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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.