Thomas J. Mackin, Ph.D.

Why Exactly Did the WTC Collapse?

Presentation by Thomas J. Mackin, Ph.D.

Introduction by John C. Snider

In the aftermath of the terrorist destruction of the World Trade Center, many people have been asking how buildings weighing hundreds of thousands of tons could have been totally obliterated by the impact of a single airplane. While it's true that the Twin Towers were intended to withstand earthquakes, hurricanes - and yes, even aircraft impacts - it is also true that no building can be designed to survive every possible occurrence. Thomas J. Mackin, Associate Professor in the Department of Mechanical Engineering at the University of Illinois, has created an 8-slide presentation to explain exactly how and why the WTC collapsed.

Dr. Mackin created this analysis at the request of his school's administration, as part of an overall effort to help students and faculty discuss and understand the tragic events of September 11. One of his students emailed the presentation to some friends, and pretty soon it was carried all over the world. When we received it, we contacted Dr. Mackin and obtained his permission to post it on scifidimensions.

The slideshow we have posted is actually a corrected version from the one that has spread out across the globe. Dr. Mackin freely admits: "In my haste and in my depression I made a typo which turned into a numerical error. It does not affect the conclusion, but it does have an effect on the numbers." He also expressed amazement at how far and wide his work has traveled. "I had no idea that my simple presentation was going to make its way around the world!... Regardless, this WEB is an interesting story as well...how a simple lecture for my class ended up being e-mailed by a student, then shipped round the world, and generated a huge amount of incredibly stimulating interaction."

We thank Tom Mackin for his fortitude in tackling such a grim task. When asked what engineers could do to prevent something like this from happening again, his reply is simple. "Nothing."

Click here to view Prof. Mackin's presentation (requires Adobe Acrobat).

Another paper by Mackin.

LU Xinzheng & JIANG Jianjing

Simulation for the Collapse of WTC after Aeroplane Impact

Proc. Int. Conf. on Protection of Structures Against Hazard, Lok TS eds. Singapore Nov. 2002. 57~60

LU Xinzheng & JIANG Jianjing

Department of Civil Engineering, Tsinghua University, Beijing, 100084

Download PDF version

Abstract: Mechanical simulation and parameter discussion for the collapse of WTC (World Trade Center) after aeroplane impact are presented in this paper with the dynamic FEA software of LS-DYNA. The simulation results are very close to the real situation, which means that such type of special damage process can be recurred on the computer with proper parameter and numerical model. The results show that the direct reason for the collapse is the softening of steel under fire and the chain reaction damage of floors under the impact load of upper floors. If improve the fire resistance and the ductility of the structure, the collapse may can be avoided.

Keywords: dynamic FEA; world trade center; collapse

1. Introduction

The two towers of world trade center in New York were impact by two aeroplanes on Sep. 11 of 2001. The two towers were collapsed completely and more than 3 thousand people died in this affair. This affair obtained everyone¡¯s focus and many explanations are put forward for the reason of this collapse, including fire damage, heaped load, second-damage and so on. Because the collapse of the towers is very complex and it is difficult to recur by test, most explanations are based on qualitative analysis. However, the computer technology applied us a chance to simulate the collapse in the computer, so that the reason for the collapse can be discussed, as well as the methods can be raised to avoid this disaster.

2. Numerical model

This simulation is based on the dynamic FEA software of LS-DYNA, which is developed by the Software Company of Livemore.

The WTC is topical tube-in-tube structure system, whose outer tube is dense-columns-deep-beams system. The width of the column is 476.25mm while the space between columns is just 558.8mm. The thickness of beams is 1219.2mm. In order to simplify the numerical model and decrease the degree of freedom, the outer dense-columns-deep-beams tube system and inner steel truss tube system are approached with shell elements. The thickness of shell elements is established with equivalent section method. Such approximate tube has the same stiffness of global bending and axial deformation as the real structure, while the local bending stiffness of structure elements is not consisted with the real condition. Since the primary deformation of tube-in-tube structure system is global bending and axial compression deformation, this approximate method should be feasible.

The material of inner and outer tube is set to be steel in the numerical model, with the constitutive relationship of Material 3, Plastic Kinematic model in LS-DYNA. The density of steel is 7800Kg/m³. The Yang¡¯s modulus is 200GPa. The Poisson ratio is 0.27. The yield strength is set to be 310MPa, according to Steel A440 in U.S. The hardening modulus is set to be 2GPa, 1% of initial modulus. As the fracture of steel is considered in this analysis, the failure plastic deformation is discussed as parameters, whose value is set as 0.5%, 1% and 5% respectively

The material of floor is set to be RC material, with the constitutive relationship of Material 3, Plastic Kinematic model, too. The density is 2500Kg/m³. The Yang¡¯s modulus is 30GPa. The Poisson ration is 0.2. The yield strength is 30MPa. The hardening modulus is 0. All the fracture plastic deformation is set to be the maximum compression strain of concrete, whose value is 0.38%.

Because of the weak fire resistance capacity of steel, the high temperature caused by the burning of aeroplane oil will soften the steel. So the material of steel under fire is needed to be setup, whose other parameter is the same to normal steel except that the Yang¡¯s modulus and strength is set as 1/20 of normal ones, to approach the performance of steel at 700¡æ.

As the collapse analysis is very complex, the Single Face Erosion contact module of LS-DYNA is used in this case. This contact model can search the contact face automatically to establish the contact relationship, and those complex border conditions such as penetration, erosion can be considered, too. The friction factor of material is set to be 0.25.

Since the LS-DYNA is explicit dynamic software, when applying the gravity load, the structure will vibrate for a short time. It is not consisted with the real conditions. So the whole computation should be divided into two stages. In the first stage, the towers have not been impacted. The gravity load is applied to the model and relatively large damp ratio is applied to the towers, whose value is 10%. Computing the model until the vertical vibration disappeared. Then, the second computation stage starts. In the second stage, some elements in the tower are ¡°killed¡± to approach the hole of airplane impact, and the material property of some survival elements is modified to simulate the fire influence, too.

3. Numerical results

3.1 Collapse of North tower

Here the impact position was on the center of tower, so the collapse is vertical collapse basically. The simulation results are show in figure 1.1, 1.2.

3.2 Collapse of South tower

Here the impact position was near to the corner of tower, so the collapse is inclined. The simulation results are show in figure 21, 2.2.

Fig. 1.1 Collapse of north tower----perspective

Fig. 1.2 Collapse of north tower----vertical

4 Conclusions

The following conclusions are obtained from the simulation.

1) The reasons for the entire collapse of the towers are the structure elements¡¯ soften of fire and impact of the upper layers¡¯ collapse. From the numerical results, the towers does not collapse immediately after the impact. The north tower can go on standing. Likely, the south one dose not collapse, too, though there are some large deformations in it, which are caused by the asymmetric damage. This is consisted with the real situations.

2) Improving the structure fire resistance ability or control the fire influence area will avoid or delay the structure collapse, efficiently. We simulate the fire influence by adjusting the material property of elements. From the numerical results, even though the structure has been damaged seriously by the impact, if the influence area is smaller than 20%~25% of the survival section in the tower, the collapse still can be avoided. When more than 30~50% of the survival section near the impact zone fails, the collapse will start.

3) When the towers go into the collapse stage, the reason for the chain failure of un-impact layers is the impulse of upper collapsing floors. The impact force of upper floors is much larger than the heap load. And because there are a lot of bump and eject on the contact surface of collapsing floors and lower floors, the fragment of structure falls consecutively so that there is no chance to form a lot of heap load. So the heap load is not the critical reason for the collapse.

4) Improving the ductility of structure elements is an efficient way to avoid the chain collapse happens. In the simulation above, if the fracture plastic strain of steel structure is 0.5%, the chain collapse will take place entirely. However, if the fracture strain is improved to 1%, the impact energy of upper floors will be absorbed by the lower structures and the chain collapse will be stopped at about 100m under the airplane impact zone. When the fracture strain is improved to 5%, only part of the structure near the airplane impacting zone will be damaged, and no chain collapse will take place. Hence, if the structure has enough ductility to absorb the energy of upper floors¡¯ collapse, the chain damage will be controlled. Even though consider the influence of heap load, the towers still have much larger chance to escape from the entire collapse.

Fig. 2.1 Collapse of south tower----perspective

Fig. 2.2 Collapse of south tower----vertical

Reference

1. Wang Guozhou, Qu luqian. Steel structure----principles and design. Beijing: Tsinghua University Press, 1993

2. Bao Shihua, Fang Ehua. Design of Tall Buildings. Beijing: Tsinghua University Press, 1994

3. Jiang Jianjing, Gong Xiaonan, Wang Yuanqing, Cui Jinghao. Analysis and dispose for building accidence. Beijing: China Building Industry Press, 1998

4. Manual of ANSYS/LS-DYNA, ANSYS Agency in Beijing, 1999

Henry Petroski

A Q&A with Professor Henry Petroski

December 1, 2001

Henry Petroski, Aleksandar S. Vesic Professor of Civil Engineering and professor of history, is an expert in the implications of failure for engineering. In his book, To Engineer Is Human: The Role of Failure in Successful Design (1985), Petroski explored how engineers learned from engineering failures. In a recent interview with Dialogue, Petroski discusses how the collapse of the World Trade Center towers has changed engineering thinking.

Q. In the immediate aftermath of the World Trade Center attacks, you said you expected this would be the end of tall high-rises, and indeed the new plans for the WTC area don't include any building higher than surrounding ones. Do you expect this to continue? Are there any good economic or engineering reasons to drive buildings any taller?

Petroski: I do not expect that there will be any supertall buildings built in the United States for the foreseeable future. (That is not to say that such buildings will not be built in parts of the world where terrorism is perceived to be less of a threat to the infrastructure.) There never were good economic or engineering reasons to build as tall as the Twin Towers of the World Trade Center or the Sears Tower, and since those structures were completed in the early 1970s no taller skyscraper was constructed in America. In fact, there are economic disincentives to build as tall as the Twin Towers and the Sears Tower. As buildings rise higher, more space inside the structure must be devoted to elevators to move people up and down, and the more space devoted to elevators the less there is to rent and recoup the investment in the building. Supertall buildings have been built not so much for economic or engineering reasons as for reasons of civic or corporate symbolism.

Q. The collapse of the buildings has received a lot of study. Have any of the results of these studies been of particular interest? What has been surprising?

Petroski: Among the most interesting results of engineering failure analyses of the collapsed towers has been the incontrovertible evidence that fire and the heat that accompanies it can trigger the collapse of a structure the way they did in New York. There had been fires in skyscrapers before, but none had collapsed, because the fire and the attendant structural damage was confined to a floor or two and thereby localized in their effect and the structural damage they caused. In the case of the World Trade Center, the massive structural damage due to the impact of the hijacked airplanes, in combination with the intense heat of the resulting fires, produced a theretofore incredible combination of forces on the buildings. Such combinations of forces are, obviously, no longer incredible.

Q. Has engineering changed because of what we learned from the buildings' collapse?

Petroski: The collapse of the World Trade Center Twin Towers will have an enormous and long-reaching effect on structural engineering as it relates not only to tall buildings but to any structure susceptible to terrorism. There have already been calls for changing what building codes require in terms of fire protection, evacuation routes, and the ability for a structure to withstand the massive damage that can result from a terrorist attack. It is likely to take some time before these changes are incorporated into formal building codes, but in the meantime engineers will no doubt design more terrorist-resistant and more escapable structures.

Q. What, if anything, is different about how engineering has adapted to the WTC collapse compared to other structural disasters, such as the collapse of the Hyatt skywalk?

Petroski: Engineering adapts in pretty much the same way after any catastrophic failure. There is typically a moratorium on designing and building anything that resembles the structure that has collapsed, not only because it would be unwise to do so until the causes of the failure were fully studied and understood but also because of the psychological reason that people would be disinclined to want to use a structure that so reminded them of one that collapsed. The Hyatt Regency skywalks were not rebuilt as elevated walkways hanging by slender steel rods from the ceiling of the hotel. Rather, a single elevated walkway supported from below by massive concrete columns was constructed over the lobby. It conveyed a sense of strength and stability that was reassuring in the hotel lobby that had been the scene of such a tragedy.

Dr. Charles Thornton

Engineer explores structural reasons for WTC collapse

By Brian Henn
Princetonian Contributor

Noted structural engineer and visiting University lecturer, Dr. Charles Thornton, spoke yesterday about the engineering analysis of the September 11 terrorist attacks on the World Trade Center. Thornton, chairman of the engineering firm responsible for assessing the damage at the WTC site, explained the reasons for the catastrophic collapse of the twin towers.

Thornton confirmed the widely reported structural explanation for the towers' collapse. "No aircraft alone will knock over an office building" as massive as a tower at the World Trade Center, he said.

The burning jet fuel weakened the building's structural supports, but as to what specifically failed, he admitted, "I don't know if anybody will ever know for certain."

He noted several measures that could have improved the buildings' chances for survival under such an assault. "I might use a concrete core [rather than the steel in the WTC], or I might encase the beams in concrete," Thornton said.

Reinforced concrete can survive intense heat far better than steel, he said. The beams of the twin towers were made of steel with an applied fire retardant coating.

Thornton referred to the Petronas towers in Malaysia, which his company — the Thornton-Tomasetti Group — designed. Those twin towers, currently the tallest in the world, have concrete beams arranged in a circular pattern, which gives added strength against wind and other stress. "There's no way an aircraft goes through a thirty-inch concrete wall," he added.

ABOLHASSAN ASTANEH-ASL

WORLD TRADE CENTER COLLAPSE, FIELD
INVESTIGATIONS AND ANALYSES

ABOLHASSAN ASTANEH-ASL

Professor, Department of Civil and Environmental Engineering
University of California, Berkeley, 781 Davis Hall, Berkeley, CA, 94720-1710, USA

ABSTRACT: The main structure of the World Trade Center consisted of an external tube, internal gravity columns and steel truss Joists connecting the interior columns to exterior columns.
A field investigation of the World Trade Center structural remains and an ongoing nonlinear dynamic analysis indicate that the many innovations that were used in design and construction of the WTC may have affected the structural performance in a positive way. However, the performance under fire was not as good resulting in eventual collapse.

INTRODUCTION
On September 11, 2001, terrorists flew two passenger planes into the upper floors of the two 110 story towers of the World Trade Center in New York City. The planes broke the exterior columns, entered the building, caused damage to interior structure of the towers and exploded inside the buildings. Although the towers did not collapse during the first impact, as a result of ensuing fire which had started by the jet fuel in the planes, both towers collapsed and more than 2800 innocent occupants lost their lives. A week after the collapse of the towers, the author arrived at New York and started collection of perishable data and a field investigation of the collapse.

The World Trade Center consisted of seven buildings; two of them were the 110-story towers.
This paper focuses on the two towers and presents a summary of the main architectural and structural features and the post collapse field investigation conducted by the author. In addition, a summary of an ongoing non-linear analysis of the impact of the planes on the towers and partial results are provided. Finally, lessons that are learned from this tragedy and could be applied to other skyscrapers to save lives in the future will be presented. Many innovative concepts were used in architectural as well as structural design of the WTC towers. The emphasis herein is on those aspects and new concepts that were used in the design of the towers and may have had significant effect on the performance of the structure during the impact and ensuing fire as well as on the survivability of the occupants.

ARCHITECTURAL AND STRUCTURAL ASPECTS OF WTC
The towers of the World Trade Center were completed in 1972 and 1973 and were 417 m and 421 meters high. Figure 1 shows plan view of the floors of the World Trade Center. The main features This study is dedicated to the memories of all victims of September 11, 2001 terrorist attacks and particularly to the firefighters and other first responders who so heroically sacrificed their lives to save others.

Emerging Technologies in Structural Engineering
Proc. of the 9th Arab Structural Engineering Conf., Nov. 29 – Dec. 1, 2003, Abu Dhabi, UAE of the towers were the perimeter tube, the core area and the open (column-free) office space created between the perimeter tube and the core service area. The perimeter tube consisted of closely spaced steel box columns and horizontal plates at floor levels acting as spandrel beams.
The center to center distance of columns in the perimeter tube was about one meter. Figure 2 shows details of the perimeter column, the window attachments and the fire-proofing. The box columns generally had 46 by 46 centimeter outside dimensions and varying thicknesses. The window opening between the two adjacent box columns of perimeter frame was about 56 centimeter. The fireproofing consisted of sprayed-on material. Aluminum façade sheets were added to the three outside faces of the exterior columns, Figure 2. According to architectural drawings and as shown in Figure 2, on the interior face, the steel columns were covered with plaster wall panels as finished surfaces of the offices.
The core area of each floor was generally used for elevators, stairwells, ducts, pipelines for utilities and restrooms. In design of this building, in order to accommodate large number of elevators needed to reach the upper floors, the elevators were stacked in three theirs, each tier serving approximately one third of the height of the building as shown in Figure 1. There were three stairwells, all near the center of the building. The walls around the stairwells were gypsum
boards. The floors were generally made of lightweight concrete slab cast on steel corrugated deck.

The concrete floors were supported on the truss joists, which were about 80 cm deep. The floorto- floor height of each story was 3.65 meters.
Figure 1. A sample of typical floor plans of the World Trade Center and view of elevators The structure of the towers was a new system made of three main elements: (a) an exterior steel tube with closely spaced column to carry gravity and lateral load, (b) interior steel core columns and beams to carry gravity only and; (c) the light weight concrete on steel deck floors supported on simply supported steel truss joists. Figure 3 shows a typical framing plan for the upper floors. Figure 4 shows cross section of a typical upper floor. The World Trade Center was constructed using 3-story pre-fabricated welded units. Figure 5 shows the units used for exterior tube. The 3-story units were bolted to each other at the site using end plate connection shown in Figure 5. The end plates had 4 or 6 bolts. Both ends of the spandrel plates beams on each prefabricated unit had one or two rows of holes. During erection process, two ends of the spandrel plate in adjacent pre-fabricated units were connected by adding doubler plates on both sides and bolting the resulting splice. The horizontal spandrel plates were about 80 cm deep and had varied thicknesses depending on location. The horizontal plates acted as spandrel beams and were almost at the same level as the floor truss joists as shown in Figure 4.

FIELD INVESTIGATION OF THE COLLAPSE
One week aftert the collapse of the World Trade Center, the author, armed with a research grant from the National Science Foundation, arrived in New york and started collection of perishable data and investigating the remains of structural steel from the World Trade Center buildings. The main goals of the author’s field investigations were:
a. To visit the site and map the collapsed structure and the debris.
b. Inspect quality of construction
c. Collect samples of material for further studies
d. Collect drawings and information on design, construction and maintanence
e. Establish failure modes and formulate a hypothesis for causes of collapse.
Figures 6 shows views of various components of the World Trade Center Towers after collapse. By inspecting the remains of the steel structure visually, it appeared that the construction and fabrication of the steel structure was of high quality and no apparent flaws could be observed. Several components of the steel structure appeared to be from the impact areas although at the time of inspection it was not possible to identify the location of these pieces. Such pieces were preserved and later were turned over to the National Institute of Standards and Technology for testing and identification.

A HYPOTHESIS FOR WHY THE TOWERS COLLAPSED
Studying the remains of the collapsed WTC towers and the architectural and structural drawigs of the towers, it is the author’s opinion that the following sequence of events might have resulted in the final demise of the towers:
1. When the planes hit the towers with very high speed, the impact of the plane broke many exterior columns and their bolted splices with relative ease without much damage inflicted to the plane itself. This was due to the fact that the box columns of the exterior tube were relatively thin at the higher floors where the planes hit. The shock of impact must have shaken the sprayed-on fireproofing off the structure at least in the floors near the impact area.
2. When the planes entered the towers their fuselodge and wings had not been significantly damaged and the planes had the bulk of their jet fuel delivered inside the building. After entering the towers, the planes hit the floor slabs in 3-4 floors and most likely demolished relatively light joists and the floor slab. It is likely that the floor slabs must have inflicted serious damage to the plane or its wings cutting through them. At this time most likely the jet fuel was spread inside the buildings and the fire must have started.
3. After entering the towers and demolishing the floors,the damaged planes continued to move inside the buildings until they hit the relatively heavy structures of the core. It appears that at this point the damaged planes must have been brocken apart and in case of the South tower, the plane exploded. After planes exploded inside, the heavy parts like engines still continued flying inside the towers and in fact one engine was found several block away from the towers. At this time all the fuel was spread within the open space of several floors. Notice that as Figure 1 shows, the towers had very large open spaces without strong firewalls to compartmentalize the large open space. As a result, the fire spread very rapidly throughout the entre floors that were impacted.
4. When the planes hit the core of the building, they must have demolished the stairways on their path. As indicated earlier, the stairways in the tower had only relatively light and weak gypsum board walls. It is established that only one of the three stairwells in the south tower survived the impact of the planes and was partially open for the occupants’ escape. In the North tower, all three stairways apparently were demolished and closed preventing the occupants from escaping before the towers collapsed.
5. After the planes hit the core and broke apart, the jet fuel spread throughout the open floors which conatined furniture and particularly very flamable paper material. The fire that most likely had started earlier, continued with more intensity as more and more contents reached flash point. Since probably there was not much of fireproofing left on the steel elements, the fire started warming up the relatively thin and exposed floor joists as well as the exterior columns. The properties of steel and concrete under high temperature is well known. As shown in Figure 7, when the temperature reaches about 500 to600 degree Celsius, the yield strength as well as modulus of elasticity of steel drops rapidly. As the fires went on, it is likely that the floor joists collapsed frst resulting in elimination of bracing that they were providing to the columns. Since the unbraced length of the columns now had increased significantly, the columns buckled and initiated the final collapse.
6. As a result of buckling of exterior columns, and perhaps some of the interior columns, the top portion of the towers, above the impact floors dropped on the lower portion and under the pull of the acceleration of gravity ponded the lower floors down to complete vertical collapse.

ANALYSIS OF THE PLANE IMAPCT ON WTC TOWERS
In order to study structural performance of the WTC towers when hit by a passenger plane similar to Boeing 767, a non-linear model of a 10-story segment of the north tower was built and is being subjected to simulated attack by a nonlinear model of a plane. Figure 8 shows snap shots of the simulated plane flying into the simulated model of the upper floors of the north tower. The studies so far have indicated that indeed the exterior columns were not able to resist the inertia force of the plane and either the columns or their bolted splice connections fail upon impact by the plane.

The main objective of the ongoing analytical studies is to learn as much as possible from this tragedy and try to find answers to at least some of the many unanswered yet very important questions.
Some of the questions to be answered are:
1. What would have happened if the exterior tube system of the World Trade Center was not the closely spaced yet relatively light steel columns and instead it was more traditional perimeter moemnt frame with relatively heavy steel or composite columns spaced 5-7 meters apart as is done in most high rises? Could these stronger exterior columns inflict enough damage to the plane that the plane will drop its wings and the jet fuel within them outside the buildings? Or, due to impact of the plane a few of these strong columns will collapse and result in immeidiate progressive collapse of the towers?
2. What would have happened if the floor beams instead of being the truss joists connected to the exterior and interior columns by simple seat supports, were traditional rolled wide flange girders or traditional trusses connected to the columns by moment connections?
3. What would have happened if the walls around the elevator shafts and specially the stairwells instead of being gypsum boards were masonry, reinforced concrete or composite shear walls? Could the stronger walls resist the impact of the plane and protect the stairways so that the occupants above the impact area could escape to safety?

TENTATIVE CONCLUSIONS
Based on the field investigation and study of drawings and other design related documents, it is the opinion of the author that the highly redundant exterior tube of the World Trade Center with many closely spaced columns was able to tolerate the loss of many columns and support the gravity while almost all occupants who could use a stairway escaped to safety. The collapse of the towers was most likely due to the intense fire initiated by the jet fuel of the planes and continued due to burning of the building contents. It is also the opinion of the author that had there been better fireproofing installed to delay the steel structure, specially the light weight truss joists and exterior columns from reaching high temperature until the content of the buildings burned out, probably the collapse could be avoided and the victims above the impact area rescued. Finally, in the opinion of the author, if the walls around the stairwells were stronger and the stairwells were not all located at one place, many of the victims who were trapped in the floors above the impact area probably could find a useable staircase and escape to safety.

ACKNOWLEDGMENTS
The project summarized here was sponsored by the National Science Foundation of the United States of America. Partial support was also provided to the author by the American Institute of Steel Construction and the University of California at Berkeley. The generous donation of the powerful analysis program Patran/Dytran by the MSC Corporation to the University of California, Berkeley is sincerely appreciates. Casey Heydari, Vijay Tunga, Professor Jay Shen and Qiuhong Zhao participated in the analyses. Their contributions are acknowledged and appreciated

REFERENCES
1. A. Astaneh-Asl, “World Trade Center Post-Disaster Reconnaissance and Perishable Structural Engineering Data Collection”, Report Number: UCB/CE-Steel-05/2002, Final Report to National Science Foundation, Department of Civil and Environmental Engineering, University of California, Berkeley, CA, December 2003.

Ronald Hamburger

Stanford Report, December 3, 2001

Structural engineer describes collapse of the World Trade Center towers

BY MARK SHWARTZ

Vulnerabilities in the design of New York's World Trade Center (WTC) are likely to have contributed to the collapse of its two main towers and adjacent buildings, according to Ronald O. Hamburger, a structural engineer currently investigating the Sept. 11 disaster.

"These buildings were incredibly strong, especially with respect to resisting dead loads and wind loads, but they also had a number of vulnerabilities," Hamburger told a packed auditorium on Nov. 29 when he delivered the second John A. Blume Distinguished Lecture -- an annual event sponsored by Stanford's Blume Earthquake Engineering Center.

This graphic shows buildings in the New York Financial District affected by the Sept. 11 terrorist attack. Gray buildings were destroyed, red are in danger of collapse, blue suffered major structural damage and others shown were damaged, but can be repaired. Graphic courtesy of: Ronald Hamburger/ABS

"What New York City experienced on Sept. 11 was very much like an earthquake," he told the Stanford audience. "Life loss exceeded anything we in the United States have experienced in an earthquake, and the financial loss exceeded anything we've experienced -- and it all occurred within one square mile."

As chief structural engineer and senior vice president of ABS Consulting Inc. in Oakland, Calif., Hamburger is a member of an engineering team commissioned by the Structural Engineers Institute of the American Society of Civil Engineers (ASCE) to assess the performance of the WTC and surrounding buildings in the aftermath of the terrorist attacks.

He pointed out that four buildings were immediately destroyed in the WTC assault, and three others suffered irreparable damage and are in the process of being razed. Another half-dozen buildings were harmed structurally but can be repaired, and more than 50 others were damaged by the enormous debris cloud and the burning material that followed the collapse of the twin towers.

"Thirty million square feet of commercial space were affected, including 10 million square feet that were taken out permanently -- the equivalent of all the financial office space in the city of San Francisco," he noted.

Hijackers' plan

Using photographs of the WTC taken before, during and after the Sept. 11 assaults, Hamburger presented a brief chronology of events. He noted that the first jetliner banked into the north tower at a 45-degree angle, damaging floors 92 to 95. About 40 minutes later, the second jet crashed into the south tower, hitting floors 78 to 84.

An aerial view of downtown Manhattan. Credit: spaceimaging.com. Copyright spaceimaging.com All rights reserved. Online or news media distribution or publishing requires permission from Space Imaging.

"I believe that the hijackers flew the aircraft into the lowest part of the buildings they had access to," Hamburger commented. "If there had been no nearby structures, they would have hit the towers lower."

One member of the audience asked Hamburger if he believed that the pilots intentionally banked the planes at an angle to take out as many floors as possible.

"Yes," he replied.

According to Hamburger's preliminary analysis, the impact of the jetliners shattered and fractured two-thirds of the support columns on one face of each tower, causing the partial collapse of several floors. Debris penetrated each building's core and may have damaged the core columns located in the center of the 110-story structures.

"The damaged columns held up the weight of the building, so logic would dictate that the building would fall," said Hamburger, "but that didn't happen. Because of its great structural redundancy, the load was distributed to other parts of the building. We have reason to believe that, without the fire, the buildings could have stood indefinitely and been repaired. But we did have a fire."

Born of fire

Hamburger noted that the fuel in both jetliners burned off rapidly, despite media reports that the aircraft continued burning long after the crash.

"The impact probably caused a failure of the fireproofing in the affected areas," he said. "We think that the fuel ignited several floors in the building," he added, which had a devastating effect on the steel support beams.

"Steel is born of fire," Hamburger explained. "As it's reheated, it expands and loses its rigidity. Above 1,000 degrees Fahrenheit, it loses a significant amount of its strength."

He said the extreme heat from the fires might have caused the steel floors to expand and bow, which may have caused the support columns to bend inward and buckle. Heat also may have caused the steel flooring to separate from the columns, or the columns themselves may have heated up and buckled outward.

Hamburger and his colleagues have not yet determined which of these scenarios occurred on Sept. 11, but there is little doubt that the collapse of the upper floors of the WTC towers brought down both structures.

"Think of the impact of dropping a 25-story building straight down," Hamburger told the audience. "It was like a pile driver, which is why it collapsed as it did."

Vulnerabilities

While acknowledging the many innovations that went into the design of the WTC towers in the 1960s -- including one of the earliest applications of computer stress analysis -- Hamburger also cited several features that made the buildings vulnerable to the intense fires that ultimately caused their collapse.

"The floor trusses [joists] were relatively flimsy. As the tower collapsed, the trusses just fell apart," he observed, noting that trusses are difficult to fireproof.

Hamburger noted that each tower was constructed using a novel tube frame system designed to resist winds of up to 80 miles per hour. But the connections of the tube frame were weak, causing them to break apart and become three-pronged missiles that crashed into the street and into nearby buildings.

Hamburger also discussed the collapse of WTC Building 7, which housed the offices of Con Edison, the FBI and the CIA.

"WTC-7 was a 47-story building and became a two-story pile of rubble," Hamburger said, "making it the first major structure in the United States to collapse because of fire."

Future designs

He pointed out that fires frequently occur in high-rise buildings and noted that between 1994 and 1998, 30 fires occurred in the United States in buildings that were 50 stories or taller.

"The question is, should fire protection standards be changed in some significant way in the aftermath of Sept. 11," Hamburger asked, "and should structural engineering designs include consideration of fire load and the response of structures? Right now, structural engineers know very little about fire."

He and his colleagues will attempt to answer those questions when the ASCE report is made public next spring.

Does it make sense to design buildings to withstand such events as Sept. 11 in the future?

"In my opinion, no," Hamburger concluded, although he does support the decision implemented after the 1995 Oklahoma City bombing to redesign federal buildings to withstand the accidental loss of major structural elements.

The Blume Lecture series honors John A. Blume, often called "the father of earthquake engineering," who earned his undergraduate and doctoral degrees at Stanford. In 1974 he helped underwrite the John A. Blume Earthquake Engineering Center, which promotes earthquake engineering research and education. Today at age 92, Blume remains a consulting professor in Stanford's Department of Civil and Environmental Engineering.