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The Structural Failure of New York’s Twin Towers: Their Concept, Engineering, and Impact
Haydon Osborne
Global Scholars Program
Park Tudor School
Indianapolis, Indiana
Mentor: Ravi R. Talwar Date: March 24, 2010
http://kinless.files.wordpress.com/2009/
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Acknowledgements
I would like to begin this paper by acknowledging certain people who have helped me or
supported me since my junior year of high school on this project. Firstly, I would like to thank
my mentor, Mr. Ravi Talwar. From day one, Mr. Talwar always showed an interest in my topic
and guided me in narrowing it down. His detailed yet understandable explanations made the
most difficult concepts that I came across while researching easy to understand, and his caring
attitude made me feel as though I was his friend and not just a student that required his
assistance. Also, I would like to thank his wife, Mrs. Eleanor Talwar, for opening her home to
me on numerous last minute occasions so that I could meet with Mr. Talwar.
In addition to thanking Mr. Talwar, I would also like to thank my panel members. These
include Mr. David Kaszko, in charge of security at Park Tudor School, who helped me come up
with the topic of my project and allowed me to use his own books for reference; Dr. Dario
Untama, Physics teacher at Park Tudor School who has shown an interest in my topic and has
taught me many important physics concepts that will help me in my future studies; Mrs. Deborah
Everett, the Director of the Upper School at Park Tudor School and former Speech and Debate
coach whom I have had the privilege of knowing since 5th grade; Mr. Donald Weymuth, Middle
School math teacher at Park Tudor School who remembers me as his student and always likes to
keep in touch; and Erin Tuckman, a great friend of mine that I have known since freshman year
and have been able to get to know very well in these past few weeks from our musical rehearsals.
I would also like to thank my friends and family for supporting me in this project. They
each understood the magnitude and importance of it to me, and always seemed interested in my
topic. This made the project more exciting for me to research and write about. I also wish to
thank Dr. Jan Guffin, who coordinates the Global Scholars Program. Dr. Guffin is one of the
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kindest and wisest men that I know, always showing an interest in his teachings and the lives of
his students. The personal relationship that I have developed with Dr. Guffin over these past two
years in itself has made the Global Scholars project worth it.
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Abstract
The Twin Towers of the World Trade Center were two structurally revolutionary
skyscrapers which became icons for the city of Manhattan. Eventually, however, when they both
fell to the ground as a result of a structural failure due to impacts by two commercial jetliners on
the tragic morning of September 11th, 2001, they instantly became a lasting American symbol of
both pride and sorrow. The structural failures of the Twin Towers, in turn, led to a cascade of
important and controversial political and social changes in our country that still can be seen
today. The objectives of this paper are not to examine the legacy of the fall of the Twin Towers,
but rather, to examine and better understand their structures in order to formulate a conclusion as
to why they failed structurally on September 11th, 2001. In this paper, I hope to leave the reader
with a better understanding of the concept, engineering, and impact of the Twin Towers.
This paper is divided into six major sections. The first section begins with an
introduction to structure and architecture, which defines key terms related to the project and the
duties of both an architect and a structural engineer. The next section discusses the history of the
World Trade Center. This includes why they were built and what steps were taken to ensure that
they would be successful.
The next two sections are about how the towers were designed and how they were built to
be structurally stable. These sections deal with important engineering aspects of the towers, and
they introduce concepts that will be important for later discussion on why the towers failed
structurally.
The final two sections are over the actual construction of the Twin Towers and the
theories of why they fell on September 11th, 2001. The section on the construction of the towers
will introduce more key elements that pertain to the structural failure of the towers. The final
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section about the theories of why the Twin Towers fell introduces many ideas and facts that will
be important in understanding my own conclusion to this paper.
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Outline
Research Question: What was the structural failure inside New York’s Twin Towers on September 11th, 2001?
I. Introduction to Structure and Architecture
A. Development of construction of high rise buildings
1. Reinforced concrete
2. Fabricated structural steel
B. Duties of architects and structural engineers
II. History of the World Trade Center
A. David Rockefeller
B. Port of New York Authority
1. Austin Tobin
2. Port Authority Trans-Hudson Railway
C. Opposition to World Trade Center
III. Designing the World Trade Center
A. Choosing an architect
B. Minoru Yamasaki
C. Exterior design
D. John Skilling
1. Tube Structure
2. Exterior column panels
2. Bar-joint trusses
IV. Structural Analysis
A. Building safety
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B. Effectiveness against wind
1. Viscoelastic dampers
2. Vierendeel and Hat trusses
V. Building the Twin Towers
A. Demolition
B. Digging the “Bathtub”
1. Battery Park City
2. Slurry wall system
3. PATH
C. Steel
1. Obtaining fabricated structural steel
2. Preconstruction
3. Grillages
D. Elevator system
1. Herb Tessler
2. Subway system design
E. Floor-by-floor assembly
1. Core columns
2. Staggered exterior panels
3. Floor trusses
4. Finishing off
F. Building “up”
1. Derricks
2. “Kangaroo Cranes”
3. Hat trusses
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G. Completion of towers
1. Daily life
2. Catastrophe on September 11, 2001
VI. Theories of Collapse
A. Melted steel theory
B. Weakened steel theory
C. Sagging floor theory
D. Hat truss theory
VII. Findings
E. Conclusions based on research
F. Questions which remain
G. Implications
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Introduction to Structure and Architecture
Before the actual structures of the Twin Towers are examined, it is extremely important
to understand certain key aspects of building any type of skyscraper. Specifically, these aspects
include the different types of loads that affect reinforced concrete construction, usage of steel in
a high rise building, and the duties of both engineers and architects.
Reinforced concrete is a key component to any building, and when combined with the
resilient strength of steel, it is able to be distributed into a number of different shapes and forms
with the purpose of distributing the weight, otherwise known as the load, of the building. Loads
are divided into three categories: dead loads, live loads, and dynamic loads. A dead load is a
weight that is, in a sense, permanent to the building. The dead load is the heaviest load of the
structure for it includes the weight of all the stationary columns, trusses, beams, floors, etc. that
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are present in a building.1 These structures must all support their own weight in addition to the
weight of the different sections of a building.2
Live loads, on the other hand, are the exact opposite of dead loads, meaning they are
anything in a building that is not permanent, like furniture or people. Live loads can be
constantly changing, and in order to combat the fluctuating weight of a live load, building codes
all over the world require a structure to be built according to the heaviest allowed weight of a
live load to prevent any sort of structural accident. These restrictions are noticeable to the
common eye when one sees a sign in a building that says “Maximum Occupancy Allowed” with
a number next to it. Thus, live loads vary based on a buildings purpose and individual structure.3
Both dead loads and live loads change weight very slowly, if at all, during a building’s
lifetime. Hence, they are categorized into what are called static loads. Rapidly changing loads
are known as dynamic loads, and are loads created as a result of things like wind or, in the case
of the twin towers, a sudden explosion due to an impact over an extremely short period of time.
Essentially, dynamic loads are more difficult to combat in engineering a building due to their
arbitrariness and unpredictability.4
These different types of loads are distributed throughout a building by reinforced
concrete and/or fabricated structural steel. In the case of the Twin Towers, reinforced concrete
was not a heavily used technique in the main construction of the buildings; however, this is
briefly how it works. Reinforced concrete is created when water and a cement paste harden in
between grains of sand and stone, acting as a sort of glue. The extraordinary aspect about
reinforced concrete is that it has the ability to be lightweight in order to reduce a heavy dead load
or it could be very heavy depending on the necessities of the engineer. To vary the weight and
strength of reinforced concrete is very simple, for it only depends on the ratio of water to cement
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and cement to the aggregates (the sand and stones). If the aggregates are larger and harder, then
the concrete will be stronger, and if there is a greater amount of water in the mixture, it will be
weaker. To distribute loads, “bars of steel are embedded in the concrete in those areas where
pulls will develop under loads.”5 This is so the steel accepts the tension, or the lengthening due
to pulling, like an outstretched rubber band. On the other hand, when a material is pushed
together, it is in compression, meaning it is shortened due to a force.6 The concrete is what
accepts the compression, for the steel is never completely as lengthy as the concrete it sits in.7
Equally important to constructing a high rise structure is the development and usage of
fabricated structural steel. Steel is an alloy made up of carbon and iron, and is very inexpensive
and strong. Structural steel is primarily used in a building for the columns and beams, which are
crucial to creating a stable and safe building. Steel begins to become malleable, or soft, at the
high temperature of about 400 degrees Celcius, which will be examined later when describing
the impact of the airplanes on the towers. Steel is also “fatigued” when stress is applied to it,
meaning it transitions from tension to compression multiple times due to factors like wind in a
high rise building or shifting of weight. The typical structural steel present in the body of the
Twin Towers had the ability to hold up to 36,000 pounds of stress per square inch; however, the
main cores of the buildings were comprised of high-strength steel, which could yield 50,000
pounds of stress per square inch.8
Both architects and structural engineers play key roles in the development of a high rise
building. Their duties vary, and it is of the utmost importance to understand their differences in
order to better understand the topics discussed later in this paper. Architects, according to
Jobbankusa.com, are “licensed professionals trained in the art and science of building design.”
In other words, an architect is in charge of designing the building not only in terms of aesthetics,
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but also in designing the building to be “functional, safe, and economical” and also to “suit the
needs of the people who use them.” An architect is involved in all aspects of a building project
and must be well trained in skills such as “designing, engineering, managing, supervising, and
communicating with clients and builders.” Architects also are in charge of creating blueprints
which include not only the structure of the building, but also things like electrical systems, air-
conditioning and heating systems, plumbing, etc.9
According to Whole Building Design Guide, the structural engineer is “the individual
responsible for ensuring that buildings will remain standing while carrying out their intended
functions.” The structural engineer looks at factors like the type of construction carried out
based on building zones and codes and available material as well as building specifics like
column locations that will effectively carry out the job of the building while minimizing costs.
Also, they are in charge of things like floor-to-floor heights, where they must leave “adequate
space” for not only the structure, but also the piping, lights, etc. inside the building. The
structural engineer must be able to determine how to combat the strength of the building along
with all the outside forces that will be a burden on the building, and the structural engineer, along
with the architect, is present throughout the entire building and design process of a structure.10
History of the World Trade Center
The World Trade Center never would have been created if not for the industrialist David
Rockefeller, a leading figure for Chase Bank from 1946 until 1980 and a grandson of Standard
Oil tycoon John D. Rockefeller. Throughout Rockefeller’s career with Chase, he partook in
much international travel in order to promote the bank so that it “could use its global deployment
to benefit both its shareholders and society at large.”11 For this, many believed he was acting as
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an imperial aristocrat; however, Rockefeller combated these accusations by saying he was only
trying to fix the overall standard of living at the time.12
David Rockefeller released his first official report of a “center for world commerce” in
the fall of 1958. He proposed developing this in lower Manhattan with the hopes that it would
reinvent the area, which at the time was depleted. The official proposal was for what was
initially called a “World Trade and Finance Center” and was meant to be a place
“where the United States and foreign business and financial interests can meet to do business; where representatives of the United States and foreign governments are available for consultation and aid; and where facilities are available to expedite business transactions. ”13
The belief was that a center like this in bustling New York would “accelerate the development of
international business and act as a symbol of this country’s growing world leadership in the
international business community.”14 These small ideas, however, were only the beginning of
what would soon become known as a World Trade Center.
As Rockefeller’s plans for the project grew, he decided to commission a study to
determine whether or not a World Trade and Finance Center would be successful or a failure.
For this, he hired a consulting firm, McKinsey & Co., in 1959 to analyze the outcomes of the
center’s development. After examination, McKinsey & Co. determined that the World Trade
Center would be a “serious financial bust.” According to the firm, the major risk of developing a
center of this magnitude would be the fact that neither the mission nor the targeted clients of the
project were guaranteed at the time. They determined that “major corporations had already
started exploiting international trade and would gain little real advantage from a World Trade
Center.”15 They also believed that it would fail just due to its location in lower Manhattan, for at
that time, all business in New York City was in midtown Manhattan. They believed the location
would be an inconvenience for the businesses and make them not want to join in partnership with
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the World Trade Center. A stubborn Rockefeller refused to accept these findings, and fired
McKinsey & Co. just before the executive meeting with the Downtown-Lower Manhattan
Association about initiating the project. He hired a new firm, Skidmore, Owings & Merrill, and
within two weeks they had developed a small, rough project design. On January 25th, 1960,
Rockefeller held a public discussion with reporters in which he proposed his $250 million dollar,
13.5 acre plan to the world. In his proposal, the main building was to be a seventy-story hotel
with “a six story international trade mart, an exhibition hall, and a securities exchange building,”
which Rockefeller hoped would someday house the New York Stock Exchange. David
Rockefeller used this meeting to gain publicity for his project, and its success can be summed up
with the New York Times headline the next day: “A World Center Of Trade Mapped Off Wall
Street!”16
Rockefeller’s strategy of gaining media attention for the World Trade Center fulfilled its
sole purpose: to grab the attention and support of the Port of New York Authority, commonly
called the Port Authority. Since 1921, the job of the Port Authority was to “develop, coordinate,
and oversee all transportation-related activities in the twenty-five mile radius of the Statue of
Liberty.”17 This meant that anything related to transportation, whether it be subways, railroads,
ferries, bridges, etc., were under the control of the Port Authority as long as it was in the proper
area. As the years progressed, however, the Port Authority began to focus more on gaining
profit for themselves without public approval rather than build civic means of transportation like
tunnels and bridges. As a result of this, during the 1960s they were very unpopular with the
public; more specifically because they failed to revitalize a railway tunnel under the Hudson
River connecting New Jersey to Manhattan. For this, business was being diverted from New
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York City to suburbs of New Jersey, causing many to lose jobs along with harm to the economy
of New York.18
The Port Authority had, in a sense, complete freedom in funding any project they deemed
profitable partly because they were not government operated or had any elected officials.19
Austin J. Tobin, the executive director of the Port Authority, did indeed take notice of the news
headlines about Rockefeller’s ideas for a center for world commerce. He became extremely
intent on initiating the project, for it would, without a doubt, financially benefit the economy
and, most importantly to Tobin, New York’s Hudson River port.20 Tobin had joined the Port
Authority in 1927, and now, in the 1960s, he understood that he would need to find some legal
way of making the Trade Center a project for the Port of New York Authority. In a sense, he
had to “sell the World Trade Center as a port without water.”21
Tobin ordered a young Port Authority employee named Richard Sullivan to conduct a
report on “the feasibility of the World Trade Center,” similar to how Rockefeller ordered
Skidmore, Owings & Merrill. Sullivan’s report, released on March 10, 1961, stated that the
advantages of developing a World Trade Center were numerous, for it would
1. “Simplify and expedite the processing of administrative and procedural matters involved in arranging for the movement of export and import cargo through the Port, resulting in savings in time and money as well as improved service.
2. Centralize and improve the trade information services now located in scattered areas of the Port.
3. Provide a marketplace for the United States products available for export through the Port, which would attract foreign buyers from around the world.
4. Provide an international marketplace for import products for United States buyers.
5. Establish a central location for agencies of the United States and foreign governments concerned with the Port’s commerce, thereby making it possible for them to serve the world trade community more effectively.”
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Concerning the actual building layout of the Trade Center, the Port Authority generally accepted
Rockefeller’s original modest plan. The conclusions of whether or not the project would be
successful went against those stated by the firm of McKinsey & Co., and the estimated cost of
developing a World Trade Center was at $355 million.22
The Port Authority needed to make sure, based on its charter, that any project they
decided to fund would be beneficial to and supported by both New York and New Jersey.
Luckily for David Rockefeller, New York’s governor was Nelson Rockefeller (his brother), who
agreed to support the plan of a World Trade Center. The problem arose with New Jersey, for
their governor had already come up with a project for the Port Authority. This was to revitalize
the exhausted, no longer in use trans-Hudson railway. Now, the Port Authority wanted to switch
from that project to one that seemed to benefit solely Manhattan. Because New Jersey governor
Robert Meyner wanted the Port Authority to focus spending their money on this railway rather
than a non-advantageous project for his state, Tobin needed to develop a plan in which both New
York and New Jersey would benefit from
a World Trade Center or else the whole
project would be called off. Tobin’s plan
was such that they would combine the
ideas for the World Trade Center with the
ideas for the trans-Hudson railroad
(Hudson & Manhattan Railroad) by setting
the location of the World Trade Center on
the west side of Manhattan facing New Jersey rather than the east side of Manhattan (Figure 1).
Thus, the railway, now known as Port Authority Trans-Hudson Railway, or PATH, would gain
Figure 1: http://www.nyc-architecture.com
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revenue for New Jersey by transporting businessmen across the Hudson from New Jersey to the
World Trade Center. The only change necessary was that of location.23
By 1960, the Port Authority was attaining annual revenue of around $79 million, and this
number was continuing to increase. As a result of this, Tobin decided to invest his surplus into
creating a larger, more enormous World Trade Center than was ever imagined by anybody.
Although his idea seemed flawless to anybody on the inside, in the city it was met with a great
deal of skepticism.24
Robert Wagner Jr., the mayor of New York City, opposed the World Trade Center
because the Port Authority operated “tax free.” This meant that the World Trade Center would
not have to pay into New York City’s operating costs even though it would greatly impact the
city. Wagner Jr. did, however, finally agree to the project, even though still opposed, after
forcing the Port Authority to “reimburse the city for the projected losses in tax revenue.”25
Another sadder story of opposition to the World Trade Center took place at Radio Row, the new
area where the World Trade Center was to be built. The lower east side of Manhattan was
nicknamed Radio Row due to the endless rows of family owned shops selling electronics.
Although the region was indeed visually despondent, it can be stated without hesitation that
Radio Row represented the hard working lower class of America. Although the inhabitants of
Radio Row did all they could to stop to Port Authority in terms of lawsuits and protests, their
efforts would not stop the bulldozers from crushing their stores.26
Designing the Towers
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In 1962, Austin Tobin, head of the Port Authority, endorsed Guy Tozzoli, a successful
member of the Port Authority, as general manager of the World Trade Center Project.27 This
meant that Tozzoli was in charge of essentially building the entire World Trade Center complex.
Tozzoli’s first project was to hire an architect, and he knew of no more qualified man than
Minoru Yamasaki. Featured on the cover of Time
Magazine on January 18th, 1963, architect Minoru
Yamasaki had begun making a name for himself
with his architecture, filled with Gothic, Italian,
Indian, and Japanese influences (Figure 2).28
When the members of Yamasaki’s young, eight-
year-old architectural firm located in Michigan
received a letter from the New York Port
Authority asking if “Yamasaki was interested in
serving as chief architect for what the letter said
was a $280,000,000 project called the World
Trade Center,”29 they did not know what to think. At this point in his career, Yamasaki had
created a number of notable works like the McGregor Memorial Community Conference Center
at Wayne State University, the Federal Science Pavilion for Seattle’s World Fair, and the
Michigan Consolidated Gas Tower; however, he felt this project was too great for him. After
persuasion from his team members, Yamasaki did indeed decide to accept the job after teaming
up with a New York architectural firm, Emery Roth & Sons. Guy Tozzoli was delighted, for the
project was about to get underway.30
Figure 2: http://www.time.com/time/ magazine /0,9263
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Yamasaki began what would be the largest project of his lifetime by walking through the
streets of Radio Row (where the Trade Center would soon stand) and the streets of Manhattan to
find inspiration. He realized that, somehow, he would need to combat the busy and overcrowded
streets surrounding the large skyscrapers in the city by creating a space large enough for people
to rest and take refuge from their busy lifestyles. In other words, he realized that it would be of
utmost importance to create a plaza for the World Trade Center. Yamasaki then began to gather
a team of engineers and architects, and started to design the World Trade Center.31
Yamasaki’s first problem on his agenda was to determine how many buildings would be
created and what they would look like.
After creating a scale model of lower
Manhattan in his office, Yamasaki and his
team checked model after model of possible
building designs to see which would work
the best with the Manhattan skyline (Figure
3). Yamasaki understood that the Port
Authority wanted to create “the most
dramatic project in the world” which would become “a symbol of New York,” so Tozzoli and he
knew the project could not just be a few small scattered buildings. The determined architect was
realized that he would be expected to create the tallest buildings in the world- buildings which
would stand out in lower Manhattan and be revolutionary in their structure. “If a building is too
strong or brutal, it tends to overpower man. In it he feels insecure and uncomfortable. A
monument to the ego of a particular owner or architect is contradictory to the principle that each
man who uses the building should be able, through his environment, to have the sense of dignity
Figure 3 : http://www.hokubei.com/files/images/mino
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and individual strength to carry on his hopes and aspirations.”32 This was the mentality that
Yamasaki carried going into this project of a lifetime, and he sums up the importance of the
project with his statement, “It’s going to be the grandest project ever. The grandest project
ever.”33
Yamasaki came up with the idea of twin towers from his idol, Mies van der Rohe, who
had created two like buildings along Lake Michigan. Yamasaki developed two separate designs
for his “twin towers:” two 80 story towers and two 110 story towers. Only the 110 story towers
would break the world record. Considering the latter of the two, Yamasaki was worried that they
would look too much like a “fat lady” that is “too tall.” The other problem with the 110 story
towers was that they would require an elevator shaft so huge that it would take up most of the
floor space used for the businesses. Tozzoli was fond of the twin tower design, and refused
Yamasaki’s objections. “Yama, I
have something to tell you. President
Kennedy is going to put a man on the
moon. You’re going to figure out a
way to build me the tallest buildings
in the world.”34
The basic size of the towers
was decided, and now Yamasaki
needed to find a design for them that
would be aesthetically appealing,
economical, and productive. Yamasaki, ironically, had a terrible fear of heights. To contest this,
he decided the towers would have very narrow windows. Yamasaki’s first designs for the towers
Figure 4: City In The Sky
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were very intricate and flamboyant in terms of their aesthetics, as seen in Figure 4. As their
designs progressed, however, more and more of the decoration was stripped away in order to
create the simple and effective design that was used for the final project.35 Now, a basic design
for the towers was finished; however, a final design would never actually be written down on
paper. This was due to the sheer enormity of the project. The Port Authority now needed to find
a lead structural engineer to design the structural aspect of these gargantuan towers. Yamasaki
and the Port Authority hired John Skilling, a brilliant and imaginative man who would “almost
never say no” when given a task and “knew how to melt their creative hearts and design a great
building at the same time.”36 When looking at Yamasaki’s design, Skilling had in mind “a high-
rise structure that did not have a matrix of vertical columns piercing each floor every fifteen or
twenty feet in order to
provide support, as
traditional skyscrapers did.
Instead, a tight series of steel
columns would run vertically
along each of the twin
towers’ facades, like a picket
fence around an empty lot.”37
These would be 22 inches
apart with 61 on each facade,
the windows placed in between each column. Yamasaki’s closely placed steel columns would be
present not just to make thin windows, but also to bear the vertical load of the weight of 40% of
the structure itself. This had never been seen before in a building, and would take away from the
Figure 5: http://3.bp.blogspot.com/_a1mlmmnPZvE/Rh
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necessity of columns inside the buildings for as far as 60 feet from the edges.38 The other 60% of
the vertical loads of the building would be supported by “a cluster of columns in the building’s
core, where the elevator and stair shafts would be.”39 These would be the large steel core
columns. The steel trusses that would hold up the floors would accept the horizontal loads
created by things like wind. This innovated design to distribute the weight became known as a
tube structure (Figure 5).40
The outer steel columns would be added to the floors of the buildings in what were called
“prefabricated column panels, which were 3 stories tall staggered panels of the steel columns to
provide strength to bear the huge loads of the building.”41 These were connected to the vertical
columns by “spandrels,” which were horizontal steel plates. These outer columns would not
only support the loads of each building, but also provide the aesthetics for the outsides of the
Figure 6: http://www.thewebfairy.com/killtown/images/wtc-gallery/nist1d/1-4_perimeter-column.jpg
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buildings with Yamasaki’s simple, Gothic cathedral influenced design. In a sense, they were
killing two birds with one stone by employing the tube structure method. These vertical exterior
columns also supplied the rigging upon which each floor truss would be attached, and were
sprayed with a lightweight fire proofing material then covered with an aluminum coating. The
windows would then be placed in between each vertical column (Figure 6).42
In order to support the floors of the towers, Skilling put to use the idea of bar-joint
trusses, which were “web-like networks of thin steel bars and angle irons” on which “corrugated
decking would be placed atop... so that concrete could be poured on it to create the floors.”43
This would be reinforced concrete, with the corrugated steel acting as the reinforcer by
providing it with stability. The corrugated steel, which is slanted, allows more steel surface area
to be touching the concrete, providing a greater strength for reinforcing. These trusses were
made of thin and lightweight steel, which was a new and economical design that was just
becoming popular in the 1960s. The trusses would serve as a support system to divide the
weights and even the loads between the core columns and exterior columns (Figure 7). They
spanned the length of either 18 meters or 11 meters in connecting the core columns to the
Figure 7: http://willyloman.files.wordpress.com/2009/08/wtc_floor_truss_system1.jpg
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perimeter, and would separate each floor by 12 feet.44
Structual Analysis
In deciding the safety
of the buildings in the event of
structural damages like fires or
airplane crashes, an analysis
was created by the firm of
Worthington, Skilling, Helle &
Jackson. Concerning an
airplane crash, the firm
determined that “the buildings have been investigated and found to be safe in an assumed
collision with a large jet airliner (Boeing 707- DC 8) traveling at 600 miles per hour. Analysis
indicates that such collision would result in only local damage to the building and would not
endanger the lives and safety of the occupants not in the immediate area of impact.”45 They
Figure 8: A Nation Challenged: A Visual History Of 9/11 And Its Aftermath
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decided this by comparing the impact of the plane, which was the largest at the time the towers
were being created, to the force produced by the largest windstorm that the buildings would be
able to withstand. They also took into account the type of puncturing the impact would cause on
the buildings and how that would affect their structures. The engineers realized that an impact
would create what is called a Vierendeel Truss, an arch over the hole that would change the
spread of the load to the columns closer toward the edges of the buildings. These Vierendeel
Trusses created would be so effective that “the tower would still be strong enough to withstand
100-mile-per-hour winds.” In Figure 8, the Vierendeel truss created on September 11th can be
seen. The engineers failed to take into account, however, the probability of a plane larger than a
Boeing 707 hitting the buildings with thousands more gallons of high-octane jet fuel. Skilling
was aware that the greatest problem of an airplane impact would be the spilling of ignited jet fuel
throughout the building, and he told the Port Authority of this. The Port Authority failed to take
his cautions of a fire into account for fear that it would risk the completion of the project. The
Twin Towers were also going to be created with multiple thin steel elements with a large surface
area, present in the trusses and interior girders. This large surface area of steel meant that they
would need to use a strong fireproofing material; however, the Port Authority decided to spray
the steel with a lightweight, foamy ineffective fireproofing product that would dry in place rather
than a heavier heavy-masonry guaranteed fireproofing product. 46 This was called SFRM, which
stands for sprayed fire resistant material. It would only be sprayed .5 inches thick on the trusses
throughout the buildings, yet would be upgraded to a thicker 1.5 inch thick coating in 2001
before the attacks. Only Tower 1 was completely upgraded to the thicker coating in time for
September 11th. The Port Authority chose the economical way of solving the fire-proofing
problem, which was a disastrous mistake in the long run.47
Osborne, 26
The next difficulty with the creation of the Twin Towers was determining their
effectiveness against the wind. In 1965, Paul Edkildsen, an engineer and practicing optometrist,
was put in charge of carrying out a series of “free eye exams” in Oregon. As people came into a
room for Edkildsen to give them their eye exams, as he would analyze their vision, the room, to
the subjects unknowing, would sway from side to side. The room itself was placed on a series of
hydraulics which would shake the room with a greater and greater intensity as the eye exam went
on. The room was soundproof so that the subject receiving the eye exam would be unaware of
what was going on, and when they would begin to feel sick, Edkildsen would stop the exam and
tell them they were being tested for the effects of wind sway in a high rise building. This study
continued to show negative results, for the people inside would feel sick or disoriented after a
certain amount of time, which is what the people working on the top floors of the towers would
feel. The Port Authority refused to accept these results for fear that they would compromise the
entire project, and so they decided to hire a man in charge of designing a system that would fix
the problem.48
Osborne, 27
Jack Cermak, a distinguished fluid mechanics engineer, was hired by the Port Authority
in charge of everything to do with the Twin Towers’ resistances to wind. Cermak realized that
the towers needed to increase their so called “creakiness,” which is the sideways sway of the
buildings based on its large fluctuations in tension and compression. In order to do this, he
realized a sort of braking effect would have to be created to absorb some of the sway of the
buildings. Thus, they developed viscoelastic dampers, which were “sets of three flat metal
pieces, about two feet long, held
together like a double-decker
sandwich with epoxy and a tough,
rubbery glue called polyacrylic
(Figure 9).” “The two outside
plates would connect to an
exterior column, and the middle
plate would be fixed to the
underside of a steel truss.”49
These plates would slide against one another as the building swayed in the wind, and the
polyacrylic glue would absorb some of the shock. About 100 of these were put on each floor, or
about 10,000 per tower. They also decided to alter the number of columns on the outside in
order to help the swaying of the buildings. This, however, was a simple task that only changed
the number of columns from 61 per side to 59. These viscoelastic dampers were put in place to
help lower the dynamic loads of the buildings. They also decided to place hat trusses at the tops
of the buildings that would attach the core of the buildings to their exterior, and the hat trusses
would also decrease the sway of the building. These were large diagonal pieces of steel that
Viscoelastic Damper
Figure 9: http://www.designcommunity.com/discussion/images/wtc_damper_nn_fig2.jpg
Osborne, 28
would span the height of three floors, and would also help the buildings support the weight of
large antennas. The concept of a hat truss will be examined later in this paper. The problem of
wind had been solved by an innovative group of engineers (Figure 10).50
Building The Twin Towers
After five long years of intense debate and discussion, it was time to break ground in
lower Manhattan to make room for the World Trade Center. The task would be extraordinarily
difficult, for to build any sort of building among numerous others in a densely packed region is
Figure 10: City In The Sky
Osborne, 29
daunting in itself. Also, the land upon which the Twin Towers would soon stand was horrible
soil, full of abandoned cables and pipes for a project of their magnitude. Against any odds, the
team of the Port Authority, Yamasaki, and Skilling were determined to meet any challenge head
on in order to make sure the project would succeed.51
The first job was demolishing the buildings inside the 16 acre area between Vesey Street
(the north), Church Street (the east), Liberty Street (the south), and West Street (the west). On
March 21, 1966, the first building was torn down. This continued all throughout Radio Row,
although during this time, legal challenges were still being pursued by its inhabitants against the
building of the World Trade Center. However, they eventually all gave into the commands of
the Port Authority and the area was cleared.52
When the construction for the World Trade Center began, it called for more than
200,000 pieces of steel fabricated all across the country 3,000 miles of electrical wiring 425,000 cubic yards of concrete for the flooring 2.2 million square feet of aluminum cladding for the tower facades 7,000 plumbing fixtures 170 miles of connecting pipe 40,000 doors 43,600 windows 6 acres of marble for the plaza and lobbies “$1,547,800 tandem of automated window washers designed to crawl up and
down the face of the towers.”53
Osborne, 30
The foundation, or basement, of
the building, now being dug out, became
known as “the biggest ditch in
Manhattan.”54 A problem discovered
while digging was how they would get
the excavated dirt out of bustling
Manhattan. Normally, it would be carted
away by an army of dump trucks to a
landfill in New Jersey; however, this
project was so large and expensive that it
would require about 100,000 truckloads
to move it all out of the city. Austin
Tobin and the Port Authority
compromised with the mayor of New
York into moving the soil to the
southwest tip of Manhattan and dumping it into the water around old, unused loading docks.
This would create a new 6 block manmade “landfill” that would be able to be sold as real estate
and create roughly $90 million of tax revenue for New York City. This landfill area would be
called Battery Park City. The World Trade Centers were changing not only the skyline of
Manhattan, but also the shoreline (Figure 11).55
The soil where the Twin Towers would stand was actually created by an accumulation of
“garbage, wrecked boats and piers, construction fill” and “other discarded materials that had
been dumped at the lower end of Manhattan for centuries, building up the total size of the
Figure 11: City In The Sky
Osborne, 31
peninsula.”56 The land was only 3 feet above sea level, but the bedrock was 65 feet below the
ground. The sheer size of the project made it necessary for what became known as “the bathtub”
to travel all the way down to the bedrock, which would provide a natural support for the weight
of the towers. Engineers were aware, however, that as they would begin to dig, the hole would
cave in when they reached water level. They also knew that it was necessary for the walls of the
gargantuan eleven acre hole 65 feet in the ground to be a barrier for the watery Hudson River soil
as well as provide a strong foundation for both of the entire buildings. In order to resolve this,
they created a system called the “Slurry System.”57 This system involved pumping a mixture of
water and a natural clay called bentonite into the watery hole while it would be dug out by a
large clam digger. The porous bentonite would absorb the water and then swell up into a thick
yet liquidy mixture that would be strong enough to keep the hole from caving in or collapsing.
Osborne, 32
This system would be completed in sections three feet wide by twenty two feet long, and when
the bedrock was reached, “a twenty-five-ton, seven-story-high steel cage was lowered into the
slurry.” Then, concrete would be poured into the cage at the bottom of the hole, forcing up the
slurry into a sucking vacuum that would allow it to be used for the next section of excavation.
This created the concrete wall necessary for the support and containment against the Hudson
River, and the wall was stabilized by tiebacks that were drilled into the soil connecting the
concrete to the bedrock. This revolutionary concept allowed the work site for the World Trade
Center to be forever dry (Figure 12).58
In the original agreement between Austin Tobin and the governor of New Jersey, the
World Trade Center would only be chartered if they agreed to revitalize the Trans-Hudson
Railway, now called the PATH (Port Authority Trans-Hudson). Two of these railways traveled
Figure 12: http://www2.massgeneral.org/pubaffairs/Graphics2008/071108slurry.jpg
Osborne, 33
through the soil where the Twin Towers would stand, and it was the engineers’ job to determine
a way to not disrupt the subways while digging out the bathtub. Thus, they needed to figure out
how to “suspend the tubes in midair” while creating their giant basement (Figure 13).
In June, 1968, the north railway saw its first light of day since it was buried in 1909. The
engineers were aware that if the subway tube shifted even slightly during the process, the trains
(which continued to run all throughout the construction) could be in danger of derailing. To
support the tubes as they dug, the “crews had sunk heavy vertical elements called caissons every
forty-two feet on either side of each tube, then ran stout steel supports along the caissons so that
the cast-iron cylinders looked as if they were enclosed in cattle chutes.”59 Small cuts were made
into the tubes in order to allow for contraction and expansion of the newly exposed metal to the
sun. The workers began to dig, wedging pieces of wood between crossing steel straps every ten
Osborne, 34
or so feet. As they dug, they began to find a number of old artifacts like cannonballs, a time
capsule from 1884, clay pipes, old fishing boats, etc. In 1968, they finally reached bedrock.60
A problem arose with determining where to get the huge amount of steel needed for the
project for the lowest price. Originally, Tobin expected either U.S. Steel or Bethlehem Steel, the
two largest steel companies at the time, to provide the materials necessary for a reasonable price.
However, both gave prices 50% higher than expected for creating the some 200,000 steel pieces:
$118.1 million for Bethlehem and $122.2 million for U.S. Steel. Tobin came up with an
ingenious plan of hiring a number of small, cheaper independent companies that would provide
steel for certain sections of the tower. For example, Dreier Steel of Long Island City, New York
provided the steel for the grillages, Des Moines Steel provided the “tridentlike forks...that would
sit on the base columns and run to where the regular pinstripes began on the ninth floor,”61
Pacific Car & Foundry Co. in Seattle was creating the 5,828 steel panels with the 3 steel columns
connected by spandrels, Laclede Steel in St. Louis was creating the thirty-two thousand floor
trusses, and Granite City Steel was fabricating the corrugated steel deck and ductwork.62 The
grand total for using fifteen small steel companies rather than one industrial giant was only $85.4
million, a little less than $20 million cheaper. It was difficult for the Port Authority to deal with
all of these separate companies, so in order to make the process more efficient, Tobin hired a
“middle man contractor” who was in charge of overseeing the installation and delivery of the
steel by each of the companies. This was the Karl Koch Erecting Company.63
The team in charge of building the World Trade Center next needed to figure out a way
to bring the enormous amount of steel into bustling Manhattan. Due to space constraints, it
would be necessary for the steel to already be pre-constructed before it would reach the
construction site, so that it would be ready to be installed and not take up the already limited
Osborne, 35
space where the workers were laboring. The steel was pre-fabricated and assembled in a one
hundred acre railroad yard in Greenville, New Jersey (just across the Hudson River) and then
brought over by trucks, boats, and even helicopter. However, the method of transporting steel by
helicopter was quickly abandoned when a helicopter carrying a heavy piece of steel lost its
control due to the large weight and had to drop the piece of steel into the Hudson River, where it
still remains today. The method of preconstruction was very efficient, yet difficult due to the
necessity of a piece of steel to arrive to the site on time. If it was not there on time, the
construction would be delayed and costs would increase. This led to the next revolutionary
invention in the construction of the Twin Towers.64
In order to make the construction of the gigantic towers efficient and less confusing for
the workers, each piece of steel, sometimes weighing 50 tons, would be marked with a series of
numbers that any worker would understand.65 For example, one would read PONYA A-251-92-
95. PONYA stood for Port Of New York Authority, and the A meant it would be located in
Tower A, the north tower. The buildings faces were numbered 1-4, and the 2 in front of 251
meant it would be located on the east side. The 51 in 251 was the 51st column placed out of the
59 that were on each side of the building, and the steel panel would span the length of 3 floors
from 92-95. This saved time and effort and made the building process run very smoothly for a
project of such magnitude.66
Osborne, 36
The first steel grillage created by Dreier Steel was placed into the bathtub in the early
morning of August 6th, 1968. Once determined level, a grout was placed between the concrete
slab and the steel grillage, and these would set the foundation to bear the weight of 116 floors,
counting the basement, of the soon to be tallest buildings in the world. In other words, these help
support the dead loads.67 Grillages were “tremendous steel feet that would support the big
columns at the twin towers’ bases.”68 In other words, a grillage is a series of crossed steel beams
to create a base pad with the ability
to distribute the weight of huge
loads over as wide of an area as
possible. Each 34 ton, fifteen feet
by eleven feet grillage was
mounted directly onto a concrete
base connected to the bedrock, and
28 grillages were necessary for
each tower.69 This enormous system can be more clearly visualized in Figure 14.
Figure 14: http://www.howstuffworks.com/wtc1.htm
Osborne, 37
As previously stated, the Twin Towers would require no interior columns throughout the
floors of the buildings except for the unseen core columns surrounding the elevator shafts, which
will soon be mentioned. This was due to its tube structure, which uses the sides of the buildings
to divide up a portion of the weight with the other portion held up by the center core columns.
The buildings’ cores would be large, but include the elevator shafts to help create a larger area on
each floor for office space. Each tower had four core columns made up of a number of small
steel pillars to divide the loads. This allowed just enough space for the innovative elevator
system without compromising the towers’ structures. Herb Tessler, an architect working with
Yamasaki on the elevator designs, came up with his elevator plan based on what he observed in
New York’s subway system. Tessler realized that New Yorkers usually had to change subways
multiple times before arriving at their destination. He figured that elevators could use the same
principle, where not all elevators have to go from the bottom floor directly to the top floor.
Tessler came up with the idea of creating “three sets of elevators in addition to the nine freight
elevators in the building plans.”70 The first set would be express elevators that went to or near
the top of the towers, where the viewing platform would be located on the South Tower and the
Windows on the World restaurant on the North Tower. Tessler also designed a new way of
minimizing the discomfort that would be expected in these speedy accelerating elevators, which
would be going sixteen hundred feet per minute rather than the standard eight hundred feet per
minute.
Figure 15: http://static.howstuffworks.com/gif/wtcelevators.gif
Osborne, 38
The second set of elevators would divide the building into three different “zones.” There
would be a lobby on the ground floor as well as two others- one at the forty-fourth floor and one
at the seventy-eighth floor. One elevator from the ground floor would go directly to the lobby on
the 44th floor, whereas one from the 44th floor would go to the 78th floor. The third set of
elevators were called “local elevators,” which would take people from a lobby to any floor above
it within the restrictions of the lobby above it. This innovative elevator system consisting of 198
elevators is used in numerous skyscrapers today, and the Twin Towers’ system can be seen in
Figure 15.
The advantage of this elevator was that “one set of shafts could be used for all the local
elevators.” They could all be present at the center of the building surrounded by the core
columns, and their system would be like taking the subway,
for as people went up, depending on their floor, they may
have to take a number of different elevators. The other
revolutionary design created by Tessler for the Twin
Towers was the elevator “entry and exit procedure for the
passengers.”71 Each elevator had the capacity to hold fifty-
five people, who would enter through one side of the
elevator and exit through the other. Although this is seen in
many buildings today, it had never been done before up
until this point, and maximized efficiency of boarding and
exiting the elevators for the building workers. Tessler’s
system allowed the building to maintain its tube structure,
and altogether, seventy-two local elevators and twenty-
Osborne, 39
three express elevators would be created. This meant 75% of the floorspace could be rented out
by offices, a number considerably larger than the average skyscraper percentage of 62%.72
As the towers began to rise, they became a sort of vertical assembly line. Each floor
began with the addition of steel columns to the core columns. As the core columns were built
(usually two to three stories at a time), the outer steel columns were placed to match it, for they
were both in charge of carrying the weight of the floors that would soon be placed in between
them. As previously stated, these outer steel panels, each weighing over twenty tons, were 3
floors tall and were staggered, connected by welded horizontal spandrels to provide stability.
After the support systems were created, the floor was installed. The floor was installed with the
support of the trusses below it, which were the thin triangular thirty-two inch deep frames that
provided the stability upon which the concrete floors were poured. These were supported by the
lightweight steel transverse trusses, and connected the outer perimeter to the core columns.
The thirty-two inch spaces under the trusses included conduits, which would contain the
necessary cables for office floors to function smoothly. At the bottom of these trusses, ceiling
tiles were added, for the floor would also be the ceiling of the floor below it. As each floor was
being completed, electricians, plumbers, etc. would be present to install the necessary equipment
for the towers to function properly as office buildings (Figure 16).73
Osborne, 40
As each floor was completed, the final step of each was installing the exterior wall cover.
The exterior steel beams of the building were first coated with fireproofing which would protect
the sturdiness of the building in maintaining the loads in case of an exterior fire. For the first 35
floors of the North Tower, they sprayed it with asbestos, a hazardous, unsafe material. This was
determined lethal during the time of the building, so they removed all the asbestos and recoated
the outer columns with a safer substance then covered the beams with aluminum. The aluminum
reflected the outside sun, which gave the towers the gleaming, shiny look to the outside world.
Ten inches in from the outside of these columns were placed 43,600 windows, which were extra-
heavy duty to protect from the wind
(Figure 17).74
Moving steel material up 110
stories is no easy feat, and the way
this was carried out in building the
Twin Towers was yet again another
Figure 16: http://911review.org/Wget/guardian/wtc/fig-2-9.jpg
Osborne, 41
revolutionary concept. Usually, when
creating a building, a crane has the
ability to be present on the ground and
lift steel up to high levels. When
building a skyscraper, this is not
possible due to the sheer size of the
building. The typical procedure when
building a skyscraper involved a type
of crane machine known as a derrick.
A derrick, which is an extremely heavy
crane, would be situated on the building
being constructed, adding to the difficulty of construction, for they would have to be moved to
the higher floors as they were being built. The raising of a derrick was very time consuming,
difficult, and inefficient, for it took at least a day and a half to move a derrick up one floor.75
For the World Trade Center, the alternative to using derricks came in the form of the
“Kangaroo Crane (Figure 18).” Developed in Australia, the Kangaroo Crane looked like a
normal crane; however, it was anything but. In building the Twin Towers, these cranes were
“perched atop each tower, one crane at each corner of the elevator core. It was stabilized by a
long base wedged deep into the core that functioned like a sword in a sheath.” The cranes
reached over the edges of the buildings and picked up steel from the ground, a maximum
capacity of 50 tons. This was possible because, at the end of the boom of the crane, there was
located a large counterweight that would even out the heavy weight of the steel. The cranes were
called “Kangaroo Cranes” due to their ability to, in a sense, jump up floors. Each crane had a
Figure 18: http://heiwaco.tripod.com/nist0.htm
Osborne, 42
hydraulic system that would lift itself up twelve feet at a time after completing the necessary
three floors. This took two hours, a huge improvement over the two days it would take a
derrick.76
Once the top floors were reached, a set of steel braces were put into place from the 107th
floor to the 110th floor in each tower. These were the hat trusses, and they were diagonally
placed through the floors by large steel beams. As previously stated, the hat trusses could be
used to support a large antenna on top of the buildings (one was eventually put on the North
Tower). They also allowed for a greater load distribution by providing another connection
between the perimeter columns and the core columns. The hat truss structure can be seen in
Figure 19.
The buildings were steadily getting taller and taller, and as soon as the north tower
completed its first sky lobby, the building up until that point was furnished and prepared to move
in businesses. The elevators and escalators were installed, the lobbies were finished, and the
flooring, painting, and tiling were completed. The final “topping out” of the north tower
occurred just before Christmas in 1970, when the last framework of steel was placed on the 110th
Figure 19: http://911research.wtc7.net/wtc/arch/doc
Osborne, 43
floor of the building. The second tower was completed on July 19th, 1971, 7 months after the
first tower. The only way of getting the Kangaroo Cranes down from the top of the building was
by disassembling them and carrying them down piece by piece. After about four years of
innovative labor, the 1,368 foot towers were finally complete. The process was surprisingly safe,
and Rockefeller’s and Tobin’s towers changed Manhattan forever.77
What went on in the Twin Towers after their completion was what many would consider
to be normal for Manhattan workers. They would arrive to the buildings, either by the newly
renovated PATH or by taxi, walk through the gorgeous marble lobby, grab a bite to eat, and take
Tessler’s numerous elevators up to their floor and begin their typical workday. For lunch, they
would eat out in Yamasaki’s outdoor plaza (named Austin J. Tobin Plaza), or would go down
into the lower levels of the World Trade Center, which was equipped with a fully functional
mall. Life in the towers was normal; the World Trade Center was a success, and any controversy
surrounding its creation was quickly forgotten.78
On the clear, sunny Tuesday morning of September 11th, 2001, four fully-loaded
commercial airline jets were hijacked by terrorist members of Al Qaeda. Two large Boeing 767s
were propelled into each of the towers, the North Tower first and the South Tower second.
American Airlines Flight 11 crashed at 8:46 into the center of the north tower between the 94th
and the 99th floors, where the financial and insurance firm Marsh and McLennan was located. It
carried 181 passengers and roughly 10,000 gallons of jet fuel in the moment it crashed into the
northern side of the tower at approximately 440 miles per hour. All floors from the area of
impact up to the 110th floor were trapped. The impact led to these estimated effects in the
structure of the tower:
35 exterior columns severed, 2 heavily damaged. 6 core columns severed, 3 heavily damaged.
Osborne, 44
43 of 47 core columns stripped of fireproofing material. 60,000 square feet total of stripped fire insulation.
The North Tower stood for 102 minutes after the impact until a structural failure occurred and it
plummeted to the ground.79
Less than twenty minutes later after the North Tower was hit, United Airlines Flight 175
crashed into the South Tower from the 77th to the 85th floor, also trapping everyone above. The
plane crashed going 540 miles per hour, 100 miles per hour faster than the plane which crashed
into the north tower. The airplane hit just right of the center of the building on its south side
while flying at a downward angle, allowing it to damage more floors than the airplane that struck
the north tower. This is the estimated damage in the south tower:
33 exterior columns severed, 1 heavily damaged. 10 core columns severed, 1 heavily damaged. 39 of 47 core columns stripped of fireproofing material. 80,000 square feet total of stripped fire insulation.
The South Tower stood for 56 minutes after the impact until a structural failure occurred and it collapsed. 80
Within two hours of the first impact, the buildings that had taken over four years to
construct were gone, along with the lives of approximately three thousand people.81 The world
watched in horror as the south tower fell first (even though it was second hit) and the north tower
second. These two incredible buildings were gone forever; however, their legacy would be
carried on in the hearts of Americans and sympathizers all over the world. The Twin Towers
were two buildings that changed the course of world history on September 11th, 2001.
Theories of Collapse
There are many theories as to what exactly happened to the structure of both towers once
they were struck by the commercial airplanes. There are many “conspiracy theories” which
Osborne, 45
suggest that the American government was aware of or involved in the collapse of the Twin
Towers; however, these are so far-fetched that they will not be analyzed. There are a number of
credible ideas as to what caused the structural failure inside the Twin Towers, and these will
briefly be mentioned.
There is no doubt that the fires played a critical role in the structural failures of the
towers, for when the airplanes hit, they easily damaged the sprinkler systems in the buildings to
the point that they were useless. One theory, however, states that when the planes hit the
buildings, the large amount of jet fuel from each fuselage caused an ongoing fire that caused the
steel inside the towers to melt. The steel was able to melt because the spray fire resistant
material, or SFRM, would have been stripped of all trusses and core columns. This in turn
would make the steel fully exposed to the burning jet fuel. In this theory, this exposure would
cause the steel to “melt; however, this theory can be disproven easily when looking at the
properties of jet fuel and steel together.82 Jet fuel burns from 1,100 to 1,200 degrees Celsius,
which is significantly less than the 1,510 degrees Celsius required to melt steel.83 This leads to
the next, and perhaps most credible, weakened steel theory.
The weakened steel theory suggests that the steel became malleable as a result of the jet
fuel igniting and burning inside the floors of the Twin Towers. 84 Steel begins to lose strength at
a temperature of 400 degrees Celsius, and loses approximately 50% of its strength around 600
degrees Celsius.85 The steel in the Twin Towers became malleable, but did not melt, after
becoming subject to the intense heat of the fires. This made it difficult for the trusses and
columns to distribute the loads from the sections of the buildings above the impacted floors
evenly. Eventually, the trusses and columns began to buckle until finally collapsing.86 This
Figure 20: http://www.forrestmarketing.com/worldtradecenters/south-tower-implodes.jpg
Osborne, 46
theory can be divided into two sub-theories: the “sagging floor theory” and the “core collapse
theory.”
The sagging floor theory suggests that the floors, which were connecting the outer
columns to the core columns, began to, in a sense, sag due to the softening of the steel from the
fires. These trusses then became so weak that they would become unlatched from the core
columns and the visco-elastic dampers which held them in place to the exterior columns. The
floor would then fall on top of the next one, creating a “pancake effect.” The core collapse
theory states the opposite, which is that the core columns became so soft that they begin to
collapse downward. This would cause the whole entire floor to go with it, for the core columns
connect to the floor trusses which connect to the exterior columns. This too would generate the
“pancake effect.”87
There is also a theory known as the “hat-
truss theory,” which states that the loads of the
building were transferred around the areas of
impact, like the Veirendeel truss, due to the
placements of the hat trusses on top of each
tower. As the steel in the hat truss would soften,
it would begin to buckle inwards, bringing along
with it the exterior spandrel columns. The top-
heavy weight of the hat truss would cause it to
dip slightly sideways as it pummeled to the
ground, most noticeably in the south tower
(Figure 20).88
Osborne, 47
Findings
Conclusions Based On Research
From David Rockefeller to the terrorist attacks on September 11th, 2001, the concept of
the World Trade Center was economical, innovative, and recreated the Manhattan skyline.
Minoru Yamasaki’s Twin Tower design set a new standard for high rise buildings, and the Port
Authority was able to take his design and make it profitable for both New York and New Jersey.
They were built based on simplicity and resourcefulness, and their structure is ultimately what
led to their collapse on September 11th, 2001. The reason the Twin Towers fell based on their
structure can be determined when analyzing their business and engineering aspects.
When the World Trade Center idea was being built, the Port Authority tended to overlook
numerous problems with its design plan. In a way, Austin Tobin was so focused on finishing the
towers so that he could profit financially from the end product that he failed to take the
appropriate building safety measures in numerous open circumstances. He tended to take every
cheap and economical opportunity that was presented to him, whether it was in his choice of
steel, fireproofing, truss system, etc. In a way, these seemed like small and unimportant details
at the time, however, they made all the difference on September 11th. Although there were
indeed critical errors in building the Twin Towers, Yamasaki and his team of engineers deserve
loads of credit for being able to construct such an amazing, innovative, and spectacular style of
skyscraper that the world had never seen before.
In the engineering sense of the buildings, both the North Tower and South Tower
collapsed in different fashions. In the North Tower, American Airlines Flight 11 first crashed
through the steel exterior columns with ease. These staggered steel spandrels would have
Osborne, 48
instantly shredded the airplane, spewing its 10,000 gallons of jet fuel throughout the impacted
floors. The remains of the airplane continued straight through the center of the tower until they
reached the core columns. As the airplane traveled through the building, the fire-proof insulation
was stripped from the trusses by its debris. Once the remaining bits of the airplane reached the
core columns, a large number of center core columns became heavily damaged due to the
straight trajectory of the plane. The jet fuel at this point would have ignited because of the
presence of oxygen and combustibles from the building. This fire continued for a long time,
being fueled by couches, papers, leftover jet fuel, etc. After the initial impact and fireball, the
fire steadily grew hotter and hotter. This made the steel from the trusses of the floor above it
become softer due to the absence of the SFRM. The trusses were also made of lightweight, thin
steel with large surface area, which would have allowed a quicker absorption of the heat from the
fires. The trusses then began to dip downwards, which produced more strain on the core
columns and the exterior columns that they attached to, similar to the “sagging-floor theory.”
The softened steel trusses were weighed down by the concrete placed above the corrugated steel
section of the truss, which in turn produced stress on the viscoelastic dampers that connected the
floor trusses to the spandrels. As this was occurring, the exterior columns surrounding the
gaping impact hole in the building would have been distributing the load of the floors above it in
the manner of a Veirendeel truss without problem; however, the wind and exposure to the
outside air would allow a substantial amount of oxygen to be present that would fuel the fire.
Then, the South Tower collapsed next to the North Tower. As the South Tower fell, a rush of air
was sent toward the north tower, also contributing to a more dangerous fire. This heat continued
to soften the weak trusses to the point that the perimeter columns and the core columns began to
Osborne, 49
buckle inwards very slowly. Eventually, they reached the point where one floor became so weak
that it fell, leading to the collapse of the whole tower.
When the South Tower was hit, the impact was very different compared to the impact in
the North Tower. A comparison of these impacts can be seen in Figure 21. When the airplane
hit the South Tower, it easily punctured the exterior steel columns, causing its jet fuel and debris
to scatter throughout the area of impact. All of the SFRM was scraped away, and the jet fuel
instantly ignited due to the presence of oxygen and combustibles. This trajectory of the airplane
upon impact was not perfectly straight, but rather more towards the east side of the tower.
Although this caused damage to
fewer core columns than were
damaged in the North Tower, the
south corner core column was
severely damaged. This produced a
great stress on the remaining corner
core columns as well as the hat truss
above it in trying to distribute the
weight of the loads evenly. Also,
the airplane was at a slight downward angle when it hit the tower. This caused more floors to be
damaged than were damaged in the North Tower. After impact, the fire softened the steel
trusses, which were weighed down by the concrete placed on them. This, however, was not as
much of a problem for the building’s structure as it was in the North Tower. Instead, the greater
number of damaged exterior columns and damaged corner core column would have made it
difficult for the hat truss above it to distribute its load evenly to the perimeter columns. Also, the
Figure 21: www.doujibar.ganriki.net/english/e-0-map&data.html
Osborne, 50
greater surface area of damage from the impact compared to the North Tower would have
allowed more steel to become subject to the intense fires, causing more trusses to sag and a
greater tension on the exterior and core columns in a shorter time frame. The hat truss was
unable to evenly distribute its load, causing it to topple slightly as the trusses and core columns
began to buckle and fall toward the ground.
The North Tower was hit from the 94th to the 99th floor, very close to the top of the
building in relation to that of the South Tower, which was hit from the 78th to the 85th floor. This
meant that the South Tower had a heavier load to handle above the area of impact than the North
Tower, which led to more load stresses for both the exterior and core columns. This, along with
the trajectory of the impacts of the planes in each tower, contributed to the difference in why
each tower reacted differently to the terrorist attack. Both towers stood for a period of time after
they were hit by the airplanes, meaning it was not the actual impacts of the airplanes that caused
their structural failures. The buildings stood due to the presence of the Veirendeel Truss;
however, the fires that lingered on after the impact are what ultimately weakened the steel trusses
and caused the structural failures in the Twin Towers.
Question Which Remain
The topic of the structural failures of the Twin Towers presents a number of questions,
many of which can never be answered. No eyewitnesses inside the Twin Towers at or above the
sites of the airplane impacts were able to survive the collapse of the buildings, thus any theory as
to why the buildings collapsed is just that- a theory. As I began to research the Twin Towers for
writing this paper, I came up with a number of questions, which I tried to answer in writing this
paper. Some, however, are more abstract and do not pertain directly to the structures of the
buildings.
Osborne, 51
It amazes me how two buildings had the ability to change the course of history forever.
Ultimately, it was the structural failure of these buildings that led to a cascade of political and
social changes throughout the world. This causes me to wonder exactly how those people who
were influential in the creation of the World Trade Center reacted to the terrorist attacks on
September 11th. Did they feel guilt for creating such amazing structures that benefitted
Manhattan and New Jersey (and many would argue the world itself) economically and socially?
This would be a very interesting question to answer; however, I left it out of my project due to its
non-relation to the structure of the towers.
Another question I came up with while researching was how other countries reacted to
the attack on American soil. When a disaster occurs in countries other than the United States, I
will sympathize with them, but only think about it during the short period of time that it is a “hot
topic” in the news. Perhaps, the fact that the attack on the Twin Towers occurred during my
lifetime and in my own country led to my great interest in it. Do other countries still think about
September 11th? Did they push it out of their minds after a certain length of time as we
Americans tend to do with their tragedies?
I would also be interested in knowing why so many people today feel that the terrorist
attacks on September 11th, 2001 were a government conspiracy. This sort of thinking divides us
as Americans, and produces false understandings of a situation that should not be second-
guessed. To me, it is similar to those who deny the horrors of the Holocaust.
One of the ultimate questions concerning the Twin Towers is this: Why did the hijackers
commit such a terrible act? Obviously, due to hatred for America and for religious reasons;
however, did it really accomplish anything good for their countries and religion? This leads to
my question about the legacy of September 11th, which asks how and in what way has the fall of
Osborne, 52
the Twin Towers truly affected us today? These questions are very abstract and could be
answered in many different biased ways, and I feel these would be an excellent subject of
another project.
Concerning the engineering aspect of my paper, I would like to know exactly how the
Twin Towers affected the sorts of building designs of skyscrapers after they were finished in
1973. Did the revolutionary tube structure catch on and become popular in building
skyscrapers? Specifically what buildings did the Twin Towers influence? I find it amazing that
even today, skyscrapers are able to be built higher and higher; however, I have noticed that these
skyscrapers all tend to become thinner and come to a point near their tops. This is because they
cannot handle the powerful winds at such a high altitude. What fascinates me is how the Twin
Towers were able to maintain their same box-like structure all the way up to the 110th floor and
still be able to withstand the wind. Did any engineers take notice of this and put to use the
towers’ innovative concepts?
I also wonder what would have happened if the fires were able to be extinguished before
the buildings collapsed. Would the United States still have gone to war? How would the Twin
Towers be fixed structurally? I also cannot help but wonder what memorial will be placed at
Ground Zero in remembrance of those whose lives were lost on September 11th. It has been
roughly nine years since the towers’ collapses, and I wonder why we have not yet been able to
create a proper monument in their memory.
Implications/Recommendations
Gestalt psychology states that “the whole is greater than the sum of its parts.” This can
be demonstrated perfectly when looking at how every little fine detail in building the Twin
Osborne, 53
Towers, whether it be the elevator system, columns, trusses, basement, etc., came together into
the final product of two fascinating buildings. Every little structural part played a big role in the
creation of the Twin Towers, and ultimately contributed to their failures after being attacked by
terrorists on September 11th. The whole concept of the World Trade Center came together only
as a result of numerous small parts working together to create a social and economic harmony;
however, this harmony was shattered on the morning of September 11th.
The terrorists that attacked the Twin Towers did so believing that they were reaching
self-actualization, the highest level in Maslow’s hierarchy of human needs. Socially, the
structural failures of the Twin Towers caused America to rightfully band together in search of
those responsible. Today, we still are in present in the Middle East in search of terrorism,
causing a political division between Americans because of the effects our occupation has on
economics, human lives, and morality. Although we may not realize it, we feel the effects from
the structural failure of the Twin Towers in every aspect of life, whether it be in the recession,
airport security, or daily gas prices. History changed forever on September 11th when the towers
experienced a structural failure, and the world will always feel the effects of September 11th,
whether it be consciously or sub-consciously.
Now, we live in a Post-Modern time period. According to Huston Smith, this period of
humanity is guided by science with nothing connecting to anything else. Smith explains that we
are lost as human beings, and it is our job to fix our disorientations as time goes on. In our
actions after the fall of the Twin Towers, we were trying to reorient ourselves as Americans, and
still continue to do so presently. Today, are trying to create order from this chaos by fighting
wars in the Middle East, hoping that, by doing so, we can prevent this disorder from occurring
again in the future by dissolving terrorism. Our actions have stirred up much internal debate in
Osborne, 54
our country, however, which disorients us even further. For this, we live in the Post-Modern
period, and will continue to do so until we find the answer to the truths of humanity. Although
this disorientation in humanity was present before September 11th, 2001, the falls of the Twin
Towers only caused a greater confusion among humankind that Smith, and I, hope will one day
be fixed.
To expand upon this project, I would recommend that one looks into the lasting effects of
the structural failure of the Twin Towers. In other words, how has their collapse affected life
today? This could include major topics of politics, economics, military and civilian lives lost,
etc. One could also mention the impact the Twin Towers have had on skyscraper architecture
around the world along with the business aspect of it. There would be many scholarly options in
pursuing this topic further, each of which I would find extremely interesting.
Endnotes
1 Salvadori, Mario. Why Buildings Stand Up: The Strength of Architecture. New York: W.W. Norton & Company, Inc., 1990. Print, 66.
2 Ibid, 43-44.
3 Ibid, 44-45.
4 Ibid, 45-47.
5 Ibid, 66-68.
6 Ibid, 60.
7 Ibid, 68.
8 Ibid, 64-66.
9 "Job Descriptions, Definitions, Roles, Responsibility: Architects, Except Landscape and Naval." JobBank USA. N.p., n.d. Web. 28 Jan. 2010. <http://www.jobbankusa.com/career_employment/architects_except_landscape_naval/job_descriptions_definitions_roles_responsibility.html>.
10 Schmidt, John. "Structural Engineering." Whole Building Design Guide. National Council of Structural Engineers Associations , 2 June 2009. Web. 28 Jan. 2010. <http://www.wbdg.org/design/dd_structeng.php>.
11 Glanz, James, and Eric Lipton. City in the Sky: The Rise and Fall of the World Trade Center. New York: Times Books, 2003. Print, 32.
12 Glanz, 32.
13 Ibid, 32.
14 Ibid, 33.
15 Ibid, 33.
16 Ibid, 34.
17 Corona, Laurel. The World Trade Center. San Diego: Lucent Books, 2002. Print. Building History Ser, 29.
18 Ibid, 29.
19 Ibid, 30.
20 Glanz, 40.
21 Ibid, 39-40.
22 Ibid, 53-55.
23 Ibid, 55-60.
24 Ibid, 61.
25 Corona, 30.
26 Ibid, 31-32.
27 Ibid, 34.
28 Glanz, 94-96.
29 Ibid, 100.
30 Ibid, 101-102.
31 Ibid, 102-104.
32 Ibid, 105.
33 Ibid, 106-107.
34 Ibid, 107-108.
35 Ibid, 110-112.
36 Ibid, 118.
37 Ibid, 119.
38 Ibid, 119-120.
39 Ibid, 120.
40 Ibid, 120.
41 Ibid, 122.
42 Ibid, 122-124.
43 Ibid, 124.
44 Ibid, 124.
45 Ibid, 131.
46 Ibid, 131-138.
47 Kirk, Jeremy. The World Trade Center Disaster. Massachusetts Institute Of Technology, June 2005. Web. 5 Mar. 2010. <http://dspace.mit.edu/ bitstream/handle/1721.1/31114/61145960.pdf?sequence=1>.
48 Glanz, 139-142.
49 Glanz, 153.
50 Glanz, 154-156.
51 Corona, 38.
52 Corona, 38-39.
53 Glanz, 176.
54 Glanz, 176-177.
55 Corona, 40.
56 Ibid, 41.
57 Ibid, 42.
58 Ibid, 43.
59 Glanz, 182-183.
60 Ibid, 183-185.
61 Ibid, 186.
62 Ibid, 186-187.
63 Corona, 60.
64 Corona, 61-62.
65 Corona, 63.
66 Glanz, 190.
67 Glanz, 192.
68 Glanz, 186.
69 Corona, 46.
70 Ibid, 55.
71 Ibid, 56.
72 Ibid, 56-58.
73 Ibid, 70.
74 Ibid, 70-71.
75 Ibid, 66-68.
76 Ibid, 68-69.
77 Ibid, 72.
78Ibid, 74-78.
79 Dunbar, David. Debunking 9/11 Myths. Ed. Brad Reagan. New York: Hearst Books, 2006. Print, 117-120.
80 Dunbar, 120-124.
81 Dunbar, 124.
82 Kirk, 15.
83 Dunbar, 38.
84 Kirk, 15.
85 Dunbar, 38.
86 Kirk,16
87 Glanz, 328.
88 Kirk, 16.