New Evidence Is Reported That Floors Failed on 9/11

New Evidence Is Reported That Floors Failed on 9/11

By JAMES GLANZ

Published: December 3, 2003

AITHERSBURG, Md., Dec. 2 — Federal investigators said here Tuesday that new evidence supported earlier suggestions that the floor supports in the World Trade Center began failing in the minutes before the towers fell and might have played a major role in their collapse.

The investigators, who are carrying out a two-year, $16 million analysis of the collapses, made it clear that they had not yet settled on a final explanation. They said, though, that their findings gave new weight to a theory that the failure of the floors weakened the towers' internal structure to the point that the entire buildings came down.

S. Shyam Sunder, who is leading the investigation for the National Institute of Standards and Technology in the Commerce Department, said, "We are seeing evidence of floors appearing to be sagging — or that had been damaged — prior to collapse." Still, Dr. Sunder said, "The relative role of the floors and the columns still remain to be determined in the collapse."

According to an alternative theory of the collapse, the planes that smashed into the towers damaged the towers' vertical structural columns so severely that the buildings were virtually certain to fall. In that view, none of the buildings' many structural novelties — the towers were daring engineering innovations in their day — would have played a significant role in the collapses.

Last spring, the standards institute found the first photographic evidence on the east face of the south tower that a single floor — with its lightweight support system, called a truss — had sagged in the minutes before it started collapsing. Now, detailed analysis of photos and videos has revealed at least three more sagging floors on that face, said William Pitts, a researcher at the institute's Building and Fire Research Laboratory.

In addition, Dr. Pitts said, sudden expansions of the fires across whole floors in each tower shortly before they fell suggested internal collapses — burning floors above suddenly giving way and spreading the blaze below.

Finally, an unexplained cascade of molten metal from the northeast corner of the south tower just before it collapsed might have started when a floor carrying pieces of one of the jetliners began to sag and fail. The metal was probably molten aluminum from the plane and could have come through the top of an 80th floor window as the floor above gave way, Dr. Pitts said.

"That's probably why it poured out — simply because it was dumped there," Dr. Pitts said. "The structural people really need to look at this carefully."

The investigators also said that newly disclosed Port Authority documents suggested that the towers were designed to withstand the kind of airplane strike that they suffered on Sept. 11.

Earlier statements by Port Authority officials and outside engineers involved in designing the buildings suggested that the designers considered an accidental crash only by slower aircraft, moving at less than 200 miles per hour. The newly disclosed documents, from the 1960's, show that the Port Authority considered aircraft moving at 600 m.p.h., slightly faster and therefore more destructive than the ones that did hit the towers, Dr. Sunder said.

The towers did withstand the plane strikes at first, allowing thousands of people to escape, but then the fires, stoked by burning jet fuel, softened the steel of the towers. Potentially challenging other statements by Port Authority engineers, Dr. Sunder said it was now uncertain whether the authority fully considered the fuel and its effects when it studied the towers' safety during the design phase.

"Whether the fuel was taken into account or not is an open question," Dr. Sunder said. It is also unclear, he said, "whether the extent of the loss of human life as a result of that" was taken into account.

The studies of the floor trusses and the design of the towers are just two elements of the investigation, which is carrying out computer calculations of the collapses, rebuilding pieces of the towers in order to test them in real fires, and piecing together a highly detailed chronology of the response to the attack.

In one set of laboratory tests concerning the floor trusses, researchers used earthquake simulators to violently shake assemblages much like the ceilings in the twin towers. The shaking was meant to simulate the impact of the aircraft.

The findings, said Richard Gann, a senior research scientist at the Building and Fire Research Laboratory, showed that many of the fire-protecting ceiling tiles near the impact probably crumbled, exposing the undersides of the trusses directly to the fires.

World Trade Towers design exceeded wind load codes

World Trade Towers design exceeded wind load codes

Single frame of a computer simulation (containing 90,000 elements) of the WTC North Tower as it responds to wind loads. Full size image available through contact

The National Institute of Standards and Technology (NIST) reported on Sept. 17 that it has done additional analysis of the wind "loads" that the World Trade Center (WTC) towers were originally designed to resist--critical data to help the agency better assess the overall strengths and baseline performance of the two buildings before they were brought down by the terrorist attacks of Sept. 11, 2001. The work is being conducted as part of NIST's federal building and fire safety investigation of the WTC disaster.

NIST recently completed its review of the original 1960s-era source documents containing wind tunnel test data and wind load estimation methods used for the towers, calculated the wind load estimates based on a clearer interpretation of this information, and determined the values actually used in the design of the buildings. These clarified original design wind load estimates all exceed those established by the New York City building code prior to 1968 (when the WTC towers were designed) and through 2001 (when the towers were destroyed). The values also are higher than those required by other selected building codes of the era, including the relevant national model building code.

Wind load capacity is a key factor in determining the overall strength of a tall building and is important in determining not only its ability to withstand winds but also its reserve capacity to withstand unanticipated events such as a major fire or impact damage.

The NIST investigation team's final report is scheduled for release as a draft document in December 2004. For more information on the new findings, go to http://www.nist.gov/public_affairs/releases/wtc_wind_loads.htm.

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Design choice for towers saved lives

Design choice for towers saved lives

• 13:14 12 September 2001
• From New Scientist Print Edition.
• Eugenie Samuel and Damian Carrington

A lucky choice of design for the World Trade Center towers reduced the death toll caused by their destruction, say engineers.

Each tower was struck by a passenger aeroplane, hijacked by suicidal terrorists, but remained upright for nearly an hour. Eventually raging fires melted the supporting steel struts, but the time delay allowed hundreds of people to escape.

"Most buildings would have come down immediately," says John Hooper, principal engineer in the company that provided engineering advice when the World Trade Center was designed.

Hooper added that engineers at Skilling, Ward, Magnusson, Berkshire, based in Seattle, had been devastated by Tuesday's events. "We're just trying to get through this," he told New Scientist.

Skyscrapers like the World Trade Center are not built to withstand direct hits by large aeroplanes, he says. Furthermore, the fire suppression system in the towers did not include the foam sprinklers that could to deal with the jet fuel fires. Both the crashed aeroplanes were fully fueled for trans-continental flights, making them ideal "flying bombs".

Steel tube

However, the design of the 415 metre WTC towers did help prevent a greater catastrophe. The outside of the towers consisted of closely spaced steel columns forming a giant "steel tube". The columns were less than a metre apart in the upper stories and could withstand the entire load placed on the building by high winds.

This meant that the building's internal columns bore only the load of gravity. Whether it was the external frame or the internal supports that kept the building standing for an hour is not known, but the "double support" saved many lives.

Many skyscrapers have vertical columns up to six metres apart and rely on combined diagonal struts to bear loads. Destroying these structures would probably collapse a building immediately.

Classic demolition

The collapse of the WTC towers looked like a classic controlled demolition, said Mike Taylor of the National Association of Demolition Contractors in Doylestown, Pennsylvania.

"If there's any good thing about this it's that the towers tended not to weaken to one side," said Taylor. "They could have tipped onto other buildings or into the river across the West Side highway."

The collapse of the WTC towers mirrored the strategy used by demolition experts. In controlled demolitions, explosives are placed not just on the lowest three floors but also on several consecutive floors about a third of the way up the building.

The explosions at the higher floors enable the collapse to gain downward momentum as gravity pulls the full weight of unsupported higher floors down into lower floors in a snowballing effect.

On Tuesday, the impacts of aeroplanes on the higher floors replaced the explosives. The collapse of the higher floors caused the floors below to be crushed. "It cascaded down like an implosion," says Taylor.

The lack of collapse in higher stories was one reason why the 454 kilogram bomb detonated in the underground garage of the World Trade Center in 1993 failed to destroy the building.

Brent Blanchard

		©Michael Golden / Protec

THE WORLD TRADE CENTER COLLAPSE
Questions & Answers

Implosionworld.com has received numerous inquiries from around the world requesting information and commentary relating to the terrorist attacks of September 11th, 2001, and specifically the felling of the World Trade Center towers. We have been contacted by media outlets, structural engineers, schoolteachers, conspiracy theorists and many others who are searching for answers and some “perspective” regarding these significant events that have evoked deep emotions and undoubtedly changed our world forever.

The editors of implosionworld.com have created this page to answer a few of the most frequently asked questions that fall within our area of knowledge and expertise. But first we’d like to be clear in stating that any conversation relating to “implosions” and what causes structures to fail is undertaken with reverence and respect to those who perished as a result of this event. As many of our frequent web visitors are aware, Implosionworld.com’s offices are located close to New York City, and several of our employees were personally touched by this tragedy. Our thoughts and prayers remain with the families of those lost and injured, and our intent here is to help foster a constructive base of knowledge and understanding through education, while dispelling false rumors related to the attack.

DID THE WORLD TRADE CENTER TOWERS ACTUALLY “IMPLODE”?
No. They collapsed in an uncontrolled fashion, causing extensive damage to surrounding structures, roadways and utilities. Although when viewed from a distance the towers appeared to have telescoped almost straight down, a closer look at video replays reveal sizeable portions of each building breaking free during the collapse, with the largest sections--some as tall as 30 or 40 stories--actually “laying out” in several directions. The outward failure of these sections is believed to have caused much of the significant damage to adjacent structures, and smaller debris caused structural and cosmetic damage to hundreds of additional buildings around the perimeter of the site.

WHY DID THEY COLLAPSE?
Each 110-story tower contained a central steel core surrounded by open office space, with 18-inch steel tubes running vertically along the outside of the building. These structural elements provided the support for the building, and most experts agree that the planes impacting the buildings alone would not have caused them to collapse. The intense heat from the burning jet fuel, however, gradually softened the steel core and redistributed the weight to the outer tubes, which were slowly deformed by the added weight and the heat of the fire. Eventually, the integrity of these tubes was compromised to the point where they buckled under the weight of the higher floors, causing a gravitational chain reaction that continued until all of the floors were at ground level.

DID THE TERRORISTS PLANT ANY BOMBS IN THE BUILDINGS IN ADVANCE TO GUARANTEE THEIR DEMISE?
To our knowledge there is no evidence whatsoever to support this assertion. Analysis of video and photographs of both towers clearly shows that the initial structural failure occurred at or near the points where the planes impacted the buildings. Furthermore, there is no visible or audible indication that explosives or any other supplemental catalyst was used in the attack.

HOW DOES THIS EVENT COMPARE WITH A NORMAL BUILDING IMPLOSION?
The only correlation is that in a very broad sense, explosive devices (airplanes loaded with fuel) were used to intentionally bring down buildings. However it can be argued that even this vague similarity relates more to military explosive demolition than to building implosions, which specifically involve the placement of charges at key points within a structure to precipitate the failure of steel or concrete supports within their own footprint. The other primary difference between these two types of operations is that implosions are universally conducted with the utmost concern for adjacent properties and human safety---elements that were horrifically absent from this event. Therefore we can conclude that what happened in New York was not a “building implosion.”

ARE THERE ANY PLANS TO EXPLOSIVELY DEMOLISH THE REMAINS OF NEARBY BUILDINGS?
Not at this time, and probably not in the future. Engineering officials have expressed concern over the risk of causing additional damage to sensitive underground liabilities such as subway tunnels and below-grade retaining walls. Therefore any future demolition activities will likely be performed piecemeal, using heavy equipment.

Editor's update 12/20/01- With the removal of the 8-story U.S. Custom's House yesterday morning, all condemned structures have now been removed from the site. Explosives were not used in these operations.

HOW HAVE THE EXPLOSIVES-USING INDUSTRIES BEEN AFFECTED?
Immediately following the attack, the U.S. Department of Alcohol, Tobacco and Firearms (ATF) and other international regulatory agencies suspended all transportation and delivery of explosives. This caused temporary disruptions within the quarrying and construction industries as well as the postponement of several high-profile explosive demolition projects in the United States and Europe (although it should be noted that all of these projects, including the DFW Hyatt Hotel, Jordan Thorpe Towers and Roby-Huntington Bridge, among others, were eventually rescheduled and successfully completed in October 2001). In the interim, the ATF issued warnings related to the reporting of suspicious activities near explosives storage and distribution areas, as well as an advisory regarding the transportation of hazardous materials and the commencement of non-routine visits to persons and locations where explosives are stored and used. It can also be reasonably assumed that other safeguards will be implemented that are not announced to the public.

HOW WILL THIS EVENT AFFECT EXPLOSIVE DEMOLITION IN THE FUTURE?
Although the full effect of these events remains to be seen, certain changes are already taking place. In the short term there will be tighter security on jobsites and stricter regulations on the use and transportation of explosives as mentioned above. Liability insurance will also likely become an issue, as an anticipated rise in rates may affect the economic viability of explosive demolition as an alternative to conventional methods. But perhaps the largest question involves public perception and society’s continued acceptance of building implosions in general, particularly as “entertainment.” It is recognized that there will always be select situations where explosive demolition is viewed as the safest and most effective way to raze a given structure. However industry experts will be watching to see whether the compelling visual allure of these events continues to be successfully exploited as promotional spectacle like many are today, or if the thunderous noise, energy and dust emanating from giant structures crashing to the ground bring forth latent mental images of terror and suffering. History tells us that strong emotions dissipate with time, and that they eventually come to pass. But history has never experienced an event--or an era--quite like this.

October 2001

A more recent paper by Blanchard

©Brent Blanchard / Protec

Engineers Set the Record Straight

Engineers Set the Record Straight on Trade Center Study Results 11/04/02

By Nadine M. Post

Engineers have bombarded several media outlets with letters recently in an effort to correct errors in coverage of the aftermath of the World Trade Center collapse. The issue is exceptionally sensitive, they say, because of pending lawsuits against the developer-owner, the Port Authority of New York and New Jersey.

The first incident involved an Oct. 22 New York Times article and its front-page summary comparing a recent engineering report on the collapse to an earlier one. It called the findings contradictory and implied there was controversy and dispute between the two study teams. W. Gene Corley, senior vice president of Construction Technologies Laboratory Inc., Skokie, Ill., and the leader of the first engineering study team, says there is no contradiction between the two reports and no dispute or controversy. "We did not say there was any flaw in the design of the twin towers or that the trusses contributed to the collapse. We said more study was needed." The second report followed up on that recommendation.

The first investigation was a building performance assessment organized by the Structural Engineering Institute of the American Society of Civil Engineers. The second report was a private study of the destruction carried out for Silverstein Properties Inc., the New York City leaseholder of the trade center.

The other incident concerns an Oct. 27 article in the New York Post that was picked up by the Associated Press, which disseminated the story nationally. The story claimed single-bolt connections in the framework of the World Trade Center's twin towers contributed to their collapse in the Sept. 11 attacks, and attributed the statement to the findings of a team of top engineers from the Massachusetts Institute of Technology.

Corley clarifies that there were "no single-bolt connections." The connection of the exterior columns to the floor trusses was welded, not bolted, directly to the exterior columns through gusset plates at the top chord of the trusses. A damper and two bolts connected the bottom chord to the exterior column. In addition, steel straps at exterior columns were embedded in the concrete topping of the floors, says Corley. The two bolts at the top and bottom chords, used for erection purposes, remained in the final assembly. The truss top chord to core-column connection consisted of a seat with a stiffener plate that contained two bolts, connected to a channel welded to the core column.

Eduardo Kausel, an MIT professor of civil and environmental engineering and the source for the Post story, says the phone interview was about a 160-page unpublished book on the World Trade Center that he and his colleagues wrote that was finished last spring. When the reporter asked Kausel how the towers failed, Kausel discussed the theory that the floor trusses failed first. "I never said one bolt or two bolts," says Kausel. The book's future is uncertain, and it has become out of date because it does not consider the more recent studies of the collapse sequence.

At least two newspapers, including The Times and the Boston Globe, have carried or have expressed plans to print corrections of various errors.

Engineers from the Structural Engineers Association of New York say that errors, innuendo and quotes out of context in the media do a disservice to the public and the engineering profession. They are concerned about the possible impact on a lawsuit filed against the port authority that alleges that design flaws in the trade center led to the deaths of the trapped occupants and firefighters.

BBC reports on WTC investigation

Thursday, 4 October, 2001, 17:37 GMT 18:37 UK

WTC collapse forces skyscraper rethink

By BBC News Online science editor Dr David Whitehouse

The rapid collapse of the twin towers of the World Trade Center (WTC) will force major building design changes to be adopted in the US, and may reduce the number of new high-rise buildings being constructed, according to experts.

There is going to be a debate about whether or not the World Trade Center Towers should have collapsed in the way that they did

Bob Halvorson, architect

The shocking events of 11 September have prompted a major reassessment of the accepted practices employed in erecting skyscrapers.

British and European architects have criticised the current US regulations for ignoring the valuable information that came out of the 1968 Ronan Point tower block disaster in London.

A detailed analysis of WTC collapse suggests that the failure of fire protection systems used in the towers' central columns contributed to their rapid demise.

'Piledriver' effect

In the days following the New York atrocity, engineers were quoted as saying that the towers behaved as they would have expected given the extraordinary circumstances.

The buildings stood for about an hour after being struck

The towers' central steel spines were weakened by the intense heat from the burning aviation fuel, and eventually gave way when they could no longer support the weight of the floors above the crash zones - that is the accepted analysis. When those upper floors began to fall, they forced everything below them to collapse in a "piledriver" effect.

But should the twin towers have collapsed so quickly? Architects and engineers have been looking at the events in more detail, asking what actually happened and what lessons could be learned.

According to some experts, US engineers have never really taken seriously the idea that buildings can collapse in this way. Khalid Dinno, of the Canadian consulting firm Walters Forensic, said that the US waited for 25 years before introducing regulations that had been adopted in Europe.

After the 1968 Ronan Point tower block partial collapse in London, in which four people died, new British Standard Structural Design Codes for concrete were introduced to prevent it happening again. According to Dinno, these codes were ignored in America.

Lack of oxygen

Professor Wilem Frischmann, of the Pell Frischmann Group and the City University, London, was part of the inquiry into the Ronan Point disaster. He said the twin towers should not have collapsed so quickly. "Prior to 11 September, I scarcely believed that this icon was vulnerable," he said.

He added that the impact of the Boeings, puncturing the outer steel shell of the towers would not in itself have caused the towers to fall. "My current analysis of the collapse sequence [suggests that] damage caused to the outside would not have triggered collapse."

Many high-rise buildings in the USA and Britain will need to be re-evaluated because this flimsy dry-lining type of protection has become commonplace

Professor Wilem Frischmann, City University

Although the explosion caused by the fuel-laden aircraft would have been intense, the lack of available oxygen inside the towers would, according to Professor Frischmann, have limited the fireball's temperature to less than 1,000 Celsius. This was within the design limits that the towers were supposed to withstand.

In an analysis of the events that led to their collapse called Obituary For The Two Tall Towers, a report replete with unanswered questions, Professor Frischmann said that sprayed-water fire protection should have maintained the buildings internal strength for several hours, allowing a more complete evacuation.

But the steel supports in the central cores supporting the towers were protected from fire by plaster that had been sprayed on to them. This plaster could have been cracked by the impact, exposing the structural steel to the fire at an early stage.

"Many high-rise buildings in the USA and Britain will need to be re-evaluated because this flimsy dry-lining type of protection has become commonplace," said Professor Frischmann.

Point of impact

According to some experts, the suicide pilots hit the buildings in just the right place to bring about their collapse.

The north tower was struck at the 80th floor; the south tower at the 60th. Had the aircraft crashed lower down, around the 30th floor, it is possible that the stronger steel shell at those levels may have prevented collapse.

The lessons from the collapse will affect future construction

The regulations concerning the construction of future high-rise buildings are likely to be reviewed. According to Tod Rittenhouse, of Weidlinger Associates, who has written a report called Designing Terrorist Resistant Buildings, the design against progressive collapse must be "top of the agenda".

But Bob Halvorson, of architects Halvorson and Kaye, thinks it may prove too expensive to modify existing buildings.

He said: "There is going to be a debate about whether or not the World Trade Center Towers should have collapsed in the way that they did."

The post-mortem on the twin towers will not be swift and will rely on the plans of the buildings, records of its construction, the testimonies of survivors, video of the collapse and forensic examination of the wreckage.

"We are operating well beyond realistic experience," said Halvorson.