As I anxiously watched the TV coverage of the terrorist attack on the World Trade Center towers, my training in Structural Engineering instantly elicited in me visions of doom, and a feeling that the towers were in imminent danger of collapse. Still, knowing that in 1993 the towers had resisted massive damage in a terrorist attack, and being unaware of similar cases of skyscraper collapse, I hoped against reason that they might survive yet again. To my horror, I then witnessed the unthinkable unfolding in front of my eyes. In retrospect, I should have been 100% sure that they would fail, but the idea was so disgusting that I allowed my wishful thinking to prevail instead. Soon after the tragedy occurred, cooler thoughts and the engineer in me returned, and I began to ponder about the mechanics that led to the catastrophe.
Why did they collapse?
There were three causes for the massive structural damage that led to ultimate failure: the impact of the aircraft, the subsequent explosion, and most importantly, the raging fire caused by the vast amounts of jet fuel. Burning fuel must have also cascaded down floor openings to the levels below.
The towers were reportedly designed for the impact of a Boeing 707 aircraft, the largest of its day. The takeoff weight of a fully loaded Boeing 707 320 is 336,000 lbs., including 23,000 gallons of jet fuel, while the maximum takeoff weight of a Boeing 767-200 is some 395,000 lbs., with 24,000 gallons of fuel. (The fuel accounts for roughly half the weight of a fully loaded aircraft). Thus the 767 is not vastly larger than the 707, and it carries approximately the same fuel load. In addition, both ill-fated planes were only lightly loaded with passengers, so they did not carry their full takeoff weight. The implication is that the buildings may indeed have been designed for the impact load caused by a commercial airliner, but the designers never considered the ensuing inferno from the fuel. Suggesting that the buildings were designed for the crash of an aircraft is ultimately self-delusion-and perhaps public relations-on the part of the design team, because other aspects of a crash, i.e. the explosion and fire, were not taken into account. Perhaps the probability of such an occurrence was deemed insignificant.
From information available on the web, it appears that the weight of each building was mainly carried by an inner core of columns surrounding elevator shafts and stairways, while a dense lattice of external columns spaced 39 inches on center formed an outer tube intended principally to prevent the building from overturning when subjected to strong lateral forces, such as those elicited by hurricane winds. The floors were supported by a grid of truss beams that carried the weight of the floors to the inner core, while the floors in turn provided lateral support that prevented buckling of the columns.
The North Tower was hit at 8:46 AM above the 96th floor, and remained erect until 10:28 AM, nearly two hours after initial impact. By contrast, the South Tower was hit at 9:03 AM above the 80th floor and collapsed less than an hour later at 9:59. The damage to the latter was more severe, perhaps because the second plane traversed the building at an angle and blew off external columns on two adjacent faces. This asymmetry, combined with the greater weight of the 31 stories above the crash elevation led to some tilting of the upper portion down the damaged corner, causing large overturning forces in the remaining members of the floor.
Memorial posters in the Bldg. 10 lobby filled up with tributes to those who died on Sept. 11, including seven MIT alumni. The Alumni Office has set up a web page on
The initial impact of the aircraft caused massive structural damage to the external columns, to the floors in the proximity of the impact, and perhaps also to parts of the inner core. The ensuing explosion must have significantly exacerbated this damage, possibly collapsing several floors, and setting the buildings ablaze in a virtually uncontrollable, fierce fire. Still, both buildings did not give way for a remarkably long period of time after the crash. This extraordinary capability allowed many lives to be saved, and is a major credit to the designers. Ultimately, however, the intense fire heated the structural steel elements well beyond the thermal limit of some 800° F, which caused the steel to lose resistance or even melt. Supporting members gave way, initiating the final failure of the building.
Various mechanisms may have been at play in this failure. Witnesses who escaped the buildings reported seeing large cracks develop on the walls of the staircases. This would suggest a steady redistribution of vertical forces and propagation of structural failure down the building. However, the immediate failure mechanism was almost certainly initiated locally at the elevation of the crash. Truss beams heated by the fire were probably more vulnerable than columns, and may have been the first to go. As parts of the floors then collapsed and rained down onto the floors below, the weight of the accumulating debris steadily increased beyond the support capacity of those floors, and they collapsed in turn.
At the same time, local collapse of the floors caused the heat-weakened columns to lose their lateral support, and to buckle and collapse under the intense weight of the floors above the level of the fire. At that point, the upper floors began to fall wholesale onto the structure below, and as they gained momentum, their crushing descent became unstoppable. Indeed, with two fairly simple dynamic models, I determined that the fall of the upper building portion down the height of a single floor must have caused dynamic forces exceeding the design loads by at least an order of magnitude. There was no way in the world that the columns below could have taken this large overload, and these failed in turn and collapsed, creating a domino-effect down the building. The towers then collapsed in practically a free fall.
Why did they not fall like a tree?
Some observers have wondered why the buildings telescoped down, instead of overturning and rolling to their side like a tree. Unlike trees which are solid, rigid structures, buildings such as the WTC towers are mostly open space (offices, staircases, elevator shafts, etc.). Indeed, a typical building is 90% air, and only 10% solid material. Thus, it is not surprising that a 110- story structure should collapse into 11 stories of rubble (actually less, because the rubble spreads out laterally, and parts are compressed into the foundation).
In addition, the towers did not fail from the bottom up, but from the top down. For a portion of the tower to roll to either side, it must first acquire angular momentum, which can only occur if the structure can pivot long enough about a stable plane (e.g. the stump in a tree). However, the forces concentrated near the pivoting area would have been so large that the columns and beams in the vicinity of that area would simply have crushed and offered no serious support permitting rolling. Also, both building sections above the crash site were not tall enough to significantly activate an inverted pendulum effect. Thus, the upper part could do nothing but simply fall down onto the lower part, crushing it. While photographic evidence shows the upper part of the South Tower to be inclined just as it began to collapse, it may not necessarily have rolled to the side, but instead fallen down onto the lower floors in a tilted position. (A careful review of collapse videos and additional photos should help clarify this contention.) Indirect evidence points to minimal vertical resistance to telescoping or pancaking of either tower: the duration of the collapses was nearly the same as that of an object in free fall, while any serious resistance would have slowed down the collapse. In essence then, the towers did not collapse like trees because the structures, despite their strength, were too fragile to sustain such motions.
During the dedication of the Memorial Wall on Sept. 14, members of the community set lighted candles afloat in the moat surrounding the MIT chapel. Photo: Donna Coveney/MIT
Corollary to the WTC collapse
An important lesson from the WTC collapse is that buildings are like chains in that they are only as strong as their weakest link. If the structural integrity of any floor in a building should be seriously endangered by a blast or a massive fire (perhaps excepting the very top floor or those immediately below it), that building is highly likely to collapse and pancake to the ground. However, inasmuch as catastrophic damage to all load bearing members is very rare and the vast majority of modern high rise buildings are well-engineered and designed to resist office fires (but not jet fuel fires), these buildings are and will continue to be very safe indeed.
Can we design buildings to resist collapse?
The answer to this question depends on what is meant by design. If we make buildings as solid as the containment structures in nuclear power plants, it might be possible to design not only for impact and blast forces, but also for the massive fires caused by the jet fuel. But nobody would wish to live or work in such fortresses. In addition, they would be unbearably ugly. From a practical viewpoint, the chance that any individual building out of hundreds of thousands (millions?) in the nation might suffer an attack is so small that it would not make economic sense to make them jet-crash proof. (But do not confuse this chance with the probability that some building in the US may be hit this way.) As for retrofitting existing buildings, my view is that making them jet-crash proof would make no sense whatsoever. However, it would make eminent sense to retrofit at least some buildings, perhaps as part of an overall escape system overhaul, to ensure that load bearing elements have sufficient thermal protection and the buildings can survive a fierce fire for several hours. By providing adequate redundancies in the form of both alternative escape routes and sufficient escape time, we can prevent deadly consequences to people even when we should not able to avoid ultimate structural collapse. These improvements may be needed if for no other reason other than to allay the concerns of people whose fear of a similar tragedy will persist for years to come. I, for one, would not wish to live or work in a mouse trap with insufficient escape routes.