Download Phyiscal Model of Skyscrapers with Core, Megacolumns, Belts

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.
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
4
Outriggers ............................................................................................................... 15
Connections ........................................................................................................................... 19
Assembly ........................................................................................................................ 19
5
Results ................................................................................................................................... 21
6
Conclusion ............................................................................................................................. 25
References ..................................................................................................................................... 27
iii
LIST OF TABLES
Table 5-1: Stiffness by Configuration........................................................................................... 23
iv
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
v
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
1
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.
2
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
3
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.
4
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-intube, 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.
5
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 coremegacolumns 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)
6
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.
8
Figure 2-7: 3D Printers Used
9
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.
12
Figure 3-3: Final AutoCAD Model
Foam Elements
Foam was used for the core and megacolumns due to its comparable performance to fullscale 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.
15
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.
16
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
17
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
21
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
22
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
No Lateral Bracing
Belt Trusses
Outrigger Trusses
Diagonal Braces
Outriggers and Belts
Diagonals and Outriggers
Diagonals and Belts
All Members
Force (lbs.)
0.3
3
3
3
3
3
3
3
23
Displacement (in.)
5
4
3 1/4
1 3/8
2 5/8
1 1/4
1 1/4
1
Stiffness (lbs./in.)
0.1
0.8
0.9
2.2
1.1
2.4
2.4
3.0
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
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