of cycles.
During the second set of cycles, there were many cracking noises, but the studs displayed no observable bending behaviour. Bending in the studs began as the load peaked at approximately 38 kN (Figure 5.4a). During the last set of cycles, stuttering noises appeared to be caused by movement at the hold down connection. In the remaining tests only one cycle was used for the last sets of displacement cycles where significant pinching was observed as the successive pinched loops were almost identical in this test. At failure, a stud split at the tie down (Figure 5.4b), resulting in uplift along with typical pull-out and pull-through of nails. Nails began pulling out and then pulled through as the load peaked and then degraded.
There were some over-driven nails in the interior of the Type B wall loaded in Test 7.
Cracking sounds indicating damage were detected early in the second group of cycles.
The dominant failure occurred with the pull-through of nails. The peak load was measured as 21.72 kN.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 67
Figure 5.4 Test 6 a) Bottom of studs bending after peak load was reached, b) Split stud at bottom corner.
The Type A wall that was loaded cyclically in Test 8 had better quality nailing than the
corresponding regular, multiple panel wall that was loaded monotonically in Test 4.
Through the second set of cycles the Test 8 specimen did not appear to suffer as badly as
other specimens, although movement of the top panels was apparent. The movement of the top panels was much greater than the movement of the bottom panel. Failure initiated along the blocking at the bottom of the smaller top panels (Figure 5.5).
Figure 5.5 Failure along blocking in Test 8. Top panels show more damage.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 68
5.1.2 Discussion
Important values pertaining to the static behaviour of the various types of walls appear in
Table 5.2. In addition to the peak load (Pmax), and the top displacement (drift) at Pmax, (AJ,
the table lists the yield drift (A^,eW) of each wall along with the stiffness (Su), shear strength
(G") and ductility (D) calculated for walls tested monotonically. Ayietd is the displacement
that corresponds to a shear equal to half of the peak load Pmax. The other values are calculated with the expressions in Equation 5.1 to Equation 5.3, where L is the length of the wall and H is the height of the wall. In these tests, both the length and the height of the wall are taken as 2.44 m. The last column of the table reports the post-peak load
displacement of the wall when the load had fallen to 80% of Pmax (A08Pmax).
P, max Ultimate Shear: Su= Equation 5.1 L
•H Shear modulus: G' m ax Equation 5.2 2 • Akyield •L
A, Ductility Ratio: D= Equation 5.3 A i yield
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 69
Table 5.2 Summary: Analysis of static testing results
Wall p Test Ayield G' ^0.8Pmax No. Type3 Loading max (mm) (mm) (kN/m) (MN/m) D (kN) (mm) 4 A Monotonic 17.38 57.49 8.26 7.12 1.05 6.96 80.17 8 A Cyclic 20.42 78.58 21.89 - - - -
2 B Monotonic 21.94 54.63 9.14 8.99 1.20 5.98 75.19
5 Bb Monotonic 19.44 53.78 8.69 7.97 1.12 6.19 74.48
7 B Cyclic 21.72 51.24 17.82 - - - -
1 Cc Monotonic 23.13 42.85 6.20 9.48 1.87 6.91 70.14
3 C Monotonic 35.4 60.85 12.87 14.51 1.38 4.72 97.11 6 C Cyclic 38.01 53.07 24.46 - - - -
8 Wall Types: Nail spacing at panel edges Type B for Type A,B: 150 mm. and Type A Type C Nail spacing at panel edges for Type C: 75 mm.
b This wall had vertically oriented sheathing e This wall did not have any hold downs
When loaded monotonically, the Type B wall in Test 2 (oversize panel, regular nail spacing) reached a peak load that was 26% greater than that reached by the Type A wall in
Test 4 (regular panels). The Type C wall in Test 3 reached a peak load that was 204% of that of the regular panel wall peak load (Test 4). The capacity increases of Type B and C walls over Type A walls are consistent with He's findings (He, 1997).
The peak load of the Type C wall without hold downs in Test 1 was only 32% higher than that of the regular panel wall (Test 4). In this Type C wall, the observed low peak load, significant uplift and minimal sheathing to frame separation indicated that the dead load imposed on this wall was not sufficient to ensure a true racking behaviour. Therefore, test
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 70
results from the Test 1 Type C wall should not be used to make judgements on the relative racking response of the wall types. The results do, however, highlight the importance of restricting specimens to racking behaviour. Compared to tests of the 7.3 m long walls with the same linear dead load but no hold downs (He, 1997), these results reinforce the observation that short walls are more susceptible to overturning.
Figure 5.6 displays all of the load-drift curves from all monotonic tests. These curves and
the information for Au in Table 5.2 suggest that the drift at peak load is not proportional to the peak load for this length of wall when hold downs are used. In these tests, the walls reached their peak load within a drift of 50 to 61mm even though the peak loads ranged from 17 to 36kN. This is contrary to the findings reported by He (1997) which showed that an increase in strength corresponded to a decrease in the drift at peak load.
As shown in Table 5.2, the Type A walls were weaker than the Type B walls which, in
turn, were weaker than the Type C walls, both in terms of Su and G'. The less stiff walls were slightly more ductile, when ductility is as defined in Equation 5.3. Walls with regular panels showed greater diagonal strain and less sheathing to frame separation.
These measurements were consistent with observations of significant movement along the seams where the panel edges were attached to the blocking. The increased movement here may have led to the increased ductility values.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 71
Figure 5.6 Load vs. drift curves for monotonic tests
Comparisons of the load-drift curves obtained in the monotonic and cyclic tests of the three types of walls are shown in figure 5.7. Here, the original monotonic load-drift curve for each wall type was reproduced in the negative force and drift quadrant to permit comparisons with the cyclic test results. In general, for each type of wall, there was good agreement between monotonic and cyclic tests (Figure 5.7 and Table 5.2). The biggest discrepancy was found in Type A walls (Tests 4 and 8) where the monotonically tested wall (Test 4) had a peak load of 17.38 kN, while the cyclically tested wall had a peak load of 20.42 kN. This monotonically tested wall may have been weaker because of observed poor nailing.
As discussed in Chapter 3, previous work (He, 1997) showed that a particular cyclic protocol regime can lead to a predominance of nail fatigue failures in shear walls, a
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 72
phenomenon that is seldom observed in either dynamic tests or earthquakes. The cyclic protocol adopted for this study was intended (He, 1997) to achieve nail failure modes consistent with observed failures in shearwalls in significant earthquake events, that is, either nail pull-out or nail pull-through. In cyclic tests 6, 7 and 8, nail fatigue failures were not observed, only nail pull-out and nail pull-through failures.
5.2 Dynamic Tests
As with the static tests, the dynamic test results and discussion are dealt with in separate sections. An analysis section is also included because of the increased complexity of the dynamic analyses. All of the test methods are detailed in Chapter 3.
5.2.1 Results
Summary results from each test are reported in Table 5.3. Uplift measurements are not reported because of errors in the results. Excessive uplift was not observed, and as discussed below, minimal crushing around the hold down bolt holes was indicative of minimal uplift.
Measurement of the shake table acceleration revealed the actual peak ground acceleration achieved in each test. Input motions were programmed to have a peak ground acceleration of either 0.35 g or 0.52 g. Table 5.3 shows that there were only slight variations of the actual peak ground acceleration values from these targets.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 73
Figure 5.7 Load vs. drift curves for cyclic tests compared to monotonic tests.
40
30 Tes t 8 Test 4 20
z^1 0 / i v 0 n / o
-•.10
-20
-30 Ty oe A
-40-12 0 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 Drift (mm) 40 Test 7 30 Test 2 20 // ^10 < xT 0 / n o < -•-10
-20 Typ -30 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 Drift (mm) -40 40 Test 6 30
20 Test 3 \^ ^.10 S i r •D 0 / ra o
^-10
-20 ]
-30 Typ eC
•40-12 0 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 Drift (mm)
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 74
The applied dead load was measured to ensure that it remained reasonably constant throughout each test. Figure 5.8 depicts the dead load variation for Test 10b, which showed the most fluctuation in dead load. Recall from Chapter 3 that the tension on the dead load cable had to be approximately 5.56 kN to provide a total dead load of 22.25 kN, or 9.12 kN/m. Figure 5.8 shows the deviation of the measured cable force from 5.56 kN.
Although the applied dead load did remain reasonably constant, fluctuations were largest during high accelerations, which corresponds to high demand on the wall. Improvements could therefore be made to the dead load application method.
Figure 5.8 Deviation of cable force from approximately 5.56 kN.
1.6 1.2 Z 0.8 -j- * 0.4 -0.0 J ti I rVVvwv^—^- S_-0.4 m £-0.8 -1.2 -1.6 i , i , i , i , i 10 20 30 40 50 60 Time (s)
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 75
Table 5.3 Dynamic test results
Natural frequency Natural Measured peak Test Wall without dead frequency with ground Number number type8 load (Hz) dead load (Hz) acceleration (g) of nails 10a 0.38 173 10b 0.56 173 lOretrofit 0.36 473c 11 4.1 4.2 0.36 157 12 B 4.5 4.6 0.36 109 13 B 4.2 4.2 0.35 109 14 3.9 4.0 0.37 157 15 4.4 4.5 0.54 173
a Wall Types: Nail spacing at panel edges Type B for Type A,B: 150 mm. and Type A Type C Nail spacing at panel edges for Type C: 75 mm.
b Total number of nails may deviate slightly due to retrofit procedure.
Hammer testing. An impact hammer test was applied to all the test walls except the Test
10 wall. Table 5.3 shows that in all cases, the application of dead load slightly increased the natural frequency of the wall system. Other findings with respect to hammer testing are that the walls tended to have a torsional mode very close to their first lateral mode, which may be attributed to the fact that each wall was sheathed on one side only. The frequency of the next lateral mode of each wall was at least one order of magnitude higher.
For example, the first lateral natural frequency for Test 11, Type A wall without dead load, was determined by hammer testing to be 4.1 Hz. When the dead load was applied, the natural frequency increased to 4.2 Hz.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 76
Figure 5.9 shows a spectrum, obtained from Fast Fourier transform, of the hammer test results for Test 11 after torsional effects were separated out of the response. This was achieved by using an accelerometer on each end of the wall to measure the response of the wall to the hammer impact. Lateral frequencies were found by adding the records from the two accelerograms before performing the Fast Fourier transform. Since the torsional components of the acceleration at each end of the wall were in opposition, adding the records removed the torsional component. The fundamental lateral frequency of 4.2 Hz was indicated by the first peak. The second lateral frequency for this case was 29.9 Hz.
Torsional frequencies were found by subtracting the records from the two accelerograms before performing the Fast Fourier transform. Since the lateral components of the acceleration at each end of the wall were in the same direction, subtracting the records removed the lateral component. When torsional effects were considered, the first torsional frequency was 5.5 Hz.
Figure 5.9 First lateral natural frequency for wall with dead load in Test 11. Amplitud e
0 1 2 3 4 5 6 7 8 9 10 Frequency (Hz)
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 77
Earthquake loading. Test 10 had three parts. In Test 10a, a Type C wall was subjected to
the Landers, Joshua Tree, earthquake scaled to a peak ground acceleration of
approximately 0.35 g. Minimal damage was detected visually and no failure was
observed. Nail pull-through was not apparent. There was some separation of the
sheathing from the frame, resulting from slight pull-out of nails from the frame members.
This wall was then subjected to a second test: the Landers, Joshua Tree earthquake scaled
to a peak ground acceleration of approximately 0.52 g. The response was quite violent
and the technician shut off the shake table at around 30s into Test 10b because of safety
concerns. At least 14 nails failed in fatigue during this test while at least 14 nails pulled
out of the studs. Most of the fatigued nails were along the bottom edge, with a few on the
lower side edges (Figure 5.10a). Nails pulling through the sheathing constituted the
dominant failure mode (Figure 5.10b). Some studs separated from the top and bottom plates and there was noticeable movement at the hold downs.
Figure 5.10 Test 10b: a) Nail fatigue failures, b) Nails pull-through failures.
(a) (b)
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 78
For a final test, exposed nails were removed or sawed off, and replacement nails were
applied using a nail gun. Care was taken to prevent the new nails from sitting in the holes
created by the old nails. As a result, the new nail count may have differed slightly from
the original. The old nail spacing was largely maintained, but it was offset. This
retrofitted wall was subjected to the earthquake excitation scaled to 0.35 g in Test
lOretrofit (lOr). Although pull-out appeared to begin first, pull-through of the nails
emerged as the dominant failure mechanism. Studs splitting was observed around the
frame-to-frame nails (Figure 5.11a). Some studs were poor quality (Figure 5.11b). The
bolt holes used for connecting the hold downs to the wall were checked for crushing. The
damage was not extensive (Figure 5.11c) after the three earthquake simulations despite the
relatively large movements observed at the hold downs noticed in Test 10b.
Figure 5.11 Tests 10a, 10b, 1 Or: a) Damaged stud, b) Poor quality damaged stud, c) Minimal crushing at hold down connection.
Test 11 involved subjecting a Type A (regular) wall to the Landers earthquake scaled to
0.35 g peak ground acceleration. The application of the dead load caused a slight
premature bending in the studs. Failure began along the bottom edges of the upper panels
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 79
where these panels were attached to the blocking. Pull-out of nails occurred before pull-
through. At least one nail suffered fatigue failure. It was on the bottom corner of an upper panel.
The studs of the Type B wall of Test 12 were of very poor quality. Unlike other single panel walls, pull-out and pull-through occurred first at the top edge. The peak ground
acceleration was approximately 0.35g. Pull-out and pull-through occurring first at the bottom had been observed in all other tests of single panel walls, including those loaded monotonically and cyclically. Approximately two nails failed in fatigue at the bottom. It was not clear whether others did at the top.
The Type B wall in Test 13 had to be forced into the test frame, creating a pre-load that
could have increased resistance to overturning. This wall appeared to fail equally along the top and bottom corners and edges when subjected to the ground motion with 0.35 g peak acceleration. Two nail fatigue failures were noted in the bottom corners.
Failure along the blocking was noticed first in Test 14. No nail fatigue failures were noticed in this Type A wall. Crushing of the panels occurred in the middle of the wall, where the panels shifted and came in contact with one another (Figure 5.12). Again,
failure occurred when the corner nails at the blocking first pulled out of the frame and then pulled through the sheathing. This test was performed with the peak ground acceleration at
approximately 0.35 g.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 80
Figure 5.12 Crushing of panels in Test 14.
1
In Test 15, a Type C wall was subjected to the Joshua Tree earthquake, scaled to have a
peak ground acceleration of approximately 0.52g. Again, the shake table was shut off, this
time at roughly 30s into this version of the earthquake. The typical failure modes were
observed.
5.2.2 Analysis
The UBC shake table setup was described in Chapter 3 (Figure 3.7). Due to the
complexity of the test set up, there was a need to perform a detailed free body analysis to
relate some of the experimentally measured data (such as the acceleration time history of
the inertial mass) to the forces experienced by the shear wall system. Figure 5.13 displays
the model of the table and frame on which these free body diagrams are based. Calculated
values such as the force imparted on the shake table by the actuator, were compared to the
measured values. Other calculated values, such as the shear force on the walls, were
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 81 useful for making comparisons between monotonic, cyclic and dynamic tests as well as between different dynamic tests.
Figure 5.13 Frame model for force calculations. + Concrete masses -•
Dead load • Force Acceleration
Wall specimen
260 B Shake Table West 483
76 1118
Measurements in millimetres.
Nomenclature. The following variables appear in Figure 5.13 to Figure 5.18 and
Equation 5.4 to Equation 5.34:
aK = absolute acceleration of the inertia mass.
aF = absolute acceleration at the top of the shearwall frame.
aT— absolute acceleration at the shake table surface.
B - calculated horizontal force applied to the shake table through the actuator.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 82
STOP. SBor shear forces applied to the top and bottom of the shearwall, respectively.
d = displacement at the top of the wall relative to the shake table.
dF - displacement at the top of the shearwall frame relative to the shake table.
dT = displacement of the shake table.
D = dead load force applied to the shearwall through the pulley system.
FRI FT = inertia forces at the center of mass of the inertial mass and of the shake
table, respectively.
H{ = horizontal reactions as defined in the free body diagrams.
Mi = moment reactions as defined by the free body diagrams.
K = inertial mass.
Z; = vertical dimensions as defined in Figure 5.13 and the free body diagrams.
T= mass of the shake table.
i?i = reactions as defined in the free body diagrams.
U = resultant uplift force on one side of the wall. The uplift force distribution is
assumed to be linear across the length of the wall.
V{ = vertical reactions as defined in the free body diagrams.
Wj = horizontal dimensions as defined in Figure 5.13 and the free body diagrams.
Some of these variables are known through measurement or basic geometry (the
Pythagorean theorem and the theory of similar triangles):
aF, aT, and Jrare values measured during testing.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 83
d is the relative displacement (drift) between the top of the wall and the shake
table.
. 498 + 2545 , dF= • d mm.
2545
D = 22.25 kN.
^ = 4545 kg.
Z,= 185 mm.
L2= 285 mm. L = ^98 . £ mm. 2545
2 2 LA= j2545 -d mm.
L5= 592 mm.
L6= 242 mm.
2 Z7= Vlll8 -^ mm.
T= 2043 kg.
w,= 2923 mm.
w2= 2110 mm.
2
w3= -•2440.\w3= 1627 mm (distance between shearwall uplift force
resultants, Figure 5.17).
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 84
Calculations. In the calculations, inertia forces on the steel frame are considered negligible. Then, ignoring the rotational and vertical inertia of the inertia mass, K, kinetics are applied to the inertia mass shown in Figure 5.14.
Figure 5.14 Free body diagram of inertial mass K.
a + K —• K
pi Kg
• Force H o - -• Acceleration Mr
The resulting equation of motion is:
FK= K • aK Equation 5.4
Then equilibrium is applied to determine resultant forces.
I>= 0
0= H0-FK , which, after substituting in Equation 5.4, leads to
H0= K-aK Equation 5.5
I>= 0
0= V0 - K • g where g is the acceleration of gravity.
V0= K-g Equation 5.6
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 85
o
0= FK-L,-M0
M0= K-aK-Lx Equation 5.7
Figure 5.15 Free body diagram of inertial mass platform.
Referring to Figure 5.15,
^H= 0 where H0 is defined in Equation 5.5.
Note that the horizontal forces at the pins are assumed to be equal.
0= 2-Hi-K-ak
Hx= K-aK/2 Equation 5.8
^ V= 0 where V0 is defined in Equation 5.6.
0= V, + V2-K-g
Vx= K-g-V2 Equation 5.9
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 86
The moment about point Q, ^MQ= 0 , is expressed as
0= V2-Wl+M0 + H0-L2-V0-^
where M0 = K-aK-Li , H0= K • aK and V0= K • g . So,
0= V2-wl+K-aK-L,+K-aK-Kg^ thus,
V2= K-K-X-^jc. (L] +Li) Equation 5.10 2 Wi
and by substituting Equation 5.10 into Equation 5.9,
:.V,= ^ + +L ) . Equation 5.11 2 w, 2
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 87
Figure 5.16 Free body diagram of vertical supports for inertial mass.
Figure 5.16 shows free body diagrams of both vertical supports for the shearwall frame.
The first set of calculations are for the support identified with the point labelled R:
Hx= K-aK/2
0= -H,-^-^ + H2 2
_ -K-a H K Equation 5.12
2>= 0
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 88
2 wx
0= V3-V,:.V3= Vx
V3= *L& + • (X, + L2) Equation 5.13 2 Wj
IX= 0
0= H\ • (L3 + L4) + V,-dF- H2 • L4 Equation 5.14
0= (L3+L<) + l^ + H-^. (Li+LJ .dF-H2.L4 2 v 2 w,
„2= ^ . + l) + *JL^ + Equation 5.15
2 \L4 > 2Z
K • aK • dF (L,+L2)
L4 • w,
and, from substituting Equation 5.15 into Equation 5.12.
, „ K • aK L3 K • g • dF K • aK • dF , T T . „ . - , , :.H3= ——£ • -f + —'- + ——£—£ • (I, +Z,2) Equation 5.16
I LA 2Ld LA • w.
The second set of calculations are for the support identified with the point labelled S:
I>= 0
Hi= K-aK/2 Equation 5.17
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 89
+ HA H5= —Y^ Equation 5.18
2>= o
X-£-*L£«.(Ll+L2) v2 2 w
0= V4-V2.:V4= V2
V. /4=— — - ^-^ • (I, + L2) Equation 5.19 2 w,
0= • (Z3 + L4) + V2 • dF — H4 • L4
0= ^.(Z3+L4) + r^-^-(L1+Z2)J-^-^4^4 2 v 2 w,
jy4= ^ . + {\ + K^dr Equation 5 2Q
_K-aK-dF .{Lx+Li)
L4 • wx
.H_ K_a, U + KJ_g^df _ K aK dp ^ + ^
2 L4 2L4 L4 • wx
from substituting Equation 5.20 into Equation 5.18.
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 90
Figure 5.17 Free body diagrams to determine the shear force transferred from the load transfer beam of the frame to the wall.
D U
vv3
S Top -• s (a) i^^^^^^nm |
• Force (b) -• Acceleration D and Uare resultants of distributed loads.
The inertial force is imposed on the shearwall through a load transfer beam. The free body diagram of this beam appears in Figure 5.17 (a). Note in Figure 5.17 (b) that the shear forces on the top and bottom of the wall are equal.
5>= 0
0= STop-H2-H4:.STop= H2 + H4
K- aK (L^^^K- g • dF K- aK- dF
V 1 2) 2 Vl4 J 2L4 L4-wx
K-aK (L3,A,K-g-dF K-aK-dF
2 V£4 J 2L4 I4-w,
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 91
Equation 5.22
>Bot Equation 5.23 L
Figure 5.18 Free body diagram of the shake table platform. Resultant forces in the link supports under the platform must be in the direction of the links at all times. +
v4
•f i * • Force H. - -• Acceleration
a t- t
w R 2 d,
Although the test data include a reading of the force applied to the shake table by the actuator, the reading is derived from a pressure measurement at the actuator, and it does not account for the inertia of the table. The measured values are checked by calculating a
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 92
value for the applied force using the free body diagram of the shake table appearing in
Figure 5.18:
By kinetics, the inertia force of the table is:
FT= T• aT Equation 5.24
Then equilibrium is applied:
0
0= H6 + H7 + B-T-ar + H3 + H5-SBol
B= T• aT + SBot-H3-Hs-H6-H7 Equation 5.25
2>= 0
0- V5+ V6-T-g-D-V3-VA-U+ U
Since V3 + V4 = K • g
V5= - V6 + T- g + K- g + D Equation 5.26
0= V6-w2-T-g-^ + Fr-L6-B-L6-H3(L5 + L6)
- H5(L5 + L6) + SB0I(L5 + Lt) +V$ - f) - + fj 2 2^ V2 2-
V22 22J. V2 2-
Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 93
F v6= L^ + iL£ + ^- rJ^ + ^_u.^ + ^iLi + L6)
2 2 2 w2 w2 w2 w2
S + ^(L5 + L6) - -^(L5 + L6) - - + hfc + $
w2 w2 w2\2 2J w2V2 2'
By making the appropriate substitutions for H2 ,H5,SBo„ V2, V4 and
collecting terms:
V6= I^ + ^ + R-^lih + ilh-u.^ Equation 5.27
2 2 2 w2 w2 w2
_ K • aK • L5 _ K • aK • L6 K • aK • Lx K • aK • L2
w2 w2 w2 w7
T :.V5= = ^ + ^ + D + iiJ^_BjJ^ + U-Wl Equat.on 52g
2 2 2 w2 w2 w2
+ K• aK- L5 +K • aK- L6 + K • aK- Lx + K• aK- L2
w2 w2 w2 w2
F5was obtained from substituting Equation 5.27 into Equation 5.26. Since
the forces in the shake table's support links must always be in the direction
of the links, and the links can be assumed to displace equally:
C d *L= Z = "5 '6 L-, L7 L-i From horizontal equilibrium of the table (Equation 5.25): B= T • aT + SBot — H-i — H5 - H6- H7 Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 94 B= T.aT+((K.aK)-(^ + \)+^^) _fK_a£_L1 + K-g-dF + K-aK-dF .(Li+Li)) _fK_a£ L1 + K-g-dF_K-aK-dF . iLi +L ^ _ v . ^2 L4 2L4 L4 • w{ J Z7 B= T• aT + K • aK — (T • g + K • g + D) • — Equation 5.30 Li Equation 5.30 was obtained from substituting Equation 5.29 into Equation 5.25, and collecting terms. Also by making appropriate substitutions and collecting terms: 2 2 2 W2 W2 W2 + K • aK • L5 + K • aK • L6 + K • ciK • Z, + K • aK • L2 w2 w2 w2 w2 2 2 2 w2 -^.[T-aT + K-aK-T-g + K-g + D-^ w2 v L w + TJ 3 + K • aK • L5 + K • aK • L6 + K • aK • Z, + K • aK • L2 w2 w2 w2 w2 w2 V5= {T• g + K • g + D) • (^ • ^ + -) Equation 5.31 + ^.(z1 + L2 + z5) + rj.^ w2 w, In the same manner: Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 95 C l V6= (T-g + K-g + D)-(-^- ll+- Equation 5.32 ^ w2 L7 _^.(xI+z2 + is)_c/.^ Since H6= V5 • ^ and H7= V6-^ : \= (T-g + K-g + D)-^.(L^.f:+i) Equation 5.33 L7 ^w2 L7 2/ w + ^lK.dI.(L]+L2+L5)+U. I.dI Equation 5.34 Raw data measured during shake table excitations were fed into five MathCad calculation sheets. In the first sheet all data were converted into consistent metric units. The second sheet removed offsets from data records by averaging the first twenty data points from each record and subtracting this value from every data point in the record. The third calculated values such as the relative top displacement (drift: d), the relative velocity (v), the force applied to the table by the actuator (B), and the shear force on the wall (Stop, Sf,ot). In this calculation sheet, Equation 5.30 was used to find B, and Equation 5.23 was used to find Stop, and Sbot. The third calculation sheet also included an estimate for the damping force active within the wall. The calculation assumed a 1% damping ratio (£, = 0.01) to be consistent with Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 96 Chapter 4 analyses. The damping coefficient c, was determined with the following equation: c= 2 • t, • m • 2 • n -f Equation 5.35 where/is the natural frequency (in Hz) of the wall with the dead load applied, and m is the mass. In these calculations, the mass is the inertia mass, K, applied to the top of the test frame. From above, K = 4545 kg. The natural frequencies are taken from the results of the hammer tests. For the wall in Tests 10a, 10b and lOretrofit, where no hammer tests were performed, the frequency for the other Type C wall was used (Test 15). This value is for an undamaged wall and certainly overestimates the stiffness of the wall in tests 10b and lOr. An equation of motion for a wall undergoing earthquake loading can be written as follows (Paz, 1991): m-a + c- v + k-d= 0 Equation 5.36 Here, a is the absolute acceleration at the top of the wall, while v is the relative velocity at the top of the wall and d is the (drift) at the top of the wall. The stiffness of the wall is k, the damping coefficient for the wall is c, and the mass of the wall is m. Again, the mass of the wall is taken as K, the inertia mass applied to the test frame (m = K). Rearranging: m • a + c • v= —k-d Equation 5.37 Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 97 The equation is now in the correct form for constructing load-deflection curves (hysteresis loops). If the left side of Equation 5.37 is graphed against drift, then the slope of the line at any point is the wall stiffness. So the third calculation sheet also found F = m • a + c • v Equation 5.38 for the purpose of constructing hysteresis loops. Including a damping force based on a damping ratio of 1% had little effect on the resulting hysteresis loops. As a check, the damping ratio was increased to 5%, but this also had little effect. Finally, the third calculation sheet included a calculation for the energy used by the wall. For each measurement step i, the energy E is equal to the area under the load-deflection curve, assuming that the load-deflection relationship is linear between step i -1 and step i: Equation 5.39 Where F denotes load and d denotes the relative deflection at the top of the wall (drift). The sum of the energy per step up to a given step / yields the energy dissipation of the wall up to that step. The fourth calculation sheet was used to shift the data from each test to remove data measured before the simulated earthquake motion began. The fifth data sheet extracted maxima and minima from the measured and calculated values. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 98 Figure 5.19 Verification of earthquake input and measurements for Test 10a. 0 10 20 30 40 50 Time (s) Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 99 Comparative time history plots (Figure 5.19) were used to verify that the earthquake was reproduced appropriately and that the measurements appear reasonable. All the time histories in Figure 5.19 were measured or calculated results from Test 10a, except for the "scaled ground motion", which is the actual Joshua Landers earthquake record scaled to have a peak acceleration of 0.35 g. From the plots, the shake table acceleration appears to be a reasonable approximation of the actual earthquake acceleration. Recall from Chapter 3 that the shake table will not reproduce the ground motion exactly. It is therefore necessary to independently check the measured data. In Figure 5.19, "applied force" refers to the force applied directly on the table by the actuator. The calculated applied force is consistent in form with the measured applied force, although the measured applied force peak is approximately 20% greater. This value is fairly consistent for all tests. This discrepancy seems somewhat high, but it should be kept in mind that the measured applied force was based on hydraulic line pressure. The actuator friction resistance may cause this load reduction, especially under high accelerations. The "shear on wall" time history is also very similar to both the calculated and measured applied force time histories. 5.2.3 Discussion The analysis described above yielded several values that facilitate comparison between different wall types. These values are summarized in Table 5.4. Referring back to Figure 5.19, the shake table acceleration time history with a peak acceleration of about 0.35 g has Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 100 high accelerations during the periods from 7.5 seconds to 10.5 seconds and from 28.0 seconds to 31.0 seconds. Table 5.4 lists the peak absolute acceleration at the top of the wall during each earthquake test. It also lists the peak drift and peak base shear on the wall for each earthquake test. The times of occurrence for every peak in the table suggest that the peaks all occurred in response to the high acceleration portions of the input. In Test 14, the absolute acceleration, drift, and base shear peaks occurred at effectively the same time. In Tests 10a, lOr and 11, they occur close together in the same high input acceleration range. In Tests 12 and 13, the peaks occur in either of the high input acceleration ranges. In the tests where the input was scaled to have a peak ground acceleration of approximately 0.52 g (Tests 10b and 15), the tests were interrupted by the table control system before the second high acceleration range was completed, so the time of peaks given neglects the response to this input. It appeared that the Test 10b wall reached its acceleration, drift and base shear peaks in the first high input acceleration range, while the Test 15 wall reached these peaks in the second high input range. Trends are apparent from Table 5.4. Where wall failures occurred, walls with higher measured stiffness (natural frequency) had higher absolute accelerations. The walls with higher measured stiffness also had lower drifts. It is unclear why one Type B wall (Test 12) had a higher natural frequency than the Type C wall (Test 15), while the other Type B wall (Test 13) had a natural frequency equivalent to a Type A wall (Test 11). Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 101 Table 5.4 Summary: Analysis of dynamic testing results. Peak3 Peak3 top of wall Peak Natural ground acceleration drift Peak base Monotonic freq• Test Wall acceler• (absolute) (g) (mm) / shear (kN) peak uency number type ation (g) [Time (s)] Time (s) / Time (s) shear/drift (Hz) 11 A 0.36 0.33 53.42 20.35 4.2 [28.44] [28.39] [28.43] 17.38/ 14 A 0.37 0.32 61.64 20.42 57.49 4.0 [28.46] [28.44] [28.46] 12 B 0.36 0.34 45.14 21.98 4.6 [11.77] [28.31] [11.76] 21.94/ 13 B 0.35 0.33 60.84 19.88 54.63 4.2 [28.4] [28.39] [11.82] 10ab C 0.38 0.37 43.87 21.54 4.5C [30.44] [28.65] [30.48] lOr C 0.36 0.38 61.76 24.28 - [28.37] [28.84] [28.40] 35.4/ 10bd C 0.56 0.50 76.13 30.86 60.85 - [7.80] [8.25] [7.80] 15d C 0.54 0.61 83.97 38.17 4.5 [26.43] [26.81] [28.29] a All accelerations are expressed as magnitudes of g b The wall did not reach failure in Test 10a c This value is assumed from Test 15 results d The input was stopped early in these tests The maximum base shear calculated for each of the failed walls tested dynamically showed excellent agreement with monotonic results. For both types of single panel wall (Tests 12,13,15), the maximum shear from dynamic tests was within 10% of the maximum shear from monotonic tests of the same wall type. The regular, multiple panel walls showed up to an 18% deviation. Recall, though, that the regular panel, Type A wall tested monotonically (Test 4) was observed to have lower quality construction. When the maximum shear from cyclic testing of a Type A wall (Test 8) is compared to the maximum Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 102 shear from dynamic testing (Tests 11 and 14), the dynamic shear deviates from the cyclic shear by less than 1%. Excellent agreement was found between base shears measured in dynamic tests and cyclic tests (Table 5.5). A deviation of less than 2% was found in all cases except in the comparison between the Type B walls tested cyclically in Test 7 and dynamically in Test 13. The results allay concerns that the cyclic protocol did not have enough cycles to measure a lower bound for the peak load. The lower bound may not be as critical to earthquake modelling as has been reported (Yamaguchi and Minowa, 1998). Table 5.5 Deviation of calculated dynamic peak shear from measured cyclic peak shear for each wall type. Deviation of dynamic Peak shear Peak shear shear from Cyclic test from cyclic Dynamic test from dynamic cyclic shear Wall type number test (kN) number test (kN) (%) A Test 8 20.42 Test 11 20.35 -0.34 A Test 8 20.42 Test 14 20.42 0 B Test 7 21.72 Test 12 21.98 1.20 B Test 7 21.72 Test 13 19.88 -8.47 C Test 6 38.01 Test 15 38.17 0.42 Figure 5.20 shows the drift time histories recorded in all shake table tests using a peak ground acceleration of 0.35 g. Qualitatively, the drift responses of Test 11 (Type A), Test 14 (Type A) and Test 13 (Type B) are similar. The Test 13 wall was less stiff and of poorer quality compared to the other Type B wall (Test 12). It is reasonable, therefore, for the Test 13, Type B wall with a single oversized panel to behave similarly to a Type A wall Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 103 with regular panels. The Type C (Test 1 Oa) wall with a single panel and reduced nail spacing at the edges, exhibits a response with low drifts and sustained high frequencies. These qualitative observations verify measured values for the drift and stiffness of each wall. The Type C wall from Test 10a was subjected to a second, higher acceleration ground motion in Test 10b, and then re-nailed. The re-nailed wall was subjected to the original, lower acceleration ground motion in Test lOretrofit (Test lOr). Although the stiffness of the retrofitted wall was not measured, its drift time history, notably the frequency of its response, was very similar to that recorded in Test 12 with a Type B wall. This indicated that the retrofitted Type C wall regained a significant portion of its original stiffness. Theoretically, since the load-slip behaviour of the nails governs wall behaviour, and damaged nails were replaced into fresh holes, the behaviour of the retrofitted Type C wall should be the same as in Test 10a. The retrofitted wall in Test lOr reached a peak base shear that was 64% of the Type C wall failed in Test 15 (the Test 10a wall did not reach failure loads). The less stiff, less strong behaviour of the retrofitted wall compared to an original wall can be attributed to many factors, including not all nails being replaced. Only nails that failed (or were observed to be close to failure) in pull-out, pull-through or fatigue were replaced. The other nails and the sheathing around them would still have suffered damage. Also, no retrofit was performed to the frame-frame connections, which were observed to loosen, especially in Test 10b. Increasing damage in the studs as well as Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 104 perforations and damage in the panel at original nail locations may also have contributed to the degradation in overall wall strength and stiffness. In all tests except Test 10a, where the wall was not significantly damaged, the frequencies of the drift time histories decreased indicating damage and a reduction in stiffness. In Tests 11 and 14 (Type A), Tests 12 and 13 (Type B) and Test lOr (retrofitted Type C) the frequency reduction occurred roughly 12 s into the ground motion. The Type C wall (Test 10a) which had no significant observable damage did not undergo a significant frequency reduction. Figure 5.21 shows the acceleration time histories recorded during the same tests. The differences between the Type B walls are not as apparent. The Type A walls have consistent responses. The retrofitted Type C wall still has a similar response to the Type B wall in Test 13. Again, this appears to occur at about 12 s for all walls except the previously undamaged Type C wall in Test 10a. Figure 5.22 and Figure 5.23 show the drift time histories and the acceleration time histories recorded, respectively, for the walls tested with a peak ground acceleration of 0.52 g. Both walls were Type C, although the Test 10b wall was previously damaged in Test 10a, while the Test 15 wall was undamaged. The undamaged, Test 15 wall had a higher frequency, lower drifts and higher accelerations characteristic of a stiffer wall when subjected to this ground motion. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 105 Figure 5.20 Drift time histories for tests with 0.35 g peak ground acceleration. 0) # ype-M\ o T*« m ri P CM (ft E 0) E 50 Type B 25 (A 0 O I- -25 -50 50 i ype is o 25 0 (A -25 *- -50 i_ 50 Type C ° 25 1ft 0 V v © -25 -50 Retrofit 10 20 30 40 50 60 Time (s) Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 106 Figure 5.21 Acceleration time histories for tests with 0.35 g peak ground acceleration. ^ 0.50 £ 0.25 W 0.00 •®-o.2o 5 -0.50 0.50 t- 0.25 W 0.00 £-0.25 -0.50 0.50 TypeB Type B re 0.50 Type C ° 0.25 *£ 0.00 O-0.25 •"-o.so i_ 0.50 Type C °0.25 ^0.00 ©-0.25 -0.50 Retrofit 10 20 30 40 50 60 Time (s) Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 107 Figure 5.22 Drift time histories for tests with 0.52 g peak ground acceleration. Figure 5.23 Acceleration time histories for tests with 0.52 g peak ground acceleration. Type C 3 oc ra 0) O« 0.50 i o ype u <0 0.00 3 ,2 -0.25 10 20 30 40 50 60 Time (s) Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 108 Figure 5.24 shows the hysteretic behaviour of Test 11, assuming 1% damping. Of significance is the relatively small number of large loops in the elastic as well as the inelastic regions. A stiffness reduction is observable, as the walls deteriorated signified by flatter gradients of the loops. The hysteresis loops were also symmetric about the zero displacement point indicating no significant observable permanent drift. Figure 5.24 Hysteresis loop for Test 11 (Type A), assuming 1% damping. 25 20 15 10 5 o o o 0 fll LL y i_ (0 Oi -5 x: CO J -10 / -15 v -20 -25 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 Drift (mm) To determine if the new cyclic protocol is reasonable for use in dynamic modelling, the energy dissipated in cyclic tests was compared to the energy dissipated in dynamic tests of Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 109 the same wall type. The energy was calculated to the first occurrence of the cyclic or dynamic record reaching a drift equivalent to the drift at the peak load measured monotonically for each type of wall. Since failure is typically considered when load drops to 80% of maximum after ultimate, it would be desirable to calculate the energy dissipation up to the first occurrence of a drift equivalent to this failure point. However, none of the dynamically tested walls reached this drift. Hence arises the question of why dynamic drifts are so low or what is an appropriate drift level when determining ductility in monotonic tests. The energy calculations indicate that the wall tests using the new protocol dissipated less energy compared to dynamic test results (See Table 5.6). The dynamic energy dissipation is consistently more than two times greater than the cyclic energy dissipation except with dynamic Test 12, which matches cyclic Test 7 very well. The same trends occur from type to type, in that the stiffer walls generally dissipated more energy. Type A and Type B walls show similar energy dissipation. Type C walls dissipate around twice as much energy compared to other types before reaching the approximate failure drift. Based on these calculations, the new protocol could be extended by adding a few extra cycles in the intermediate displacement range and by cycling through each displacement to reach the stabilized envelope curve. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 110 Table 5.6 Energy dissipation for walls tested cyclically and dynamically Cyclic test Dynamic test Cyclic energy Dynamic energy Wall Type number number dissipation (kN*mm) dissipation (kN*mm) 11 4900 A 8 1800 14 5700 12 2000 B 7 2200 13 6600 C 6 15 4900 9900 5.3 Summary The variety of tests and analyses performed in this study yielded consistent results. Walls were significantly more likely to fail at the bottom corners of a panel than anywhere else. Where walls were constructed with several panels, the panels with their bottom edges attached to the blocking failed first. Failure precipitated by bottom edge and corner nails beginning to pull-out of the framing, followed by pull-through of these nails through the sheathing. The same pattern was observed at top edges and corners, but the failure at the bottom edges dominated the failure behaviour of the walls. Studs appeared to deflect as straight, pin-ended members until failure level displacements were reached, whereafter the studs began bending, most often at the bottom. Single panel walls with regular nail spacing (Type B) were stiffer and stronger than regular, multiple panel (Type A) walls in monotonic, cyclic, hammer and earthquake testing as well as in analysis and previous testing (He, 1997). Their behaviour could, however, be considered similar when compared to the behaviour of single panel walls with reduced spacing. These Type C walls had roughly double the monotonic, cyclic and Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Test Results and Discussion 111 dynamic load capacity. In cyclic and earthquake testing, the single panel reduced nail spacing walls could dissipate approximately twice the energy of the other types of walls before reaching the monotonically measured drift at failure. The Type C wall was effectively undamaged and maintained its original stiffness when subjected to the ground motion which failed the other types of walls. After being brought to dynamic failure and then retrofitted, the Type C wall regained a significant portion of its strength and stiffness. In addition to specific findings with respect to the behaviour of the single panel walls, the study revealed that the cyclic protocol yields results with lower but promising compatibility with dynamic loading results. It might be suggested that a few more cycles be added to the cyclic test protocol to establish hysteresis curves at lower displacements after significant damage and stiffness reduction. Compared to the 7.2 m long walls tested by He (1997), these 2.4 m long walls did not exhibit the strong inverse correlation of load capacity and ultimate drift in comparison between walls with single and multiple panels. Due to the higher overturning moment of the 2.4 m walls, the applied dead load plays an increasingly important role in preventing uplift of the wall corners. The use of hold down brackets becomes crucial to achieve the desired racking resistance of a wall. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels CHAPTER 6 Conclusions and Recommendations Shearwalls constructed with regular and oversized oriented strand board sheathing panels were successfully tested with monotonic, cyclic quasi-static and dynamic loading regimes using the developed test methods and existing facilities. Calculations to determine the maximum base shear on dynamically tested walls were performed. The measured maximum loads from monotonic and cyclic quasi-static tests and the calculated base shear estimates from dynamic tests showed excellent agreement for the 2.44 m walls. An attempt was made to match experimental results with analytical results using the Shear Wall Analysis Program (SWAP) computer model. SWAP, however, proved unreliable. Under monotonic loading, walls built with oversized panels (109 nails) had higher load carrying capacity and were stiffer than walls built with regular panels (157 nails). Walls constructed with oversized panels and reduced nail spacing at panel edges (173 nails) showed a remarkable increase in load carrying capacity over walls with either regular panels or oversized panels and regular nail spacing. Ductility values for the different types of walls showed little variation. Seismic Response of Wood Shear-walls with Oversized Oriented Strand Board Panels 112 Conclusions and Recommendations 113 In tests with all loading types, failure precipitated by nails first pulling out of the framing and then pulling through the sheathing. As nail heads pulled through the sheathing, the ability of those nails to transfer load from frame to sheathing was lost, resulting in reduced wall capacity and reduced wall stiffness. This type of failure occurred dominantly at the panel edges below the mid-height of the wall. It was especially prevalent at the bottom corners. The observed failure mode serves to verify that the behaviour of nailed timber shearwalls is dominated by the behaviour of the connectors. The new cyclic protocol (He, 1997) was successful in achieving this type of failure mode in the cyclic quasi-static tests. The tests by He (1997) showed that other protocols result in low cycle fatigue failure of the nails which was not a dominant failure mode in dynamic testing. As such, hysteresis curve results from the new cyclic protocol may be more appropriate for use in dynamic modelling. This was verified using energy calculations to find the energy dissipated in dynamic and cyclic wall tests. The energy dissipation was calculated up to the displacement at maximum load of the monotonic envelope curve. Calculated energy values for each type of wall were of the same magnitude, with the cyclic tests tending to dissipate less energy than the dynamic tests. Cyclic test protocols with several more cycles that result in nail fatigue failures cause walls to dissipate significantly more energy than earthquake regimes. Under dynamic testing, walls with higher load carrying capacities and stiffness values experienced higher frequencies in their response as well as higher accelerations and lower Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Conclusions and Recommendations 114 drifts (relative displacement at the top of the wall) using the chosen ground motion. Walls with oversize panels and regular nail spacing behaved relatively similar to walls with regular panels. The walls with oversized panels and reduced nail spacing had maximum base shears and energy dissipation values approximately two times higher than either of the other wall types. A retrofitted oversized panel wall with reduced nail spacing regained a significant portion of its original strength (64%) and stiffness. Only panel-to-frame nails that had failed or were close to failure were replaced with new nails in fresh holes. Degradation that had occurred in the remaining panel-to-frame connections, as well as the frame-to-frame connections, may have prevented the retrofitted wall from reaching its original strength and stiffness. While the study has provided extensive information on the dynamic behaviour of shearwalls with oversized oriented strand board sheathing panels, the use and understanding of these and other timber shearwall configurations would benefit from further studies in the following areas: (1) the dynamic behaviour of shearwalls built with oversized panels with openings. (2) an improved and internationally adaptable cyclic loading protocol. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Conclusions and Recommendations 115 (3) improved and internationally adaptable monotonic envelope definitions for failure and ductility of timber shearwalls, structures or structural components. (4) an improved and accurate analytical model to predict and catalogue the dynamic behaviour of different types of timber shearwalls. (5) an optimized panel-to-sheathing connection and connection distribution scheme for timber shearwalls. 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"Seismic behaviour of wood-framed shear walls." International Council for Building Research Studies and Documentation. Proceedings CIB W18- Timber Structures, Meeting 24, September 1991. paper /24-15-3. Yasumura, M. (1996). "Damage of wooden buildings caused by the 1995 Hyogo-Ken Nanbu earthquake." International Council for Building Research Studies and Documentation, Working Commission WIS- Timber Structures, Meeting Twenty- Nine, Bordeaux, France. Yung, W.C.W. (1991). "innovative energy dissipating system for earthquake design and retrofit of timber structures." Ph.D. Dissertation, Department of Civil Engineering, University of British Columbia, Vancouver, BC, Canada. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix A Dynamic Test Frame Schematics Figure A.1 Front view schematic of dynamic testing frame (measurements in mm). HSS 127x76x6.4 H SS 105x76 x 4. B KM East West HSS 76*76x4.B Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels 123 Appendix A 124 Figure A.2 Side view schematic of dynamic testing frame (measurements in mm). Schematics from Rudolf, 1998; most dimensions from Popovski, 1998. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B Analyses B. 1 Experimental Planning Studies The primary objective of the computer analyses was to aid in developing the experimental testing plan. The focus of the testing was upon the possible advantages of walls constructed using a single panel of OSB with reduced exterior nail spacing. The length of the test wall, however, was a concern. While the previous monotonic and cyclic tests on single panel walls were carried out on walls that were 7.3 m in length, the existing dynamic test setup for shearwalls would hold walls only up to 2.44 m in length. To obtain valid conclusions from experimental tests, the di fference in seismic response between the tested wall types would have to be reasonably significant. Recall from Chapter 3 that the wall types were to be as follows: • Type A - standard panels, standard exterior nai 1 spacing (150 mm). • Type B - single panel, standard exterior nail spacing (150 mm). • Type C - single panel, reduced exterior nail spacing (75 mm). Analyses were performed with variations to the length of Type A and Type C walls in an attempt to choose a wall length for experimental study. Once the wall length was chosen, Type C walls of that length would be subjected to different earthquake ground motions. These ground motions would be scaled to 100% and also 120% of their recorded peak ground accelerations. The characteristics of the ground motions used in analysis are given Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels125 Appendix B 126 in Table B.l. Finally, a Type B and Type C wall of the chosen length would be analyzed with the chosen length and ground motion to see the effect of the different exterior nail spacing. Table B.2 summarizes important results from each analysis. Table B.l Characteristics of analysis input accelerograms. Peak 120% of Station acceleration peak accel. Event Date Location Direction Magnitude (g) (g) Landers June Joshua Tree, E-W 7.3 0.29 0.35 28, 1992 California Imperial May El Centra, N-S 6.3 0.35 0.42 Valley 18, 1940 California Chile March Llolleo, N-S 7.8 0.67 0.80 3, 1985 California Kern July 21, Taft High E-W 7.4 0.18 0.22 County 1952 School, California Figure B.l and Figure B.2 show the results from the length study (Tests No. 1 - 6 in Table B.2). First observe that the program becomes unstable in all analyses involving Type A (multiple panel) walls (No. 1,3,5). In all of these analyses, the energy balance check within the program failed, which stopped the program. Hence, the program did not reach the algorithm that calculates maximum values. These values would, however, be useless because often as the program destabilized it reported increasingly large, unrealistic values. Also, results did not mimic expected experimental results. The excessively high frequencies and accelerations along with initially excessively low drifts reported by the Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 127 program were the same type of inaccuracies reported in the Chapter 4 discussion where existing experimental test results were compared with SWAP results. Table B.2 Analyses for planning experimental testing program. Exterior Peak Wall nail Earth• ground Max. Max. Natural Un• Single or length spacing quake acceler• drift acceler• frequency stable 3 1 No. multiple (m) (mm) input ' ation (g) (mm) ation (g) (Hz) (Y7N) Length Study 1 M 2.44 150 J 0.29 - - 3.32 Y 2 s 2.44 75 J 0.29 8.25 0.57 5.13 N 3 M 3.66 150 J 0.29 - - 3.36 Y 4 S 3.66 75 J 0.29 9.63 0.63 5.53 N 5 M 4.88 150 J 0.29 - - 3.44 Y 6 S 4.88 75 J 0.29 5.55 0.57 5.69 N Input Earthquake Study 7 s 2.44 75 J 0.29 8.25 0.57 5.13 N 8 s 2.44 75 J 0.35 9.65 0.63 5.13 N 9 s 2.44 75 E 0.35 9.52 0.63 5.13 N 10 s 2.44 75 E 0.42 12.45 0.73 5.13 N 11 s 2.44 75 L 0.67 - - 5.13 Y 12 s 2.44 75 L 0.80 - - 5.13 Y 13 s 2.44 75 T 0.18 6.48 0.48 5.13 N 14 s 2.44 75 T 0.22 8.68 0.54 5.13 N Nail Spacing Study 15 s 2.44 150 J 0.35 25.09 0.59 4.23 N 16 s 2.44 75 J 0.35 9.65 0.63 5.13 N "The letter "M" denotes a multiple panel wall. "S" denotes a single panel wall. bThe "J", "E", "L", "T" denote the Joshua Tree, El Centro, Llolleo, Taft Stations, respectively. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 128 Figure B.l Drift time histories for Type A and Type C walls with different lengths. 4v88 m Type C 3v66 m Type A E E 3.66 m Type C Q 2.44 m Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 129 Figure B.2 Acceleration time histories for Type A and Type C walls with different lengths. 0 10 20 30 40 Time (s) Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 130 Figure B.3 Changes in slope occur at the peak of each hysteresis loop (highlighted). pi ^^^^^ '\J3 A The 3.66 m and 4.88 m Type C walls also showed peculiar results (No. 2,4). The results degenerated into responses that appeared like damped free vibration. A possible explanation is that the program was incapable of completing the models of the multiple panel walls because their flexibility prevented the algorithm from converging within a reasonable specified energy tolerance. Conversely, it was incapable of modelling the stiffest single panel walls without reverting to the elastic response. It may be that the program had difficulty negotiating the transition at the peak of each hysteresis loop as shown in Figure B.3 (Dolan and Foliente, 1997). The planned earthquake input analyses, exterior nail spacing analyses and sensitivity analyses were then carried out to discover if refining the model inputs would produce reasonable and therefore usable results. A wall length of 2.44 m was chosen to carry out these studies. The results of the earthquake input analyses and the exterior nail spacing analyses were no more useful than the wall length analyses. The sensitivity analyses are described in the next section. Seismic Response of Hood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 131 B.2 Sensitivity Studies Many of the parameters required as input into the computer model were taken from geometry or experimental results. It was prudent to determine the sensitivity of the model results to variations in these parameters in hopes of improving the model results. The parameters of interest were the damping ratio, the shear modulus of the panel, the time integration step size and the perpendicular to grain nail parameters versus the parallel to grain nail parameters. All sensitivity analyses were carried out using models of 2.44 m long Type C walls subjected to the Joshua ground motion. The ground motion was scaled to a peak ground acceleration of approximately 0.35 g (120%). Table B.3 shows results from these analyses. By the previous analyses, it could be said that analysis results would be more accurate if the input parameters resulted in a response with lower frequency, higher drifts and lower accelerations. From Figure B.4 and Table B.3 entries No. 17, 18, 19, 20, it is apparent that an increase of the damping ratio lowered the frequency and the acceleration values of the response, however, it also decreased drift values. Furthermore, the improvements in frequency and acceleration achieved by increasing the damping ratio from 1% to as much as 10% do not appear sufficient to improve analysis results. Unless experimental and analytical results were already extremely similar, an improved match could not be achieved by changing the damping ratio. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 132 Figure B.4 Drift time histories showing the effect of varying the damping ratio in analyses of Type C walls. 0 10 20 30 40 50 60 Time (s) In studying the integration time step size (No. 21,22,23,24), the only size that yielded reasonable results was 0.01 s. As shown in Table B.3, the larger values caused instability. As with most other unstable runs in the sensitivity studies, the program was able to complete the analysis without the energy balance check becoming too large. The graphical and numerical results, however, showed responses with such exaggerated values, that it was clear that the program was becoming unstable. When the time step Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 133 integration size was changed to 0.005 s, the analysis results degenerated into an apparent damped free vibration behaviour. The time step integration size should typically be about one tenth of the natural period of the structure (Paz, 1991). The test walls had natural periods in a range of approximately 0.20 s to 0.25 s, suggesting that the integration time step size should be in the order of 0.02 s or less. Increasing the shear modulus by as much as 500% (No. 29) decreased the maximum drift and the maximum acceleration according to Table B.3. Increasing the shear modulus by 200% (No. 28) gave similar maximum results. It is logical that increasing the stiffness will converge towards results characteristic of an infinitely stiff panel. Though the stiffer panel models did affect the maximum values, the time histories did not show that a stiffer panel had a significant affect on the overall response. Decreasing the shear modulus to 50% (No. 26) and then to 20% (No. 25) of its original value increased the frequency of the response but also caused the program to become unstable, probably because the system was too flexible for the algorithm. As in the damping study, changing the shear modulus would only improve the correlation between analytical and experimental results if these results were already very similar. There was also little difference in the responses when parallel to grain nail parameters (No. 31) were used instead of perpendicular to grain nail parameters (No. 30). The overall response in each analysis was similar. Table B.3, however, shows that the maximum drift Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 134 point value for the parallel to grain case seems much smaller and the maximum acceleration point value seems much higher than for the perpendicular to grain case. Table B.3 Analyses for completing sensitivity studies. Maximum Maximum top First natural Un-stable No. Percent of control value (%) drift (mm) acceleration (g) frequency (Hz) (Yes/No) Damping ratio study (Control: C = 1%) 17 100 (1% damping ratio) 9.65 0.63 5.13 N 18 200 (2%) 9.01 0.61 5.13 N 19 500 (5%) 8.35 0.55 5.13 N 20 1000(10%) 6.80 0.48 5.13 N Integration time step interval study (Control: A = 0.01 s) 21 50 (0.005 s) 9.80 0.61 5.13 N 22 100 (0.01 s) 9.65 0.63 5.13 N 23 200 (0.02 s) - - 5.13 Y 24 500 (0.05 s) - - 5.13 Y Shear modulus study (Control: G = 1.5 GPa) 25 20 (0.3 GPa) - - 3.37 Y 26 50 (0.75 GPa) - - 4.45 Y 27 100(1.5 GPa) 9.65 0.63 5.13 N 28 200 (3.0 GPa) 6.88 0.61 5.62 N 29 500 (7.5 GPa) 6.82 0.61 5.96 N Nail parameter study (Control: Perpendicular) 30 Perpendicular 9.65 0.63 5.13 N 31 Parallel 7.96 0.67 5.19 N B.3 Comparison of Experiment Results and Analysis Results In a final attempt to understand SWAP results, comparisons were made to natural frequency, monotonic and dynamic experimental results. Experimental and analytical values for the first natural frequencies of the wall types showed the same trends. As Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 135 expected, multiple panel (Type A) walls were less stiff than single panel walls with regular exterior nail spacing (Type B) which were, in turn, less stiff than single panel walls with reduced exterior nail spacing (Type C). The analytical and experimental natural frequency values are reported in Table B.4. SWAP analysis underestimated the stiffness of the Type A wall and overestimated the stiffness of the Type C wall. Stiff walls would be expected to show high frequencies in their dynamic response, with the stiffness degrading (and frequency lowering) later in the response. Flexible walls would have low frequencies in their dynamic response, and these would further degrade early in the response. The mode shape observed in testing was consistent with the analytical prediction. Both showed that the response behaviour is dominated by rotation of the panels and shearing of the frame. Table B.4 Comparison between analytical and experimental first natural frequencies. Test Experiment: Analysis: first (experiment): Analysis: first natural natural Wall Type number number frequency (Hz) frequency (Hz) A 11 1 4.2 3.32 14 4.0 B 12 15 4.6 4.23 13 4.2 C 15 2,16 4.5 5.13 Figure B.5 shows the monotonic curves found from both analysis and experiment. The analysis consistently predicted loads higher than found through experiment. Again the trends observed within experimental results and within analytical results were the same. Both experiment and analysis revealed that Type C walls have significantly higher peak loads than either Type A or B walls. Type B walls were also shown to have higher peak Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 136 loads than Type A walls. The discrepancy between analytical and experimental results was, however, large. Figure B.5 Comparison between analytical and experimental monotonic behaviour. Type C Solid: Experiment Dashed: Analysis 10 20 30 40 50 60 70 80 90 100 Drift (mm) Comparisons of dynamic analytical and experimental results confirmed predicted trends. In analysis, less stiff walls had higher drifts, lower accelerations and lower frequencies when analyses with the 1992 Joshua Tree Station, Landers California earthquake. These trends have also been reported as characteristic in experimental results. Yet like with the frequency and monotonic comparisons between experiment and analysis, the dynamic comparison showed poor correlation between analytical and experimental results. In all Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 137 cases, analysis results showed significantly lower drifts than experimental results. The frequency of the analytical response was consistently too high, as were the analytical acceleration values. These discrepancies, which can be observed in Figure B.6 were of the same sort noted in Chapter 4 in the attempt to match SWAP results with results from previous testing (Latendresse and Ventura, 1995). Figure B.6 shows drift and acceleration response comparisons of analytical and experimental results for a Type C wall subjected to the Joshua ground motion scaled to a peak ground acceleration of approximately 0.35g. Greater accelerations relate to greater forces, so the overestimation of peak load occurred in both monotonic and dynamic analyses. The high frequencies in all analytical responses do not seem consistent with the analytical estimates of natural frequencies. Even where SWAP underestimated the natural frequency, the response showed higher shaking frequencies than were observed in the experiments. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 138 Figure B.6 Drift and acceleration response comparisons of analytical and experimental results for a Type C wall. 0 10 20 30 40 50 60 Time (s) 0 10 20 30 40 50 60 Time (s) Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels Appendix B 139 B.4 Surnmary The SWAP algorithm was unsuccessful in modelling the natural frequency, monotonic behaviour or dynamic response of experimentally tested walls. Further, the program was not adequately sensitive to any of the input parameters to improve the correlation between experimental and analytical results by varying these parameters within reasonable limits. Investigation of SWAP's algorithms or the use or creation of an alternative analysis program is necessary to achieve accurate results. With a dependable model, a database of shearwall properties and predicted dynamic behaviours could be created. This would be an invaluable tool in the efficient design of shearwalls, particularly shearwalls of non• standard configurations such as those considered in this study. Seismic Response of Wood Shearwalls with Oversized Oriented Strand Board Panels