Physical Model of Skyscrapers with Core, Megacolumns,

Belts, Outriggers, and Diagonals

Stott Huffaker Bushnell

A project submitted to the faculty of

Brigham Young University

in partial fulfillment of the requirements for the degree of

Master of Science

Richard J. Balling, Chair

Paul W. Richards

Fernando S. Fonseca

Department of Civil Engineering

Brigham Young University

April 2016

Copyright © 2016 Stott Huffaker Bushnell

All Rights Reserved

ABSTRACT

Physical Model of Skyscrapers with Core, Megacolumns,

Belts, Outriggers, and Diagonals

Stott Huffaker Bushnell

Department of Civil Engineering, BYU

Master of Science

With the advent of the core and megacolumn design, skyscrapers have been able to soar

to even greater heights. In order to demonstrate to student the fundamentals of this design, a

physical model had been built. However, due to problems in the initial design, a new model was

needed. It was decided that the design of the overall model would resist lateral loads in one

direction for instructional purposes. Foam was used for the core and megacolumns because of its

low stiffness. To make the model more uniform, 3D printing was used to make the structural

elements, namely outrigger trusses, belt trusses, and diagonal braces. In order to attach the 3D

printed pieces to the model, multiple adhesives were tested for strength after which epoxy was

used in the assembly. Lateral loads were then applied to the model to measure which of the

structural elements added the most stiffness. Of the three, the diagonal braces contributed much

more to the stiffness than did the outrigger or belt trusses.

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TABLE OF CONTENTS

LIST OF TABLES ......................................................................................................................... iv

LIST OF FIGURES ........................................................................................................................ v

1 Introduction ............................................................................................................................. 1

2 Background .............................................................................................................................. 3

Skyscraper Design ............................................................................................................ 3

3D Printing ....................................................................................................................... 8

3 Design of Model .................................................................................................................... 11

Foam Elements ............................................................................................................... 13

3D Printed Elements....................................................................................................... 13

3.2.1 Outriggers ............................................................................................................... 15

4 Connections ........................................................................................................................... 19

Assembly ........................................................................................................................ 19

5 Results ................................................................................................................................... 21

6 Conclusion ............................................................................................................................. 25

References ..................................................................................................................................... 27

iv

LIST OF TABLES

Table 5-1: Stiffness by Configuration ........................................................................................... 23

v

LIST OF FIGURES

Figure 1-1: Current Skyscraper Model in Use ................................................................................ 1 Figure 2-1: Empire State Building under Construction .................................................................. 4 Figure 2-2: Framed Tube with Closely Spaced Exterior Columns ................................................. 5 Figure 2-3: Building Heights per Design Method .......................................................................... 6 Figure 2-4: Simplified Core and Megacolumns.............................................................................. 6 Figure 2-5: Core-supported Outrigger Structure............................................................................. 7 Figure 2-6: Building Heights per Design Method .......................................................................... 8 Figure 2-7: 3D Printers Used .......................................................................................................... 9 Figure 3-1: Shanghai World Financial Center .............................................................................. 11 Figure 3-2: Preliminary AutoCAD Model Design........................................................................ 12 Figure 3-3: Final AutoCAD Model .............................................................................................. 13 Figure 3-4: US Bank Center ......................................................................................................... 14 Figure 3-5: AutoCAD Model of Belt Truss .................................................................................. 14 Figure 3-6: Bank of China ............................................................................................................ 15 Figure 3-7: AutoCAD Model of Diagonal Brace ......................................................................... 15 Figure 3-8: Typical Outrigger Truss ............................................................................................. 16 Figure 3-9: AutoCAD Model of Outrigger Truss ......................................................................... 16 Figure 3-10: AutoCAD Model of Floor ........................................................................................ 17 Figure 3-11: AutoCAD Model of Connection .............................................................................. 17 Figure 4-1: Adhesives Tested ....................................................................................................... 19 Figure 4-2: Attaching Connections ............................................................................................... 20 Figure 5-1: Model at Rest with All Elements ............................................................................... 21 Figure 5-2: Deformation with Floor Elements Only..................................................................... 22 Figure 5-3: Deformation with Top Floor Members Only ............................................................. 22

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1 INTRODUCTION

Visual models are important for student learning. A model of a skyscraper already

existed to show students how a tall building will deform under lateral loads and with different

lateral load resisting systems. However, the current model, Figure 1-1, is not ideal for teaching.

Aesthetically, it does not look very good. Also, the structural elements are difficult to put into

place. Each piece has only one location where it fits, requiring the professor to remember where

each part fits.

Figure 1-1: Current Skyscraper Model in Use

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The main objectives of this project were to create an educational model where all parts of

the same element are interchangeable, each element has its own color coding for ease of

identification and instruction, and replacement parts can easily be obtained. By using AutoCAD

3D and 3D printing, all three of the objectives were achieved.

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2 BACKGROUND

Skyscraper Design

The desire to construct the tall buildings is not something new. It can be seen in ancient

Egypt, noted by the increasing height of each subsequent pyramid. This desire continues today

with the building of the Burj Khalifa and the future construction of the Kingdom Tower, which

will reach over a kilometer in height. As construction heights have increased, so too has the

method for reaching these heights. There have been four main methods of the design throughout

the centuries.

From the ancient Egyptians with the pyramids and the Romans with the aqueducts, to the

early European gothic cathedrals, masonry been employed for millennia in the design and

construction of tall structures. These designs relied heavily on the load bearing strength of the

masonry bricks or stone. As can be seen in the pyramids, in order to achieve greater heights,

large bases had to be used to support the taller structures.

The dominance of stone masonry waned in the late 1800s as steel became more and more

prominent in construction. Prior to the use of steel in construction, the tallest buildings were ten

to twelve stories (Leslie, 2010). As building height increased, so did the lateral loads caused by

winds as a result of the larger surface area. In order to resist these loads, more sophisticated

designs needed to be implemented. Whereas masonry relied on rules of thumb regarding wall

thickness and building proportion to resist later loads, steel frames could absorb and direct these

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loads. The Empire State Building, in Figure 2-1, was the tallest building at the time it was built.

The building used steel frames to achieve its towering height.

Figure 2-1: Empire State Building under Construction

In the 1960s, Fazlur R. Khan presented the idea of a framed tube design for high-rise

buildings (Ali, 2001). Prior to this, shear wall and braced structures relied on individual

elements to provide lateral stiffness. The framed tube incorporates the entire building plan for

resisting lateral loads (Paulino, 2010). Framed tube designs are accomplished by closely spaced

exterior columns and relatively stiff spandrels (Khan, 1965), Figure 2-2.

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Figure 2-2: Framed Tube with Closely Spaced Exterior Columns (Khan, 1965)

After the framed tube was first introduced by Khan, many tube design followed: tube-in-

tube, modular-tube, braced-tube, and bundled-tube systems (Gunel and Ilgin, 2007). As seen in

Figure 2-3, with each subsequent method the maximum height increased. However, these

increases were small until the introduction of the composite core-megacolumn systems.

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Figure 2-3: Building Heights per Design Method (Ali and Kyoung, 2007)

The current method for the design of the tallest structures is the composite core-

megacolumns system, also known as an outrigger system. This system consists of a concrete

core, outrigger trusses, exterior megacolumns, belt trusses, and occasionally diagonal braces.

Figure 2-4 shows a simplified model of this system.

Figure 2-4: Simplified Core and Megacolumns (Taranath, 2010)

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Improved lateral stiffness and decreased lateral drift are a result of “tying” in the exterior

megacolumns to the shear core via outrigger trusses (Paulino, 2010). When lateral loads are

applied to this system, all components act in unison to resist the force. As seen in Figure 2-5, the

exterior megacolumns and the core all resist the lateral load.

Figure 2-5: Core-supported Outrigger Structure (Ali and Kyoung, 2007)

With the use of core and megacolumn design, structures have been able to soar to much

greater heights. As seen in Figure 2-6, steel tube systems capped out at about 100 stories in

height; whereas, the outrigger structure can reach up to 150 stories, or even higher (Ali and

Moon, 2007). The tallest structure in the world, the Burj Khalifa, was designed using this system

and is 163 stories tall (Bowman, 2016). This system also reduces the number of perimeter

columns, increasing the architectural view to the outside.

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Figure 2-6: Building Heights per Design Method (Ali and Kyoung, 2007)

3D Printing

In order to make easily interchangeable parts for the skyscraper model, it was decided that

3D printing the different elements would be ideal since each printed piece would be nearly

identical in the dimensions. Another benefit of using the printing machines is that the piece

could easily be color coordinated by using different colored filament in the printing. Two

printers were used: the XYZprinting Da Vinci 1.0 and the XYZprinting Da Vinci 2.0 as see in

Figure 2-7.

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Figure 2-7: 3D Printers Used

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3 DESIGN OF MODEL

The first design of the model skyscraper was loosely based off of the Shanghai World

Financial Center, see Figure 3-1. Due to printing restrictions, the maximum distance between

the megacolumns could only be 6 inches. This is because the printer bed is 6 inches by 7.5

inches. Because of this restriction, it was decided that eight megacolumns instead of four would

be used. Figure 3-2 shows the first design with the eight megacolumns and supplemental belt

trusses, outrigger trusses, and diagonals. The eight columns allow for lateral resistance in both

the x- and y-direction.

Figure 3-1: Shanghai World Financial Center (Shi et. al, 2012)

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Figure 3-2: Preliminary AutoCAD Model Design

After consulting with Dr. Balling, the eight column design was considered to be too

complicated for instructional purposes. The model consisted of twelve diagonals, twelve belt

trusses, and twenty-four outrigger trusses. Having so many parts would not facilitate quick

instruction. In order to achieve this objective, a two megacolumn design was implemented as

seen in Figure 3-3. With only two megacolumns, the model is now only capable of showing the

response of the building under lateral loading from just one direction. However, since loading

from an orthogonal direction would be identical, the two megacolumn design was considered to

be adequate.

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Figure 3-3: Final AutoCAD Model

Foam Elements

Foam was used for the core and megacolumns due to its comparable performance to full-

scale concrete cores and megacolumns. In the previous model, the core dimensions were 2 in. by

2 in. by 3 ft. The new foam core dimensions were decided to be 2 in. by 3.5 in. by 3 ft. Longer

megacolumns were used in this iteration of the model, because the shorter megacolumns of the

previous model made it difficult to insert the trusses into the interior.

3D Printed Elements

The 3D printed elements were all printed with a 30% density and 2 mm layers. In the

previous model, the belt trusses were simple rectangular pieces. To better represent the

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aesthetics, the new belt trusses were models after those of the US Bank Center in Milwaukie,

Wisconsin. Figure 3-4 shows the exterior view of the belt trusses of the building.

Figure 3-4: US Bank Center

The belt trusses, along with the other printed elements, have an inch long extension on

each end, see Figure 3-5. This is soley for attaching the piece to the model.

Figure 3-5: AutoCAD Model of Belt Truss

The old diagonals braces consisted of one diagonal member each. The new diagonal brace

pieces were based off of the Bank of China in Hong Kong in Figure 3-6, and consist of two

members each resembling an “X,” as shown in Figure 3-7. Due to the limited printing bed size

for the 3D printer, the diagonal braces were only able to be 6 inches wide by 7 inches tall.

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Figure 3-6: Bank of China

Figure 3-7: AutoCAD Model of Diagonal Brace

3.2.1 Outriggers

The basis of the design outrigger trusses came from the model in Dr. Balling’s paper, as

seen in Figure 3-8. Having only two members was ideal for an outrigger truss, because the space

left between the megacolumns and the core is only 2 inches. Figure 3-9 shows the AutoCAD

rendering of the new outrigger truss.

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Figure 3-8: Typical Outrigger Truss (Balling and Lee, 2014)

Figure 3-9: AutoCAD Model of Outrigger Truss

Floors on every story provide axial stiffness, but not flexural stiffness. A few floors were

added with floor elements that were pin-connected to the core and megacolumns. Each floor

element consisted of three separate pieces: the two supports and the floor as seen in Figure 3-10.

The pieces will then be connected by a bolt so only axial stiffness is transmitted between the core

and megacolumns.

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Figure 3-10: AutoCAD Model of Floor

The belt trusses, diagonal braces, and outrigger trusses all needed to be connected easily to

the core and/or megacolumns. Figure 3-11 shows the AutoCAD rendering of the connection.

With this design, the different elements can easily slide in and out of these connections.

Figure 3-11: AutoCAD Model of Connection

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4 CONNECTIONS

Assembly

In order to attach all of the connections to the foam core and megacolumns, eight different

adhesives, see Figure 4-1, were tested to see which would create the strongest bond between the

foam and the plastic. Each adhesive was applied to a piece of 3D printed plastic and adhered to

the foam. After waiting the allotted curing time based on the instruction for each, the plastic

pieces were pulled on to test the strength of each adhesive.

Figure 4-1: Adhesives Tested

All of the plastic pieces pulled off of the foam easily due to the smooth edge of the foam

which they were glued to. Since the foam was so slick, the adhesives could not grip the foam

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sufficiently. To combat this, sand paper was rubbed on the foam to make a rougher surface.

After applying the adhesives to the plastic and placing them on the foam surface that had been

sanded, the epoxy had the greatest strength.

To ensure that all of the connections were attached at the correct location, all distances

were measured from the base and sides of the foam pieces. At all of the locations where a

connection would be placed, the foam was sanded and coated with a small amount of epoxy

before attaching the 3D printed pieces. Figure 4-2 shows the mostly completed foam

megacolumns and core.

Figure 4-2: Attaching Connections

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5 RESULTS

After the model was assembled, lateral loads were applied to ensure that the model would

behave as expected. Figure 5-1 shows the model with all of floor elements, belt trusses, diagonal

braces, and outrigger trusses before any loads were applied. With floor elements only, the model

deflects as shown in Figure 5-2 under lateral load. The deflected shape is that of a cantilever

beam.

Figure 5-1: Model at Rest with All Elements

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Figure 5-2: Deformation with Floor Elements Only

With outrigger trusses, belt trusses and diagonal braces only on the top floor, the model

deflects with double curvature as seen in Figure 5-3.

Figure 5-3: Deformation with Top Floor Members Only

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In order to measure the stiffness of the structural elements, a measured load was applied

to the top of the model, and the displacement was measured at the top as well as seen in Table

5-1. Without any lateral bracing, the model had very little stiffness. By inserting all of the

members, the model became much stiffer. As can be seen, the diagonals add the most stiffness

to the model whereas, the belt trusses add the least.

Table 5-1: Stiffness by Configuration

Configuration Force (lbs.) Displacement (in.) Stiffness (lbs./in.)

No Lateral Bracing 0.3 5 0.1

Belt Trusses 3 4 0.8

Outrigger Trusses 3 3 1/4 0.9

Diagonal Braces 3 1 3/8 2.2

Outriggers and Belts 3 2 5/8 1.1

Diagonals and Outriggers 3 1 1/4 2.4

Diagonals and Belts 3 1 1/4 2.4

All Members 3 1 3.0

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6 CONCLUSION

By using 3D printing technology, it was possible to create a model skyscraper with parts

that were interchangeable throughout the model, color coded, and replaceable in the event of

misplacement. These three aspects will make teaching about core and megacolumn skyscrapers

more effective in the classroom.

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REFERENCES

Ali, Mir M. “Evolution of Concrete Skyscrapers: from Ingalls to Jin mao.” Electronic Journal of

Structural Engineering Vol.1 No.1 (2001) 2-14.

Ali, Mir M., and Kyoung Sun Moon. "Structural Developments in Tall Buildings: Current

Trends and Future Prospects." Architectural Science Review 50.3 (2007): 205-23.

Balling, Richard J., and Jacob S. Lee. "Simplified Model for Analysis and Optimization of

Skyscrapers with Outrigger and Belt Trusses." Journal of Structural Engineering J.

Struct. Eng. 141.9 (2015): 04014231.

Bowman, Marc D. “Application of a Simplified Skyscraper Model to the Burj Khalifa.” (2016)

Gunel, M. Halis, and H. Emre Ilgin. "A Proposal for the Classification of Structural Systems of

Tall Buildings." Building and Environment 42.7 (2007): 2667-675.

Khan, Fazlur R. “Design of High-Rise Buildings.” A Symposium on Steel (1965)

Leslie, Thomas. "Built Like Bridges: Iron, Steel, and Rivets in the Nineteenth-century

Skyscraper." Journal of the Society of Architectural Historians 69.2 (2010): 234-61.

Paulino, Madison R. “Preliminary Design of Tall Buildings.” (2010)

Shi, Weixing, Jiazeng Shan, and Xilin Lu. "Modal Identification of Shanghai World Financial

Center Both from Free and Ambient Vibration Response." Engineering Structures 36

(2012): 14-26.

Taranath, Bungale S. Reinforced Concrete Design of Tall Buildings. Boca Raton: CRC, 2010.

235. Print