"We have a long history of successful steel construction in this country and, in fact, the world," says Jonathan Barnett, professor of fire engineering," on the PBS NOVA documentary, "Why the Towers Fell." "And one of the great successes is that under normal fire conditions, we don't have building collapse. In fact, until 9/11, I was unaware of any protected steel structure that had collapsed anywhere in the world from just a fire."

Indeed, with the National Institute of Science and Technology technical conference reporting their findings from the building and fire safety investigation of the World Trade Center disaster under way at the time of this writing, it is important to reexamine the effects that impact and fire had in causing the collapse of WTC 1, 2 and 7.

The new NIST report specifically highlights the need for compartmentation (Recommendation #4), according to the International Firestop Council. Compartmentation, in fact, is proven to increase the safety of buildings by slowing and limiting flame and smoke spread, so it is no surprise that NIST found compartmentation to be critical to structural integrity and life safety.

Additional information and misinformation has abounded in the past four years. Examining some of the questions and conclusions will offer anyone involved in steel construction additional insights to prevent future tragedy.

The fall of the Towers

On Sept. 11, 2001, two commercial jets struck the north and south towers of the World Trade Center, resulting in total structural failure due to impact and subsequent fire. Watching the Twin Towers' collapse, and the collapse of WTC 7 at 5:20 pm that same day, one is reminded of the dynamic of controlled demolition. In fact, numerous claims have been made that only explosives could have brought the buildings down in the manner in which they collapsed. Several demolition experts weighed in on this idea.

"Explosives used to demolish steel are called ‘linear-shape charges," says Bill Moore, of Brandenburg Industrial Service Co., and former president of the National Demolition Association from 2003-2005. "They cut steel like a hot knife through butter and leave a very distinctive looking cut plus a copper residue. Just putting explosives on a piece of steel would do nothing unless the amount was huge. That huge amount would have blown out every window in Manhattan from the sound pressure."

Both the NIST and FEMA completed analyses that were sometimes contradictory in explaining the fall of the WTC.

FEMA's initial explanation was that when the floors collapsed, this left tall freestanding portions of the exterior wall and possibly central core columns. As the unsupported height of these freestanding exterior wall elements increased, they buckled at the bolted column splice connections, and also collapsed. This is now known to be incorrect.

"It is now known that the floor system was extremely robust," adds Charlie Carter, P.E., S.E., director of engineering and continuing education for the American Institute of Steel Construction, in Chicago. "As were the connections to the exterior walls. NIST's report provides photographic evidence of this. In fact, NIST also reports the floor trusses pulling in the wall. One can't have it both ways. In the collapse, the debris descending did tend to strip the floor framing off of the wall framing but that is entirely different."

Another demolition expert who worked at Ground Zero also finds no trouble debunking the claim

of explosives.

"Our team, working at Ground Zero, including myself, never saw indication of explosive use that would have been evident after the event," says Brent Blanchard, senior writer for www.implosionworld.com. "You just can't clean up all the det cord, shock tube, blasting cap remnants, copper backing from explosive charges, burn marks along clean-cut edges of columns, etc., nor is there any evidence in the thousands of photos taken by the press and dozens of agencies over the following days. I just can't see how it happened that way."

Moore adds to the counter demolition theory.

"Implosions are initiated by weakening structural members with explosives," he explains. "The steel in the WTC buildings was weakened not only by the crash but by the intense heat from the jet fuel fire. Thus, instead of explosives, fire was used to weaken the steel. Once the steel gave, the weight of the upper floors collapsed onto the lower floors, thus creating a domino effect. As an example of the damaging effect of fire on steel, in 1967, the original heavy steel-constructed McCormick Place exhibition hall in Chicago collapsed only 30 minutes after the start of a small electrical fire."

Although the buildings were designed to withstand a collision with a 200,000-pound Boeing 707, that theory obviously was never tested before. Nor did the designer claim to take into account that thousands of gallons of jet fuel might someday be spread throughout the floors of a crash area and down both elevator and other vertical shafts, according to Moore. He was also one of the few experts who was not surprised that the buildings collapsed.

"I was more surprised the buildings didn't collapse immediately upon impact," Moore says. "The buildings were designed so that the exterior 39-inch on-center, 14-inch square steel box columns produced a building resembling a steel tube. The center steel core carried gravity loads only. One of the videos shows the plane going in one side and coming out the other, thus compromising a major portion of the structural integrity of the building. We were very fortunate that the buildings didn't fall horizontally and strike surrounding buildings."

"The structure was only designed to carry the weight of the contents, snow load, wind load, etc.," Carter adds. "Once you add the momentum of the upper floors crashing down, the lower floors would collapse, as well."

Moore also remains unconvinced of the presence of explosives.

"An analogy," he says, "is the classic cheerleader-type pyramid of bodies: If the three people on top suddenly fall all at the same time, everybody on the bottom is probably going to fall as well and the pyramid crumbles. Also, when buildings are imploded, they place explosives on the lowest floor(s) and at least one more upper floor to start the momentum. If you just shot the first floor, the rest of the building might remain supported by the second floor and not completely fall."

The unforgettable fire

Some of the steel inspected was bare, the foam fireproof completely absent from the components. The theory is that the impact from the jets "blew" the foam off the steel, making it more susceptible to heat or destroyed completely by the crashes.

"The fireproofing material I've seen sticks very well and a ‘jolt' would not knock it off," says Moore. "If the steel was directly hit knocking off the fireproofing, then the steel would have been damaged and weakened, as well."

Indeed, one thing all parties agree on is the loss of insulation would be restricted to the impact zones.

"The loss of insulation occurred throughout the impact area and most damaging, to the core columns that carried more than half of the building's weight," explains Matthys Levy, chairman of Weidlinger Associates Inc., a company that did an independent study for WTC owner Larry Silverstein, and one of the experts of the PBS documentary. "As they were heated, they lost strength to the point when they were no longer able to support their loads."

Carter reminds that fire protection material was never intended to make steel "fireproof," only "fire-resistive." Technically, there is no such thing as "fireproof."

"Even asbestos turns into a glass-like material if heated enough and concrete will spall, and eventually disintegrate with enough heat," he says. "In the example of the McCormick Place fire in Chicago, the 50-foot-high steel roof collapsed within 30 minutes after tradeshow booths caught fire, a fire certainly less intense than jet fuel."

Again, the McCormick Place that burned to the ground decades ago was unprotected-an investigation revealed two violations of the building code requirements of the time: Not only was it built with inadequate protection of the structure, its combustible contents were inconsistent with the usage that was assumed in its design criteria.

Dr. Saeed Mirza, professor of civil engineering and applied mechanics at McGill University, Montreal, Canada, agrees that the fire was enough to initiate total structural failure.

"The tube construction was designed to be strong, to take earthquakes, wind or a hit from a 707," Mirza explains. "The fact that two planes hit the two buildings and they stood before collapsing shows the design was OK.

"Pure oxygen and carbon could give temperatures of 3,000 degrees C (5,432 F)," he explains. "But when you have hydrocarbons and air, what we call ‘diffuse flame,' the maximum temperature is 1,000 degrees C (1,832 F) and this is observed all around the world in cases of fires.

"At temperatures of 120 degrees C (248 F) until 300 degrees C (500 F) or so, there is no change in the properties of steel or concrete, no loss of strength. Beyond that, steel loses strength. At 800 to 1,000 degrees C (1,472-1,832 F) it is 20 percent of what it should be.

"That being the case, the fire on these floors had a consequence: The truss floors sagged due to the temperature and pulled on the fasteners to exterior columns," he continues. "Remember, the columns can support the load itself, they are very strong. After the planes hit and eliminated some columns, the rest of the columns took the load.

"If the steel reached 1,000 degrees C (1,832 F)," he continues, "you lose support from the trusses. These columns now become ‘long,' not supported over three or four stories (due to sagged trusses) and the columns' weak point buckled. As they buckled, the floors above collapsed.

"Each floor could easily support two to three times its floor weight," Mirza continues. "If only a few floors had sagged, the building would not have collapsed. But once upper floors collapsed onto floors that had fires, the fasteners ‘unzippered,' from the floor slab, unable to resist that high of load."

An aspect of the collapse that has caused some confusion is the failure of the "angle clips" supporting the floor trusses as Achilles' heels.

"When the NOVA special aired a few years ago, I got a little angry when they said things like, ‘The pin connections had long been suspected," comments Don Allen, PE, of the Steel Framing Alliance. "Well, those pin connections, and the trusses, and the entire structural and life safety system did great under nominal and design loads. If someone whacked your legs with a baseball bat, and you fell to the ground, would it be fair for an analyst to say that part of the reason for your collapse may be your suspect kneecaps? I think not."

There are other misconceptions regarding angle clips."

"Although often called "angle clips," the fasteners were actually welded gusset-type connections at the ends of the trusses," Carter says. "The angle supports were only there for the purposes of steel erection with the welded gussets added after and carrying all the load. This also contradicts the failing of the angle-clip bolts theory. These were erection bolts and they carried no load in the completed structure."

The discrepancy noted by skeptics of the official story here is the intensity and the duration of fire, and whether it was enough to initiate total collapse.

A FEMA photograph shows survivors at the impact zone. If it was an intense enough fire to weaken steel, how could people be hanging around the impact zone, many have asked.

"Remember, fire alone did not cause these collapses," Carter emphasizes. "These fires were coincident with massive structural and non-structural damage, very different from the idea that fire alone is responsible."

Gravity of the situation

The dynamics of the collapses are unique to all three WTC buildings in that they collapsed into their own footprints in less than 16 seconds. How could these buildings collapse so quickly? Wouldn't there be some resistance to a freefall at the rate of gravity?

"We think the idea that the collapse was essentially a free fall is incorrect," comments Dr. Louis F. Geschwindner, PE and professor of architectural engineering, Pennsylvania State University. "The part of the tower above the zone of plane damage started down when the remaining strength at the damage location was exceeded. The structure below provided upward resistance but not enough to overcome the progression of failure directly under it.

"As the failure propagated down, the failed elements added to the mass acting downward, increasing the load incrementally as the mass traveled down. This pattern occurred all the way down, increasing at the rate of gravity."


Almost unnoticed by most on Sept. 11 was the collapse of WTC 7, which occurred about 5:20 pm that afternoon. Although not struck by a jet, the building suffered damage from debris-in addition to a few other factors.

"WTC 7 had small fires but big problems," explains Levy. "The building had internal oil storage that fed the fires and furthermore, the building had trusses that spanned over the Con Ed Vaults. These trusses were heated by the fire and failed after seven-plus hours."

Yet, there is still confusion regarding this collapse that NIST should provide insight to in its conference, which will be reported on in W&C in an upcoming issue. Even FEMA's report offered no conclusions about WTC 7. According to FEMA, "there is limited information about the ignition and development of the fires at WTC 7, as well as about the specifics of fuels that may have been involved during the course of the fire. It is likely that fires started as a result of debris from the collapse of WTC 1."

NIST has claimed significant damage on the South face of WTC 7 from the impact of debris from the collapsing towers and that this debris initiated the fuel leaks and fire in WTC 7, and contributed to the collapse.

"Ultimately, the failures were due to a complex interaction between structural damage (loss of structural components upon impact), non-structural damage (destruction of all fire and life safety systems upon impact) and fire effects," says Carter. "Thus, it cannot be said that the WTC is the only fire-protected steel building to completely fail structurally from fire. It was heavily damaged and had no fire protection in its damaged state."

Stay tuned to W&C for a report on the NIST meeting and its results.