The search for answers in the rubble of the Twin Towers

The search for answers in the rubble of the Twin Towers
Apr 1, 2002 12:00 PM
Paul Rothman

The crumbled remains of the World Trade Center hold clues about how to enhance fire safety, structural engineering and security for present and future high-rises.

Researchers at the National Institute of Standards and Technology (NIST) are examining the charred rubble, hoping to unearth a world of knowledge.

The NIST, a government agency, provides measurements, standards and technical advice to federal, state and local agencies and the private sector to protect U.S. citizens from terrorist, military, natural disaster and other types of threats. In a March statement before the U.S. House of Representatives' Committee on Science, NIST Director Dr. Arden L. Bement Jr., outlined the NIST's role in the aftermath of the terrorist attacks.

“The tragedy that the United States experienced on Sept. 11 was unprecedented when compared to any prior accident, natural disaster or terrorist/war attack,” Bement says. “The collapse of the twin World Trade Center towers was the worst building disaster in human history. Engineers, emergency responders and the nation did not anticipate — and were largely unprepared for — such a catastrophe.”

Bement highlighted the technical priorities in examining the remains of the towers:

  • To establish the probable technical causes of the collapse and derive the lessons to be learned;

  • To develop and disseminate immediate guidance and tools to assess and reduce future vulnerabilities; and

  • To produce a technical basis upon which cost-effective changes to national practices and standards can be developed.

While the formal investigation and research have just begun, potential benefits are easy to envision. Above all, the NIST's findings — which will come at the conclusion of a 24-month study — could mean adjustments to current fire codes and regulations.

“We put those lessons learned to work,” says NIST spokesman Michael Newman. “We provide the data to groups which can change fire codes and building standards.

“But you never go in guessing,” he adds. “Our part is to try to determine what were the conditions that led to the collapse.”

The focus of the NIST study will be the twin towers because, according to Bement, the collapse of the towers was the triggering event that caused much of the collateral damage to adjacent properties. Also, the NIST will focus on examining procedures and practices used to provide structural reserve capacity to resist blasts, explosions, accidental fires and other disasters; and to study the effectiveness of fire fighting technologies and practices for tall buildings.

According to Bement, the objectives of the NIST investigation would be to determine:

  • Why and how the towers collapsed;

  • Why the injuries were so high or low depending on location, including technical aspects of fire protection, response, evacuation, occupant behavior and emergency response;

  • Whether or not state-of-the-art procedures and practices were used in the design, construction, operation and maintenance of the towers; and

  • Whether there are new technologies or procedures that should be employed in the future to reduce the potential risks of such a collapse.

Scientists from the NIST will conduct the study in cooperation with consultants from a range of agencies that include the New York Port Authority, Federal Emergency Management Agency (FEMA), the Tall Building Council, the Society of Fire Protection, the American Institute of Steel Construction and others.

In its preliminary stages, scientists and researchers will construct a computer model of the likely conditions inside the buildings following the collisions using a program called a Fire Dynamic Simulator or one called SnakeView.

“The model is constructed using mostly visual information,” Newman says. “Using a model looking at the fire and smoke as it is portrayed visually, the researchers can try to step back and determine what would have led to the collapse.

“They try to match [the pieces] with what they see in the models, and use the computer to go backward in time,” Newman continues. “Pieces of steel can show warping and give an indication of how hot it really was inside. With that, we can determine how the heat affected the infrastructure.”

Researchers then sift through the rubble to confirm their hypotheses. Already, 50 to 60 pieces from the towers have been transported to NIST headquarters in Gaithersburg, Md.

“The last step will be to disseminate all the information and make sure these lessons learned don't go to waste,” Newman says. “It was such a tragic event, but in effect, it's an unusual learning experience. Something like this is unprecedented, and will prove very valuable in future design and code regulations.”

WTC Lessons For Future Disasters

WTC Lessons For Future Disasters

Report Says Why Twin Towers Fell, Calls For New Evacuation Plans

WASHINGTON, April 6, 2005
The south tower collapses as smoke billows from both towers of the World Trade Center on Sept. 11, 2001. (AP)


The lead investigator says the twin towers collapsed because the impact of the planes shook loose the steel skeleton's fireproofing material, which normally would be able to withstand heat as intense as 800 degrees.

(AP) The Sept. 11, 2001, attack on the World Trade Center is more than a tragic day in American history.

It continues to be an opportunity for lessons to be learned.

In a batch of government reports released Tuesday, engineers say they know why the twin towers fell down, how things went wrong during the evacuation, and believe new thinking is needed on how to evacuate people from endangered skyscrapers and how to get rescuers into them more quickly.

The reports by the National Institute of Standards and Technology also detail how early decisions played a key role in determining who died and who survived.

A total of 2,749 lives were lost when the two hijacked jets were crashed into the twin towers.

The NIST reports note that expectations of how quickly people move down stairwells have been based on "phased" evacuations, and not the full-scale evacuation of the type attempted at the World Trade Center.

"The average surviving occupants moved slower down stairs and through stairwell exits than previously recorded for a non-emergency evacuation," investigators concluded.

To underline the importance of the finding, NIST estimated that had the buildings been hit at a time when they were full, as many as 14,000 people may have died.

In Tower 1, the average survivor took 48 seconds to descend a flight of stairs, or about half the slowest evacuation speed calculated in a current fire safety handbook used by engineers in designing buildings, the report found.

The briefing in New York was conducted by Shyam Sunder, the lead investigator for the agency's building and fire safety investigation into the disaster.

The buildings would not have collapsed if the fireproofing material surrounding the steel had not been stripped away by the impact of the planes, Sunder said.

Without that stripping effect, the intense heat of up to 800 degrees Fahrenheit would not have been enough to bring the buildings down. Sunder added, however, that it would not have been reasonable for engineers to have installed fireproofing designed to sustain the impact of a fuel-laden jetliner.

The thickness of the fireproofing had been called into question during the course of the two-year investigation; Sunder said far more important was the material's ability to stick to steel. Now there are other ways to put on fireproofing to make it adhere to the steel better.

David Collins, a member of the advisory committee that offered suggestions and questions to NIST investigators, said the research showed design and construction of the building were not major contributors to the collapse.

"I think everyone took deliberate steps to try to do what was necessary to make the buildings as safe as possible," said Collins, a Cincinnati-based architect.

NIST also found the much-documented problems with radio communication and information-sharing among first responders probably led to deaths among emergency personnel.

"Lack of timely information sharing and inadequate communication capabilities likely contributed to the loss of emergency responder lives," according to the draft report.

Sunder said some of the firefighters they interviewed said they felt they would have gotten better real-time information at home watching TV than what they were told at the scene.

The findings are NIST's last step before issuing its final recommendations in June, the culmination of exhaustive research and testing that produced 10,000 pages of data.

The other findings - about the emergency response and the behavior of those who were in the building - will be part of the ongoing debate over how to improve skyscraper safety.

Investigators have determined that previous expectations about how long people would take to evacuate buildings were not borne out by events at the World Trade Center.

The report also noted that some people delayed their evacuation by "milling" in offices, deliberating about what to do, or debating how to find the next stairwell.

The previous evacuation models cited in the report are important because architects use them to calculate the capacity needed in stairwells, elevators, and other means of exiting a building.

The report also emphasizes the limited ability of rescue personnel to reach higher floors quickly to battle fires and rescue trapped civilians.

That proved critical for firefighters who climbed 70 flights of stairs carrying up to 100 pounds of gear - and then tried to battle flames or clear debris once there.

Those concerns are spurring a debate both within the NIST group and among the larger fire rescue and construction fields about stairwell and elevator design.

The debate centers around whether "fireproof" elevators, designed to resist flames and smoke, should be installed in new buildings, particularly those that rise above 40 or 50 stories, and the best width and location of stairwells.

Elevators played a critical, but contradictory, role. In some cases, they helped significant numbers of people get out quickly. For others, they became sealed containers trapping them inside a doomed building.

NIST's ultimate goal is to recommend building code improvements.

The World Trade Center: Collapse of the Twin Towers

The World Trade Center: Collapse of the Twin Towers

Terrorist Attacks on New York's World Trade Center Towers

The former World Trade Center Twin Towers, destroyed by terrorists
Dateline: September 12, 2001
This article summarizes news reports and commentary at the time of the terrorist attacks on the World Trade Center in New York City. For complete information about the World Trade Center, reconstruction plans, memorials, and other resources related to the September 11 terrorist attacks, visit our World Trade Center Resource Page.

Built in the 1970s, World Trade Center Twin Towers in New York City were designed to withstand normal fires and hurricane-force winds. According to some reports, engineers believed that even the impact of a Boeing 707 would not bring down the towers.

But the two planes which struck the Twin Towers on September 11, 2001 were much larger than the Boeing jets of the 1970s. Experts say that no engineer could have prepared for the terrorist attack which reduced the World Trade Center to rubble.

Indeed, the two towers showed remarkable strength, standing for about an hour after the air strikes. Their ultimate collapse was caused primarily by the jet fuel fire, engineers told reporters for the New York Times. Temperatures soaring from 1,000 to 2,000 degrees caused the steel columns around the tower facades to buckle. With their supports weakened, the concrete-slab floors plunged.

The South Tower of the World Trade Center was most heavily impacted on floors 87 through 93. The North Tower was impacted at floors 96 through 103. Because the jets hit the towers near the top, the buildings themselves became their own means of destruction as the weight of many floors crushed downward. Later, falling debris and more fires led to the collapse of the smaller 7 World Trade Center building adjacent to the towers.

Designed by Minoru Yamasaki and Associates, the World Trade Center towers utilized tube construction, which was a popular innovation for skyscrapers of the 1970s. Older structures like the Empire State Building use heavy internal supports and thick masonry. In 1945, when a ten-ton B-25 bomber left a 20-foot hole in the landmark skyscraper, the Empire State Building remained standing. But the heavy masonry building would surely not have survived an impact from the larger passenger jets which struck the World Trade Center towers.

The Twin Towers were constructed of lightweight steel and glass supported by exterior columns. Each concrete-slab floor was supported by steel trusses with special plates designed to lessen the effect of high winds. Stairs and elevators were supported by columns at the core of the towers. However, the primary support for the towers came from their external sheathing.

The demolition of the World Trade Center complex has altered New York's skyline forever. By all accounts, the buildings were miracles of design. Could they have been made stronger or safer? Tell us what you think.

Why the World Trade Center Towers Fell
Engineers who studied the Twin Towers after the September 11 attacks explain why the buildings stood as long as they did, and why they eventually collapsed.

Read more about the construction of the Twin Towers

See pictures of the proposed plans for reconstruction on the World Trade Center site.

World Trade Centre Collapse

World Trade Centre Collapse

Reporter: Karina Kelly
Producer: Mick O'Donnell
Researcher: Robert Hodgson

Related Info

20 September 2001
As the world still struggles to comprehend the terrorist attacks on New York and Washington, Catalyst investigates why the World Trade Centre towers collapsed the way they did.


Narration: When the World Trade Centre was built in 1973, its twin towers were the tallest buildings in the world. Part of their revolutionary new design made them resistant to strong winds and allowed for huge open office spaces. Their strength depended on a steel shell. But no skyscraper in the world is designed to withstand this.

Dr Andy Davids: Steel looses strength and stiffness as its temperature increases past about 1500 degrees. The ferocity of that fire would have been well and truly enough to push it past it's elastic limit if you like and it would have started to soften, and yield

Narration: Dr Andy Davids is one of a handful of people who designs the world's tallest buildings. He's a structural engineer and a friend of Les Robertson who engineered the world trade centre.

Dr Andy Davids: You can see clearly on the footage that that top 25 stories moves almost as a rigid object a single block - it rotates over quite a long way - actually several meters - and drops as a single object. The mass of debris just keeps feeding on itself as it drives down the building and the building just basically unzips.

Narration: So, was the lightness of the steel frame a fatal flaw?

Dr Andy Davids: The failure was due to the consuming nature of the fire and how that weakened the steel structure. I think it's not possible to design a structure that's failure proof.

Narration: Yet, 56 years ago, a near neighbour of the world Trade Centre, withstood another aerial attack. On 18 July, 1945 a B25 bomber flew into the Empire State building in heavy fog.

Dr Andy Davids: It was a much smaller aircraft, a World War two bomber and it weighed about 10 tonne as compared to the 767 as I understand hit the World Trade Centre, which weighed probably to the order of 100 to 150 tonnes.

Karina Kelly: So 10 times bigger - but the Empire state building is still standing today - why is that?

Dr Andy Davids: Yes the structural system of the Empire State is similar in one way to the World Trade Centre in that it is a steel frame, however on the Empire State those steel beams and columns had all been in filled with heavy masonry panels so the building was a very stiff building; a very dense building; had a lot of mass in which to absorb the inertia of the aircraft striking it and limit the damage.

Karina Kelly: Does that mean the Empire State building was a better made building because it could withstand this plane crashing into the side of it?

Dr Andy Davids: Man-made objects such as building are evaluated on many criteria and the striking of a fully loaded aircraft was not one of those criteria.

Karina Kelly: But how would more modern buildings cope with a plane slamming into them?

Dr Andy Davids: The buildings that we design today are probably not much better in that regard. Buildings by their very nature require support along their perimeter and also in the centre. I think the main difference is that the more modern high rise buildings that we design and build today tend to have a large solid reinforced concrete core in the centre, which is the main stability element which prevents the building from moving in response to wind and earthquakes and impacts such as from aircraft.

Karina Kelly: So if you have a core of reinforced concrete then a collapse like that wouldn't happen

Dr Andy Davids: I wouldn't say it wouldn't happen because I think the actual collapse was due to the fire in fact, rather than the impact of the aircraft. So I think that if we are to learn any lessons from this tragedy it would be that the control of massive fire in these types of buildings really needs to be reconsidered.

Dislodged fireproofing felled Twin Towers

Dislodged fireproofing felled Twin Towers

Agen├že France-Presse

Wednesday, 6 April 2005

Building report
Investigator Shyam Sunder presents the 10,000 page report into why the World Trade Center collapsed so quickly after the 11 September 2001 terrorist attack (Image: Reuters/Mike Segar)
The twin towers of the World Trade Center would probably be standing today, if the impact of the planes used in the 11 September 2001 attack had not destroyed fireproofing material, experts say.

After what it described as the most detailed examination of a building failure ever, the US National Institute of Standards and Technology (NIST) says it will be suggesting major changes to the way skyscrapers are built and managed.

The NIST report says the structural impact of the planes and subsequent jet fuel-ignited, multi-floor fires were not in themselves enough to bring the towers down.

"The reason the towers collapsed is because the fireproofing was dislodged," says Shyam Sunder, lead investigator for the NIST building and fire safety investigation into the disaster.

If the fireproofing had remained in place, Sunder says, the fires would have burned out and moved on without weakening key elements to the point of structural collapse.

He drew an analogy with the 2003 Columbia space shuttle disaster when the absence of a small piece of insulation foam, knocked off during launch, allowed fire to seep into the shuttle's entire wing span during re-entry with catastrophic results.

New alternatives to traditional fireproofing should be explored, Sunder says, citing a paint-like substance which, if applied in sufficient layers, would stick "even if a plane hit it".

Nearly 2750 people were killed in the attack on the World Trade Center by members of Osama bin Laden's Al-Qaeda network.

Roughly 17,400 people were in the skyscrapers at the time of the attack, and NIST estimates that the death toll would have been closer to 14,000 if the two towers had been filled to their 50,000-person capacity.

The report says each jet severed perimeter columns, damaged interior core columns and dislodged fireproofing as they penetrated the buildings. The weight carried by the severed columns was spread to others.

World Trade Center remains
View of the last original parts of the World Trade Center as seen in 2004, including the new pedestrian bridge (top), the last remaining original parts (upper) and the new subway station (bottom) (Image: Reuters/Chip East)
Fires caused by the jet fuel were fed by the building contents and oxygen entering through breached walls and windows.

"Floor sagging and exposure to high temperatures caused the perimeter columns to bow inward and buckle, a process that spread across the faces of the buildings," the report says.

"Collapse then ensued."

In examining the emergency services response on 11 September and the evacuation procedures, the NIST report echoes other probes in highlighting a lack of coordination and poor communications equipment.

The report cites one senior emergency services officer inside the north tower of the trade centre as saying he would have known more about what was going on if he had been watching it on television.

"The lack of timely information sharing and inadequate communication capabilities, likely contributed to the loss of emergency responder lives," the report concludes.

The evacuation of the twin towers has generally been called a success, with 87% of the occupants, including more than 99% of those below the floors hit by the planes, managing to get out.

Occupants were often unprepared for the physical challenge of evacuating from higher floors. "It's pretty demanding, especially if you want to do it fast," Sunder says.

The NIST report, running to some 10,000 pages, is still in draft form, with a final version, complete with definitive findings and recommendations, to be released in September.

Marco J. Shmerykowsky

An occasional series on science and technology

by Marco J. Shmerykowsky

This week The Ukrainian Weekly introduces a new feature, "Sci-Tech Briefing," an initiative of the Ukrainian Engineers' Society of America. The debut article by Marco Shmerykowsky was prepared for The Weekly in late October.

The collapse of the Twin Towers

It has been two months since the terrorist attack on the World Trade Center, and like most New Yorkers I still can't fully comprehend what happened. "How could two massive high-rise buildings simply vanish in an instant?" was the question a close friend, who happened to be across the street from the Twin Towers at the time of the attack, asked me that day. Although fire caused by burning aviation fuel was a major factor in the collapse, there were also other factors, which draw from the basic elements and theories of high-rise building design.

Buildings and structural loads

The structural elements of any building provide a way for loads from higher-floor levels to travel into the supporting ground. The loads supported by the building can be separated into two general categories: gravity loads and lateral loads. Gravity loads are due to forces caused by the weights of all the materials supported by the building, such as steel, concrete, plumbing pipes, electrical wire, office partitions, furniture and people. Lateral loads are due to the horizontal forces from events such as wind pushing against the walls of a building.

While lateral loads may play a comparatively minor role in the design of a sheltered, low-rise building, they have a significant impact on the design of a high-rise building. This is because a tall building provides a large surface for collecting the wind, which translates into a large horizontal force. The structural engineer also needs to ensure that the building will be stiff enough so that under normal wind conditions it does not sway like a ship caught in a hurricane, but remains comfortable for the people who occupy it.

All high-rise buildings have special portions known as lateral load resisting systems that are specifically designed to address problems due to lateral loads. These systems consist of frames composed of beams, columns and diagonal members that are specially connected to transmit lateral forces. The frames can be located anywhere within the building.

Design of the Twin Towers

The Twin Towers used a lateral load resisting system known as a tube system, in which a series of closely spaced perimeter columns and deep perimeter beams are used to create a shell that has the strength and stiffness of a large square steel "flagpole." In this system, the perimeter columns are designed to carry both lateral loads and gravity loads, and the interior columns are designed to carry only gravity loads. The net result is that the designers obtain a larger area of usable floor space.

The gravity forces on each floor level of the Twin Towers were collected by a floor system that consisted of open-web joists, which spanned 60 feet between the perimeter and core columns. An open web joist is basically a long truss that has top and bottom members (known as a chord), which are connected by diagonal members in a zig-zag pattern. These members are attractive from a design perspective because they can span long distances but weigh less than comparable solid steel beams.

In addition to supporting the floor, the open web joists interacted with the perimeter columns by providing brace points at each floor level. The lateral bracing of a column is an important concept because it has a direct relation to the amount of load a column can carry. As a column becomes longer, the amount of load it can carry decreases. If the length is too long in relation to the magnitude of the load, the column will want to bend sideways or buckle. Since beams connect to the column at each floor level and in each direction, the effective design length is considered to be the distance between floors.

Causes of the collapse

With these basic concepts of structural engineering in mind, it is possible to understand both why the towers withstood the initial impact of the airplanes and why the towers eventually collapsed.

When one of the airplanes collided with the towers, the initial impact destroyed a number of the perimeter columns, floor joists and core columns. Once these structural elements were destroyed, the building essentially re-wired itself and redistributed the loads to the remaining structural elements.

Since the perimeter columns were designed to simultaneously carry both the gravity loads of a fully occupied building and the lateral loads in extreme wind conditions, and to provide the building with enough stiffness for the tower to be comfortable for its occupants, the net result was that the members had extra load carrying capacity under normal conditions. Thus, after the collision of the airplane with the tower, there was sufficient structure remaining to continue supporting the tower.

The fire due to the aviation fuel, however, served to amplify the damage that was done. Typical high-rise construction requires that the builders provide two to three hours worth of fire protection around main structural members. The aviation fuel fire, however, burned much hotter and quicker than a normal office fire. As a result, the fireproofing material most likely disappeared quickly.

Once the steel was unprotected and heated beyond 1,500 degrees Fahrenheit, the steel began to weaken and soften. Since the floor joists are built of thin components, they were most likely damaged by the fire first. As the floor framing failed, the bracing that the joists were providing to the columns was eliminated. Suddenly the remaining already highly loaded columns had their capacity reduced because they effectively became taller. As the fire continued to burn, the combined effect of failing floor members and weakening columns created a condition where the gravity loads from the portion of the tower above the fire could no longer be supported.

Once this point was reached, the top portion of the tower acted like a hammer driving a nail into a piece of wood. The levels at the collision point collapsed, allowing the upper floors to fall and hit the first undamaged floor. This impact was too great for that level to withstand, so it too failed. This sequence of floors stacking up like pancakes kept repeating floor after floor, until the entire structure was destroyed.

The second tower to be attacked was the first one to collapse for the simple reason that the plane hit at a lower point. Thus, there was more gravity load pushing on the tower's damaged section.

The collapse of the towers was a horrific experience for every New Yorker who experienced it first-hand and for millions more who experienced it on television. What should be noted and praised, however, is that the towers were designed so that they stood for nearly an hour after the tragic attacks occurred. This application of engineering principles allowed thousands of people to escape the towers with their lives.

Marco J. Shmerykowsky, P.E., is a principal at Shmerykowsky Consulting Engineers in New York, teaches high-rise design at The Cooper Union School of Engineering, and is president of the New York Chapter of the Ukrainian Engineers' Society of America.

Copyright © The Ukrainian Weekly, November 18, 2001, No. 46, Vol. LXIX

eXTReMe Tracker