Lateral Resistance of Walls and Anchorage in Log Structures By Robert Leichti, Randy Scott, Thomas Miller, P.E., and Jeff Sharpe, P.E. Log structures are part of American examined foundation anchorage and factors including log alignment, log pro- history and the contemporary building base shear capacity for log buildings and fi le, management of settlement, length inventory. The early structures were low, the effect of construction details on lat- of logs, corner details, and unit® shear re- squat buildings with few wall perfora- eral force resistance in walls made with quirements of the building system (Log tions for windows and doors. However, manufactured logs. The manufactured Homes Council, 2003). Each of these newer log structures often are large, logs investigated were surfaced top and fasteners has installation requirements, have many and/or large wall perfora- bottom so that fl at or mated surfaces and building system performance can be tions for windows and doors, and in- are in contact. These are distinguished affected by small changes in installation clude high aspect ratio wall segments, from round and scribe-fi t logs, which and construction details (Scott et al, For- especially at the corners. Just as in older typically have a linear contact with the est Products Journal, 2005b). log structures, new log buildings incor- neighbor logs. porate interlocked corner connections, Foundation and Base Shear Capacity and the wall height changes dimen-CopyrightInter-Log Connection Foundation anchorage is an important sion during the life of the structure as The engineering purpose of inter-log component of seismic performance in the logs lose and absorb moisture. The hardware is to provide a positive load log buildings. Mahaney and Kehoe (The interlocked corners contribute integrity path for the lateral force from roof to CUREE CalTech Wood Frame Project, to the building system, but the joints at foundation and to increase the stabil- 2001) provided a literature review on the window and door openings typically ity of the wall segments. The small and the subject of foundation anchorage for permit slip to accommodate moisture simple log structures of yore could stand light-frame buildings. Log structures are response dimensional change. without inter-log connection, because typically placed on foundations of similar Log shear walls typically are also bear- logs ran from corner to corner and wall design to those used for light-frame wood ing walls and resist lateral loading through perforations were small. The inter-log and masonry construction. Shear forces a different mechanism than light-frame connectivity for contemporary log struc- that develop at the base of the wall are walls. In light-frame walls, the lateral tures can be designed using yield mode transferred from the sill log (bottom log loads are transferred from the top plate equations in Chapter 11 of the National in the wall) to the foundation by anchor to the foundation through the nailed Design Specifi cation® (NDS®) for Wood bolts. Common anchor bolt spacing is 48 sheathing. Nail bending, nail withdrawal, Construction (AF&PA 2005). inches, and anchor bolt holes are over- and nail pull-through are important ener- A plethora of fasteners for log build- sized to facilitate construction. Anchor gy dissipation mechanisms in light-frame ings has emerged in the last decade. bolts lose tightness as the log shrinks due shear walls. According to Haney (Log Spikes, lag screws, through-bolts, thread- to drying (Scott et al., Forest Products Building News, 2000), magazinelateral loads in ed log-home screws, drift pins, and wood Society Annual Meeting, 2002), and log shear walls (Figure 1) are transferred dowels, are all recognized (Log Homes anchor bolt nuts may be inaccessible so S fromT top plate toR foundation throughU C T U R E Council, 2003). Through-rods can be they cannot be tightened later in the life log-to-log friction, inter-log hardware, tightened by automatic take-up springs of the structure. In addition, the build- and inter-wall corner connections. Log- as the building shrinks or by manually ing mass is often signifi cantly greater log slip is a critical energy dissipater in tightening the nuts at the top plate, but than a light-frame building and connec- log shear walls lag screws and spikes are not accessible tion geometry is different because the log In a recent research project, Scott (MS RACTICES and are not tightened later. The choice diameter is larger than the thickness of a Thesis, Oregon State University, 2003) of inter-log fastener is affected by many typical 2-by sill plate. P TRUCTURAL practical knowledge beyond the textbook S Figure 1: Log wall including a window opening and an inter-wall connection on a rigid foundation (from Scott et al. 2005a) STRUCTURE magazine40 March 2006 Two foundation/anchorage de- tails common to log structures were explored (Figure 2). The fi rst has the log wall sitting on the fl oor diaphragm. In this case, the an- chor bolt must be long enough (or coupled) to extend from the top ® of the foundation wall through the fl oor cavity and the sill log. In the second design, the sill log is in contact with the foundation wall or sits on a treated wood plate. In this second instance, the anchor bolts pass from the foundation di- rectly into the sill log. A series of tests was conduct- ed to evaluate the effectiveness Copyright of the two foundation/anchorage designs. The tests were of assem- bled systems that included all the components of each foundation, sill log, and anchorage hardware. Static tests of each were perform- ed and these were followed by a set of quasi-static tests based on the CUREE test protocol (Krawinkler et al., CUREE/CalTech Wood Frame Project Report, 2000). The test confi guration included a Figure 2: Typical foundation details for log buildings; (a) sill log on fl oor diaphragm, (b) sill log on concrete foundation vertical load to mimic dead and live wall (from Scott et al. 2005a) loads in the designed wall system as well as the lateral loading mechanism. Details of the testing apparatus and protocol are given The UBC base shear formula is, the roof. When the upper bound is divided by Scott (2003). by 1.4 to convert from strength level to al- magazineCvI Test results (Figure 3) for each of the V = −−−−W Eq. (2) lowable stress design, E = 2050 pounds for a foundation/anchorageS T details showedR that URT C T Urepresentative wallR that is 8 feet long.E friction between the sill log and the sill plate The UBC also defi nes the upper bound for The tested foundation/anchorage assembl- is an important part of system behavior. The base shear as, ies resisted lateral forces of at least 9890 open boxy shapes of the hysteresis diagrams pounds. Thus, the ratio of capacity to design 2.5C I are typical of friction damping behaviors. V = −−−−−a W Eq. (3) is at least 4.8, which is consistent with the These tests were terminated when the lateral R factor of safety for mechanical connections. force reached 10 kips, which was before the Cv = 0.64 and Ca = 0.44 are the seismic Uang (Journal of Structural Engineering, system failure. For the sill log on the fl oor (response spectrum) coeffi cients (UBC Tables 1989) provides a method to establish de- diaphragm, the system was still accepting load 16-R and 16-Q) for type SD soil profi le and sign coeffi cients and factors (response mod- at 10 kips, but it appeared that the ultimate seismic zone 4, I = 1 is the importance factor ifi cation, system over-strength, and defl ec- yield mode would include the rim board to sill (UBC Table 16-K), T = 0.11 seconds is the tion amplifi cation) for building seismic plate toenail connection. In the system with fundamental period that is calculated from provisions. The basic formulas derived by the sill log on the foundation wall, the sill plate UBC equation 30-8 for height = 10 feet, and Uang can be used for a rational analysis of sustained damage, but the system capacity was the response modifi cation factor R depends these factors so they can be consistent with limited by anchor bolt bending. on the structural system. A specifi c value for the International Building Code (IBC) (2003). For seismic design, the Uniform Building R has not yet been assigned to log structures. As for masonry structures, several different Code (UBC) (ICBO 1997) requires design for However, R could range from 2.8 (light steel sets of design coeffi cients may be needed an earthquake load (E), frame and some gravity-force braced frames) depending on the log profi les and type of to 5.5 (light-frame walls with shear panels inter-log fasteners. The Wood Materials ρ E = Eh + Ev Eq. (1) less than three stories). The most conservative Engineering Lab at Washington State estimate for V is used assuming R = 2.8. A University and the Department of Wood The redundancy factor ρ has an upper less conservative value for V is obtained using Science and Engineering at Oregon State bound of 1.5, which is used here. Eh is the the R-value for masonry walls (Scott et al., University are developing a collaboration to load due to horizontal ground motion (base Forest Products Journal, 2005a). Calculations establish the underlying support for the de- shear), while Ev is the load effect attributed show that the upper bound for V controls for sign coeffi cients. to vertical ground motion and is zero for this log structure. Seismic dead load W=4880 allowable stress design.