THE USE of CORE and OUTRIGGER SYSTEMS for HIGH-RISE STEEL STRUCTURES Thesis Submitted to the School of Engineering of the UNIVER

THE USE OF CORE AND OUTRIGGER SYSTEMS FOR HIGH-RISE STEEL STRUCTURES

Thesis

Submitted to

The School of Engineering of the

UNIVERSITY OF DAYTON

In Partial Fulfillment of the Requirements for

The Degree of

Master of Science in Civil Engineering

Abdulaziz Alanazi

Dayton, Ohio

December 2019

THE USE OF CORE AND OUTRIGGER SYSTEMS FOR HIGH-RISE STEEL

STRUCTURES

Name: Alanazi, Abdulaziz Manqal

APPROVED BY:

Joseph Saliba, Ph.D. Elias Toubia, Ph.D. Advisory Committee Chairperson Committee Member Professor Associate Professor Department of Civil and Environmental Department of Civil and Environmental Engineering and Engineering Mechanics Engineering and Engineering Mechanics

Riad Alakkad, M.S. Committee Member Lecturer Department of Civil and Environmental Engineering and Engineering Mechanics

Robert J. Wilkens, Ph.D., P.E. Eddy M. Rojas, Ph.D., M.A., P.E. Associate Dean for Research and Innovation Dean, School of Engineering Professor School of Engineering

Abdulaziz Alanazi

2019

iii

ABSTRACT

THE USE OF CORE AND OUTRIGGER SYSTEMS FOR HIGH-RISE STEEL STRUCTURES

Name: Alanazi, Abdulaziz Manqal University of Dayton

Advisor: Prof. Joseph Saliba

With increases in building heights, understanding the effectiveness, advantages, and limits of different structural systems to resist lateral loads is crucial. Such tall buildings rely on moment frames, braced steel cores, concrete shear walls, outriggers, and bundled boxes to resist their lateral loads in addition to their gravity loads. In this investigation four structural models were created using STAAD Pro CONNECT Edition (moment frame, moment frame and a braced core, moment frame plus outriggers on the top and outriggers at 1/3 height) to compare the effectiveness of each system. According to this study, using a braced core in conjunction with a moment frame reduced the lateral displacements by 24% and 26% (with respect to the two lateral directions) as compared to using moment frames alone. Similarly, the outriggers systems reduced the lateral displacements but less than obtained by using the braced core system. When comparing the steel weight of the four different systems, the outriggers models have the least weight, therefore the cheapest. The lightest steel frame (lighter by 3184 kips) was obtained when using the outriggers at 1/3 of the building heights.

ACKNOWLEDGMENTS

I want to express my profound appreciation to my advisor Dr. Joseph Saliba, for his help and guidance.

I Also want to thank my family for their support and encouragement. My thanks to my parents for all their prayers for me and my thanks to my dear brothers Abdullah and Yousif for asking and caring.

TABLE OF CONTENTS

ABSTRACT ...... iv ACKNOWLEDGMENTS ...... v LIST OF FIGURES ...... viii LIST OF TABLES ...... ix CHAPTER I INTRODUCTION ...... 1 1.1 Definition of Tall Buildings ...... 1 1.2 Classification of Tall Buildings Structural Systems ...... 1 1.3 Outrigger Systems ...... 2 1.3.1 Historical Backgrounds of Outrigger Systems in Tall Buildings ...... 4 1.3.2 Advantages of Outriggers System in Tall Buildings ...... 8 1.3.3 Types of Outrigger Systems...... 8 1.4 Literature Review...... 10 CHAPTER II OBJECTIVES AND METHODOLOGY ...... 13 2.1 Objectives of the Study ...... 13 2.2 Methodology ...... 13 2.3 Assumption ...... 15 2.3.1 Dimensions of the Building ...... 15 2.3.2 Adding Braced Core and Outrigger Systems ...... 16 2.3.3 Section Properties ...... 21 2.3.4 Design Loads ...... 21 2.3.5 Steel Design Parameters ...... 25 CHAPTER III RESULTS AND DISCUSSION ...... 27 3.1 Overview ...... 27 3.2 Lateral Displacement ...... 27 3.3 Axial Deformation ...... 32 3.4 Material Cost ...... 33

CHAPTER IV CONCLUSION AND RECOMMENDATIONS ...... 35 4.1 Conclusion ...... 35 4.2 Recommendations ...... 36 REFERENCES ...... 37

vii

LIST OF FIGURES

Figure 1.1: Structural Systems of tall buildings...... 2

Figure 1.2: Outriggers and core interaction...... 3

Figure 1.3: Tour de la Bourse, Montreal, Canada and the tower mechanical floors layout ...... 5

Figure 1.4: 140 William Street in Melbourne and U.S. Bank Center in Milwaukee ...... 6

Figure 1.5: Conventional outriggers and Virtual outriggers ...... 9

Figure 2.1: Outriggers effect on roof level drift ...... 14

Figure 2.2: Plan view and elevation view of the building...... 16

Figure 2.3:Elevation view showing the building core ...... 18

Figure 2.4: Elevation view showing the core on the side of the building...... 18

Figure 2.5: Elevation view showing the building core and the outriggers on top...... 19

Figure 2.6: The outriggers level...... 19

Figure 2.7: The core location...... 20

Figure 2.8: Elevation view showing the outriggers at 1/3 of the building heights...... 20

Figure 3.1: Lateral displacement for Moment Frame model and Braced Core model ...... 29

Figure 3.2: Lateral displacement for Moment Frame model and outriggers on top model .... 29

Figure 3.3: Lateral displacement for Moment Frame and Outriggers at 1/3 height model ..... 30

Figure 3.4: Steel Cost of the Structural Models ...... 34

viii

LIST OF TABLES

Table 1.1: The Ten Tallest Completed Buildings Utilize Outrigger Systems ...... 7

Table 2.1: Summary on Outrigger Topology Studies...... 17

Table 2.2: Summary of the Gravity Design Loads ...... 23

Table 2.3: Wind Loads Intensity versus Height ...... 24

Table 2.4: Generated Load Combinations...... 25

Table 3.1: Summary of Lateral Displacement on Z Direction...... 28

Table 3.2: Summary of lateral Displacement on X Direction...... 28

Table 3.3: Axial Deformations Summary for each Structural Model...... 32

Table 3.4: Steel Cost Summary ...... 33

CHAPTER I

INTRODUCTION

1.1 Definition of Tall Buildings:

The definition of a tall building is dependent on the location of the building and the surrounding area. In cities like Hong Kong and New York, a 20 story building is not considered a tall building, but it is considered tall in cities that do not have many buildings at that height. However, from a structural engineering perspective, a tall building can be defined as a building whose structural system must be modified to make it sufficiently economical to resist lateral loads [1]. The capability of tall buildings to withstand against lateral loads is the reason for its existence. In tall buildings, the lateral load created by wind and earthquakes becomes more significant. That is because of the increase of overturning effect of loads, slenderness, the increase of lateral displacements, and interstory displacements. These displacements may risk the overall structural stability and cause disturbance to the residents [2].

1.2 Classification of Tall Buildings Structural Systems:

In 1965, Fazlur Khan has classified the structural systems with respect to its lateral load resistance [3]. This classification has divided the structural systems into four types as shown in Figure 1.1.

Figure 1. 1: Structural systems of tall buildings (Source: CTBUH)

1.3 Outrigger Systems:

The outrigger systems are lateral loads resisting systems that can effectively minimize the lateral loads and strengthen tall buildings. In this system, the external and the internal structure couple as a whole to resist lateral loads. Outrigger trusses work as stiff arms connecting the core of the building to the outside columns.

When lateral loads are applied on the building, the core tries to rotate inducing forces to the outrigger trusses, which create tension in windward columns and

compression in the leeward columns (see Figure 1.2). This response creates a kind of restoring moment that acts on the core at the outriggers' level, therefore the effective depth of the structure to resist the bending moment is increased. To further improve rotation restraint of the outrigger trusses, this can be done by mobilizing all the exterior columns with one- or two-story deep wall around the building known as a "belt wall".

Floor diaphragms above and below the belt truss will try to move right and left due to the rotation of the core and overturning moment. The belt truss connected to the floors will move in return and rotate itself one face up and one face down. The exterior columns will constrain this movement by developing opposing forces [4].

Figure 1. 2: Outriggers and Core Interaction. (Source: Taranath 1998)

1.3.1 Historical Backgrounds of Outrigger Systems in Tall Buildings:

Since the development of perimeter and bundled-tube frame systems in the 1960s and 1970s, major disadvantages for these systems can be overcome by using outrigger systems. Tall buildings like the World Trade Center twin towers in New York and Wills

Tower (formerly known as Sears Tower) in Chicago had closely spaced columns and spandrel beams with relative dense external frames. These frames carry most of the lateral loads with slight or no resistance contributing from the building core. Although the perimeter and bundled-tube systems are without a doubt more efficient than the outrigger systems and have a firm presence on the building exterior, the closely spaced columns in these systems limit the flexibility of architectural aesthetic in the applied buildings.

Variations on the outside columns spacing from the outrigger system designs can satisfy architectural goals.

An example for an early building that used the outrigger system is 47-story Tour de la Bourse in Montreal, Canada (Figure 1.3). This building was the first concrete building that utilized the outrigger system. The design for this building used the approach of fewer but bigger columns to concentrate dead load. The large columns are placed on each corner of the building with X braced outrigger trusses at four levels connecting the core of the building to these columns. On each side of the building there are two independent columns between the corner columns working as a backup system to the primary system.

Figure 1. 3: Tour de la Bourse, Montreal, Canada, and the tower mechanical floors layout (Source: CTBUH) The steel-framed outrigger system was first used to design 41 story 140 William

Street in Melbourne, Australia, and the 42 story U.S. Bank Center in Milwaukee,

Wisconsin (Figure 1.4). The two office tower designs are similar with conventional outrigger and belt trusses in the middle and the top of the buildings, in addition to that, belt trusses at the bottom of the buildings are placed for change in columns spacing.

Figure 1. 4: 140 William Street in Melbourne (left), U.S. Bank Center in Milwaukee (right) (Source: CTBUH)

Outrigger systems become widespread after these major projects and the development of high-strength concrete, which grants the stiffness and the strength that is needed with less cost to make the outrigger systems a suitable choice for buildings up to

60-stories [5].

Table 1. 1: The Ten Tallest Completed Buildings Utilize Outrigger Systems

Building Location Year of Stories Height(ft) Material Use Completion 1 Burj Khalifa Dubai, UAE 2010 163 2,723 Composite Multiple 2 Shanghai Shanghai, 2015 128 2,073 Composite Multiple Tower China 3 Ping An Shenzhen, 2017 115 1,965 Composite Office Finance China Center 4 Lotte World Seoul, South 2017 123 1,823 Composite Multiple Tower Korea 5 One World New York, 2014 104 1,792 Composite Office Trade USA Center 6 Guangzhou Guangzhou, 2016 111 1,739 Composite Multiple CTF China Finance Center 7 Tianjin CTF Tianjin, 2019 98 1,739 Composite Multiple Finance China Center 8 Taipei 101 Taipei, 2011 101 1,667 Composite Office Taiwan 9 Shanghai Shanghai, 2008 101 1,622 Composite Multiple World China Financial Center

10 International Hong Kong, 2010 118 1,588 Composite Multiple Commerce China Centre

1.3.2 Advantages of Outriggers System in Tall Buildings:

1. Outrigger systems can be made of using any material of concrete, steel, or

composite.

2. Uplift and tension forces can be reduced significantly throughout the columns

and foundations.

3. The spacing of the outside columns can be driven by architectural and

functional considerations, not structural considerations.

4. There are economic advantages of using the simple beam and column for the

exterior framing instead of rigid-frame-type connections.

5. Outrigger systems in rectangular buildings under wind load, can incorporate the

gravity columns of the building to resist lateral loads. In core-alone and tubular

systems, the middle columns, which carry a significant gravity load, are not

incorporated or underused [4].

1.3.3 Types of Outrigger Systems:

Based on the outrigger systems connection to the core, they can be divided into two types. The first type is the conventional or direct outrigger system. As the name implies these types of outrigger are connected directly to braced core or shear walls at the core and to the exterior columns. On the other hand, virtual or indirect outrigger and belt truss systems eliminate the direct connections of the building core walls in vertical planes

[4] (see Figure 1.5).

Figure 1. 5: Conventional outriggers (left) and virtual outriggers (right). (Source: R. Shankar Nair 1998) Choosing between these two types depends on different conditions of a building.

No doubt that the shorter loads' paths from columns to core in the conventional outriggers make them stiffer and more efficient than the virtual or indirect outriggers. In order to attain the same benefits of the direct outriggers, more indirect outriggers are required on more levels. It is also possible that the same building uses the two types of outriggers.

Virtual outriggers can be chosen to avoid complexity in connections between the core and the outside column. In fact, in some settings, certain levels in a building are not suitable for direct outriggers, and the differential shortening is more problematic in direct outriggers than indirect outriggers at certain floors [5].

1.4 Literature Review:

Po Seng Kian and Frits Torang Siahaan (2001) have worked on a comparative study and analysis of 40-story, 2-dimensional models subjected to wind load and 60- story, 3-dimisional models subjected to seismic load. The idea is to increase the stiffness of the structures by providing outrigger systems and belt truss with different configurations by changing the locations of the outriggers to every model. This research is also carried out for the optimum location for outriggers. Researchers have found that in the 2- dimensional models, 65% reduction of maximum displacement is obtained by placing outriggers at the top and the middle of the structure. Furthermore, in the 60-story

3-dimensional models, 18% of maximum reduction in displacement is obtained by placing the outriggers at the top and at the 33rd floor.

Akash Kala, Madhuri Mangulkar, and INdrajeet Jaint (2016) have carried out an analysis of a 60-story reinforced concrete building subjected to wind load using the

ETABS software program. The purpose of this analysis is to observe the effectiveness of outrigger systems in reducing the lateral displacement and inter-story drift. They conclude that the optimum location for the outrigger is 0.25 to 0.33 times the building's height. The maximum displacement at the top of the structure was reduced from 440mm to 406mm when outriggers was placed at the 20th story.

Abhijeet V. Chavan and Prof. Vaibhav V. Shelar (2017) worked on a pushover analysis for a three-dimensional, 21-story, beam and slab reinforced concrete building models using ETABS program. Five models were created for this analysis with and

without outriggers to see the deference in the maximum displacement at every level. The result from this study that models with outrigger system, core, and infilled wall have reduced the maximum deflection from 40% to 65%. Outrigger systems and the core wall are recommended to effectively control the lateral displacement as it will be more economical compared to periphery shear walls.

A study was conducted by Kiran Kamath, N. Divya, and Asha U Rao (2012) for the outrigger systems static and dynamic behavior on a 40-story reinforced concrete building. The research aim is to investigate the building stiffness against lateral loads due to seismic and wind loads. 6 different arrangements of outriggers were created for the three-dimensional building by varying the height of the outriggers (Hs/H ratio) from

0.975 to 0.4, and the relative stiffness between 0.25 and 2. Equivalent analysis for the static behavior was carried out and Time History Analysis for the dynamic behavior purpose was performed by taking the earthquake history data that occurred in the

California region as per Indian standard codes. Different parameters such as inter-story drift, lateral deflection, and peak acceleration were carried out. The result from this investigation was that outriggers placed at the top are less efficient compared to the outriggers placed at the middle height of the structure. The study also shows that up to

50% in drift reduction was obtained by placing the outriggers at the top. In addition, when taking the peak acceleration criteria for the design, the optimum location for the outrigger is at the top where up to 30% reduction was achieved.

Abdul Karim Mulla and Shrinivas B.N (2015) have published research on the regular and vertical irregular structures. The idea is to increase the stiffness of the building by using the outrigger systems of steel bracing to the reinforced concrete building. A comparative analysis was carried out of a 20 story, the 3-dimensional building that was regular and vertically irregular shaped with a symmetrical plan. The researchers observed that there was a significant reduction in base shear under the static and dynamic seismic loads when outriggers were introduced to the building. Story drift and Time Period were also minimized, which increases the building's overall stiffness.

CHAPTER II

OBJECTIVES AND METHODOLOGY

2.1 Objectives of the Study:

Most researchers have investigated the capability of braced core and outrigger systems to resist lateral loads in tall buildings. The optimum positions of the outrigger's trusses have also been studied abundantly. Nonetheless, few researchers have carried out the economic advantages of such systems. In this research analysis and comparative study, moment frames, braced core and the outrigger systems will be investigated.

STAAD.Pro CONNECT Edition software was selected to analyze and design a 3- dimensional, 40-story, steel structure. The aim of this investigation is to observe the effectiveness of each structural system on the building's overall stiffness against the lateral loads. Subsequently, the economic advantages will be carried out by measuring the amount of steel in each structural model.

2.2 Methodology:

The methodology for this study uses the STAAD.Pro CONNECT Edition software for the design and analysis of a 3-dimensional model utilizing different structural systems. This study focuses on a rectangular 40 story steel structural building.

Four structural models were created for this study. The first structural model is designed

as moment frame. The second model will have braced core, the third and fourth structural models will utilize braced core and outrigger trusses with one story high.

The numbers of the outriggers that are needed for a tall building are dependent on the building's relative height and columns size pattern. Based on a set of assumptions,

(columns' sizes, wind load distribution, bracing geometry) a single outrigger trusses are used with two different positions. The first position is placing the outrigger trusses at the top of the structure. The second location is by placing the outriggers at one third of the building height. Studies indicate that for one set of outriggers, the ideal position is at three quarters to one third of the building height, measured from top level (see Figure

2.1).

Figure 2. 1: Outriggers effect on roof level drift (Source: Choi, 2012).

2.3 Assumption:

In this investigation the four structural models are designed according to AISC

360 2010 specifications. The dimensions, the loads, and other considerations of the structural models have been taken to give a reasonable approach of the behavior of the braced core and outrigger systems in high-rise buildings. Gravity loads (dead and live loads) are assumed typical on all the building floors.

2.3.1 Dimensions of the Building:

Each story of the 40 story building has a typical floor height of 12 ft, giving a total height of 480 ft. The length of the building is 6 bays, and each bay is 32 ft long, giving the total length of building 192 ft. The width of the building has 5 bays, giving the total width 160 ft (see Figure 2.2).

Figure 2. 2: Plan view and elevation view of the building.

2.3.2 Adding Braced Core and Outrigger Systems:

Goman Ho (2016) has conducted studies on the outrigger's topology with the same space constraint and their relationship to stiffness and strength. Chevron bracing shape was selected to the structure core and outrigger trusses due to their high stiffness

(see Table 2.1). The high stiffness of the outriggers do not necessarily comply with their efficiency on restraining the building against lateral loads, therefore, balancing the wind load and earthquake distribution on the building is mandatory to find the appropriate

stiffness. In this study, the wind load is taken into consideration without seismic load, thus, stiffness becomes more important than ductility as the case will be under seismic consideration.

Table 2. 1: Summary on Outrigger Topology Studies. (Source: Ho 2016).

The core is two bays by one bay, braced for each three stories. The outrigger trusses are conventional (direct) outriggers placed at two different locations; the third structural model utilizes outriggers on the top, whereas the fourth structural model has outriggers at one third of the building height, measured from the top level (see Figures

2.3, 2.4, 2.5, 2.6, 2.7 and 2.8).

Figure 2. 3: Screenshot of the elevation view showing the building core.

Figure 2. 4: Screenshot showing the elevation view of the core on the side of the building.

Figure 2. 5: Screenshot from the software showing elevation view of the building core and the outriggers on top.

Figure 2. 6: Screenshot for the outriggers level obtained from the software.

Figure 2. 7: Screenshot for the top view showing the core location.

Figure 2. 8: Screenshot from the software showing elevation view of the building core and the outriggers at 1/3 of the building heights.

2.3.3 Section Properties:

Steel wide flange section is considered for all the members of the structure. The columns and the beams have constant sections each four stories. The bracing that will be added to the core and the outriggers when incorporating the core and outrigger systems will also have the same section shape.

2.3.4 Design Loads:

The primary loads considered for these structural models are dead, live and wind loads. The dead load is considered the self-weight of the structure, the slab weight, 8 psf miscellaneous load, in addition to 25 plf the glass walls' weight. Roof load is taken as 25 psf and live load is considered as 80 psf. All these loads are converted to Uniformly

Distributed Load (UDL) form and placed on beams on the X direction.

Dead Load:

Slab thickness is taken as 5 in and the concrete unit weight is 150 pcf. Multiplying the thickness by the concrete density, we get the slab weight:

Slab load 5in/12ft*150 pct = 62.5 psf

The glass walls' weight is calculated by multiplying the weight with the height of the floor as follows:

Wall load 25 psf * 12ft = 300 plf =0.3 klf

The tributary area for each of the beams (apart from the beams on the edges) is

32*32= 1024 ft^2. The tributary area for the beams on the edges are going to be half of that of the inside, which is 1024ft/2 = 512ft^2. Therefore, each load on the girder will be multiplied with the total tributary area of that girder to get the total weight. Then, the result is divided by the length of the girder to get the UDL form for these loads as follows:

Slap total weight = 62.5 psf * 1024 ft^2 = 64,000 Ib

UDL slap load = 64,000 Ib/ 32ft = 2000 Ib/ft = 2 kip/ft

Miscellaneous total weight = 8 psf * 1024ft^2= 8,192 Ib

UDL miscellaneous load = 8,192/32ft = 256 Ib/ft = 0.256 kip/ ft

Total roof load = 25 psf * 1024 psf = 2500 Ib/ft

UDL roof load = 2500/32ft = 800 Ib/ft = 0.8 kip/ft

Live Load:

The live load of the building is assumed to be 80 psf and converted to kip/ft as calculated below:

Total live load = 80 psf * 1024ft = 81,920 Ib

UDL live load = 81,920 /32ft = 2560 Ib/ft = 2.56 kip/ ft

Table 2. 2: Summary of the Gravity Design Loads

Load Load Name Total Uniform Load Total Uniform Load

Type (psf) (kip/ft)

Dead 1-Slab Weight 62.5 2 load 2-Wall Weight 25 0.3

3-Roof Load 25 0.8

4- 8 0.256

Miscellaneous

Live load 80 2.56

Wind Load:

The wind load is generated according to ASCE-7 2010. Set of parameters were fed to the program to generate the wind load as follow:

1. Building classification risk category: Category II

2. Wind speed: 97 mph

3. Exposure category: B

4. Natural Frequency: 2

5. damping ratio: 0.02

By using the above parameters, STAAD Pro generates the wind load pressure automatically and converts them to nodal forces applied on the building. The wind load is

applied on both X and Z directions. Table 2.3 shows the intensity of wind load versus height on each of the building's walls.

Table 2. 3: Wind Loads Intensity versus Height.

Generally, for building heights that ranked above 160 ft wind tunnel pressure is used to determine the wind load on the structure. In this study, ASCE 7-10 considerations for wind load was taken apart from the wind tunnel test.

Load Combinations:

The load combinations were established in accordance with AISC 360-10. The auto load combination generator was used to create the load combinations for the structures. Since lateral loads are applied on the structure, and P-delta analysis was carried out to perform the analysis, repeat load cases were included in generating the load combinations (see Table 2.4).

Table 2. 4: Generated Load Combinations.

2.3.5 Steel Design Parameters:

The structural models were designed according to AISC 360-10. The steel design of the structural model was carried out using the design patch in the Analytical Modeling workflow. The design parameters that are defined on the building models are as follow:

1. METHOD: LRFD (Design method.)

2. FYLD: 70 ksi (Yield strength of steel.)

3. FU: 90 ksi (Ultimate tensile strength of steel.)

4. TRACK: 2 (Printing the output design detail at the maximum level.)

5. DFF: 160 (Deflection length/ maximum allowable local deflection.)

6. DJ1: (Starting node of member for deflection length.)

7. DJ2: (Ending node of member for deflection length.)

The rest of the parameters are set as default. STAAD Pro will carry on the design process by calculating all the parameters, the defined and the parameters that are set as default.

The design commands that are assigned after the steel design parameters are as follow:

1. SELECT:

Selecting the least size of a member based on selected code specification by

the result from the most recent analysis.

2. CHECK CODE:

This command to check the members adequacy to the desired code

specification.

3. TAKE OFF:

This command to calculate the length and the weights for all the members that

are used on the structure.

CHAPTER III

RESULTS AND DISCUSSION

3.1 Overview:

In this chapter, the output data for each structural model are reviewed and compared. The selected results from the postprocessing workflow in STAAD Pro are including the lateral displacements, axial deformations, and the weight of steel for each structural model.

3.2 Lateral Displacement:

The lateral displacement was measured in Z and X directions. The braced core model and the outriggers models are compared to the moment frame model. The lateral displacement was measured by taking the maximum node displacement in each story for each structural model.

Table 3.1, 3.2 and figures 3.1, 3.2 and 3.3 illustrate the lateral displacement for each structural model in Z and X directions.

Table 3. 1: Summary of Lateral Displacement on Z Direction.

Number of Lateral Displacement in Z direction (in)

Stories Model 1 Model 2 Model 3 Model 4

10 8.088 9.819 10.071 9.694

20 19.13 19.668 20.04 19.386

30 33.315 30.362 30.768 29.672

40 51.596 39.395 39.844 38.497

Table 3. 2: Summary of Lateral Displacement on X Direction.

Number of Lateral Displacement in X Direction (in)

Stories Model 1 Model 2 Model 3 Model 4

10 6.555 6.027 6.196 6.096

20 13.380 11.009 11.150 11.320

30 20.992 15.492 15.597 15.846

40 27.819 20.445 20.484 20.902

Lateral Displacement in Z Lateral Displacement in X Direction Direction 45 45 40 40 35 35 30 30 25 25 20 20 15 15 10 10

5 5 Number of Stories NumberStories of 0 NumberStories of 0 0 20 40 60 0 10 20 30 Displacement (in) Displacement (in)

Model 1 Model 2 Model 1 Model 2

Figure 3. 1: Lateral displacement for Model 1 (Moment Frame) and Model 2 (Braced Core).

Lateral Displacement in Z Lateral Displacement in X Direction Direction 45 45 40 40 35 35 30 30 25 25 20 20 15 15 10 10

5 5

Number of Stories NumberStories of Number of Stories NumberStories of 0 0 0 20 40 60 0 10 20 30 Displacement (in) Displacement (in)

Model 1 Model 3 Model 1 Model 3

Figure 3. 2: Lateral displacement for Model 1 (Moment Frame) and Model 3 (outriggers on top)

Lateral Displacement in Z Lateral Displacement in X Direction Direction 45 45 40 40 35 35 30 30 25 25 20 20 15 15 10 10

5 5 Number of Stories NumberStories of 0 NumberStories of 0 0 20 40 60 0 10 20 30 Displacement (in) Displacement (in)

Model 1 Model 4 Model 1 Model 4

Figure 3. 3: Lateral displacement for Model 1 (Moment Frame) and Model 3 (Outriggers at 1/3 height) Lateral displacement in the Z direction:

The lateral displacement at the top for the first structural model that utilize moment frame was reduced by 24% when incorporating the braced core in the second model. The outriggers models have reduced the lateral displacement by 23% for the model that utilize outriggers at the top and by 25% for the outriggers at one third of the building heights. Outriggers model have improved the restraint against lateral load than the moment frame and braced core systems. Placing the outriggers at their optimum location in this case at one third of the building height, gave more restrain against lateral load than the outriggers at the top and the braced core models. The braced core model

and the outriggers models did not show a large deference in resisting lateral displacements in the Z direction.

Lateral displacement in the X direction:

The moment frame model shows lateral displacement by 27.82 inches at the top of the structure. This displacement was reduced in the second model by 26% to be 20.45 inches. The outrigger models gave less restraint against lateral loads than the braced core model in the X direction.

The lateral displacements on both directions were obviously large but expected given the absence of shear walls and the aspect ratios of the braced core. Such displacements are not practical or perhaps permissible. The intent was to study the effect of each individual system rather than developing a practical design.

3.3 Axial Deformation:

The axial deformations on each structural model are checked for each 10 stories.

To measure the deformations, the maximum node displacements on the vertical axis (Y axis) was calculated for each structural model.

Table 3. 3: Axial Deformations Summary for each Structural Model.

Number of Deflection (in)

Stories Model 1 Model 2 Model 3 Model 4

10 1.198 1.199 1.222 1.212

20 2.001 2.344 2.364 2.342

30 2.893 3.291 3.316 3.276

40 4.072 3.753 3.777 3.731

From the table above, Reductions in deformation was achieved for the braced core model by 7.8%. The outriggers on top and at 1/3 of the building heights have reduced the axial deformation by 7.2% and 8.4% respectively.

The fourth structural model which utilize outriggers at 1/3 of the building heights have showed the least axial deformation compared to the braced core model and the third structural model which uses outriggers at the top.

3.4 Material Cost:

The steel weight for each structural model is estimated to observe the effect of each structural system on material cost. As mentioned, steel wide flange sections are assumed for the all the members of the building models. The steel type is high strength low-alloy steel with yield strength (Fy) of 70 ksi and 90 ksi tensile strength (Fu). The cost of the steel is estimated with constant 45 cents per pound. Table 3.4 and figure 3.4 summarize the amount and cost of the steel used for each structural model.

Table 3. 4: Steel Cost Summary.

Model Weight of Steel (kip) Cost ($) 0.45$/Ib

Moment Frame 20374.129 9,168358.05

Braced Core 17222.489 7,750120.05

Outriggers at top 17202.708 7,741218.6

Outriggers at 1/3 height 17189.821 7,735419.45

Steel Cost 21000 Kip $9,500,000

20000 Kip $9,000,000

19000 Kip $8,500,000 18000 Kip $8,000,000 17000 Kip

$7,500,000 16000 Kip

15000 Kip $7,000,000 Model 1 Model 2 Model 3 Model 4

Weight Cost

Figure 3. 4: Steel Cost of the Structural Models From the graph and table above, the braced core and the outriggers structural systems have reduced the amount of steel that is needed for this building. The braced core model showed the amount of steel was vastly reduced by 3151.64 kip from moment frame model. The outriggers model showed even further reduction by 3171.421 kip for the outriggers on top and by 3184.308 kip for the outriggers at 1/3 heights all compared to the first structural model. The outriggers model at 1/3 of the building heights has the least weight of steel, therefore they are the cheapest than the other structural models.

CHAPTER IV

CONCLUSION AND RECOMMENDATIONS

4.1 Conclusion:

This study investigates the effect of the core and outrigger systems on tall steel structures. Four building models were created using STAAD Pro CONNECT Edition program to observe the lateral displacement, axial deformation, and material cost in each model. The first model is designed as moment frame, the second model uses braced core, and the third and fourth models containing outrigger systems with different configurations in their positions. The first position of the outriggers is at the top of the building and the second is at 1/3 of the building's height. The gravity loads (dead and live) are assumed typical on all the buildings floor, the lateral loads are limited to wind load and it was generated by the software in accordance with ASCE 7-10. Chevron bracing shape is used for the braced core and the outriggers due to their high stiffness.

The load combinations and steel design were carried out using the AISC 360-10 standers.

The first structural model that utilized moment frame showed enormous amounts of lateral displacement and steel weight. However, when incorporating the braced core system to the moment frame model, the lateral displacement was reduced by 24% for the z-direction and 26% for the x-direction. The amount of steel that was used for the first

model has been decreased immensely by 3184 kip when adding outrigger systems at 1/3 of the building heights.

4.2 Recommendations:

More studies are needed on the benefits of outrigger systems in tall buildings. In this study, the core and outrigger systems are tested for 40 story buildings, considering that, outriggers can work more efficiently with buildings up to 60 stories. On top of that, in this study, core and outriggers systems are used only as steel material. In tall buildings, composite material is more widely used than steel or concrete only structures. Shear wall cores are much stiffer than braced cores, thus, outriggers can function more effectively with shear walls.

REFERENCES

1. Fintel, M., (1985). Ch 10, Handbook of Concrete Engineering. Van Nostrand

Reinhold, New York, pg. 339-383.

2. Ghosh, S. K. (1995). Structural Systems for Ultra-High-Rise Buildings. Portland

Cement Association, Skokie, Illinois.

3. The Council of Tall Building and Urban Habitat, (1994). Structural Systems for

Tall Buildings, Classification of Tall Building Structural Systems Ch. 1,

Bethlehem, Pennsylvania, pg. 5-8.

4. Vikas Govalkar, P. J. Salunke, N. G. Gore, (2014). Analysis of Bare Frame and

Infilled Frame with Different Position of Shear Wall. International Journal of

Recent Technology and Engineering, Vol 3.

5. Choi., Ho, G., Joseph, L. & Mathias, N. (2017) Outrigger Design for High-Rise

Buildings 2nd Edition: An output of the CTBUH Outrigger Working Group.

Council on Tall Buildings and Urban Habitat: Chicago.

6. Kian, Po Seng and Siahaan, Frits Torang, (2001). The Use of Outrigger And Belt

Truss System For High-Rise Concrete Buildings. Dimensi Teknik Sipil, Vol. 3,

No. 1, Maret, 36-4.

7. Akash Kala, Madhuri Mangulkar, Indrajeet Jain, (2016). Optimum position of

outrigger with belt truss in tall building under horizontal load. Jawaharlal Nehru

Engineering College, Aurangabad Sinhgad Institute of Technology and Science,

Pune. India.

8. Abhijeet V. Chavan1, Prof. Vaibhav V. Shelar, (2017). Pushover Analysis of

High-Rise Building and Outrigger System with or Without In-Filled Walls.

Trinity College of Engineering, Pune, India.

9. Kiran Kamath, N. Divya, Asha U Rao, (2012). A Study on Static and Dynamic

Behavior of Outrigger Structural System for Tall Buildings. Bonfring

International Journal of Industrial Engineering and Management Science, Vol. 2,

No. 4.

10. Abdul Karim Mulla, Srinivas B. N, (2015). A Study on Outrigger System in a Tall

R.C Structure with Steel Bracing. International Journal of Engineering Research

& Technology. Vol. 4 Issue 07.

11. ASCE/SEI 7-10 Standers, Minimum Design Loads for Buildings and Other

Structures, Chapters 27-31, Wind Loads on Buildings, pg. 203- 299.

12. Bungale S. Taranath, (2012), Structural Analysis and Design of Tall Buildings

Ch1, Lateral Load Resisting Systems for Steel Buildings, Core and Outrigger

Systems, pg. 44-64.

13. Goman W. M. Ho, (2016). The Evolution of Outriggers System in Tall Buildings,

International Journal of High-Rise Buildings, Vol 5, No 1, 21-30.

14. Alshamrani, O., Schierle, G. G., Galal1 K. & Vergun, D. (2009). Optimal bracing

type and position to minimize lateral drift in high-rise buildings. Concordia

University, Montreal, Canada. USC, School of Architecture, Los Angeles, USA.

15. Gadkari, A. P., Gore, N. G., (2016). Review on Behavior of Outrigger Structural

System in High-Rise Building. MGM’s College of Engineering and Technology,

Navi Mumbai, India.

16. Xian, L., Wei, W., Henglin, L. & Guangchang, Z. (2016). Seismic behavior of

outrigger truss-wall shear connections using multiple steel angles. China

University of Mining and Technology, Xuzhou, China.

17. Bentley Systems, STAAD Pro Technical Reference Manuel,

https://docs.bentley.com/LiveContent/web/STAAD.Pro%20Help-

v7/en/STD_COMMANDS_SECTION.html.

18. Razgaitis, V. (2015). Outrigger-Braced Systems in Tall Buildings. University of

Pittsburg, Pittsburg, Pennsylvania, United States.

19. Walsh, P., Saleh, A. & Far, H. (2018). Evaluation of Structural Systems in

Slender High-Rise Buildings. University of Technology Sydney (UTS), Ultimo,

Australia. Australian Journal of Structural Engineering, Vol. 19, No 2, 105-117.

20. ANSI/AISC 360-10 Standers, Chapter L, Design for Serviceability., pg. 239-444.