World Trade Center collapse: assessing the events

The logic of a world collapsed as the WTC towers, designed to rocket to the sky, imploded into the ground. Structural engineers are seeking for explanations of how the towers failed. Our approach is not cynical technical obsession. We don't know whether it is our way of facing a new, previously unimaginable reality, or retreating into engineering's world of sense and logic.

With the limited information available right now, we are looking at a three-step failure mechanism that occurred at different scales of the two WTC towers.
1. The impact of the plane at one or several floors of the WTC: The building was designed for the horizontal impact of a large commercial aircraft, and the towers did indeed withstand the impact and the following explosion which may have destroyed one or more floors. This destruction should have locally reduced the resistance of the columns in the core (and eventually also at the perimeter) to buckling. Even so, the resistance was strong enough to carry the loads of the upper floors for approximately one hour.
2. The failure of the impacted floor system: The explosion following the impact may have destroyed some of the thermal insulation around the steel columns. The 60 tons of flaming jet fuel easily raised temperatures to 1,500° F or more. Exposed to such intense and prolonged heat, structural steel loses rigidity and strength. This may have caused a reduction of the buckling force of the connected 2-3 floor structural system. The column-bracing system was subjected to the load of the top floors. After about 60 minutes, local buckling of the columns became inevitable. Compared to a ductile failure, the floor system failed in a brittle way, explosively releasing the energy stored in the system. The tower with the higher load above the impacted level collapsed first, but both towers showed almost the same failure mechanism.
3. Dynamic crash of the remaining structure: The failure of the 2-3 floor system set a 25- to 30-floor mass free-falling onto the 80- to 85-floor structure below. The enormous kinetic energy released by this 2-3-floor downfall was too large to be absorbed by the structure below. The impact of this upper part onto the lower part was surely much higher than the buckling load of the columns below, which to this point may had been largely undamaged, and may have ultimately caused the explosive buckling, floor after floor, of the WTC towers. Similar to a car smashing into a wall, the towers crashed into the ground with an almost free-fall velocity.

We do not want to contribute to speculations of whether the collapse mechanisms could have been avoided or modified by another design. The towers were ingeniously designed for conditions prior to Sept. 11. Even on this momentous day, the towers did not significantly tilt throughout their failure, which no doubt avoided an even greater catastrophe of massive destruction far beyond Lower Manhattan.

We need to look ahead and face a new reality.

Let's say it loud and clear: all skyscrapers in the country and beyond are as safe as they were on the morning of Sept. 11, 2001, technically speaking! The materials and structures did not change their technical performance for which they were designed. What changed is the physical and social reality, and the context in which the definition of vulnerability must now be expanded. When designing buildings, one can only anticipate the worst-case scenario known at the time of construction, and try to prevent collapse by innovative engineering design of materials and structures. The impact of a large commercial aircraft followed by a fire was a potential threat when the WTC were designed. A deliberate attack by a fuel-laden plane at a height of 85 stories was not anticipated. But will it be in the future?

Technically speaking, structural engineers can design skyscrapers to withstand a fully fueled aircraft attack at level X of any building. But there is a social and political context in addition to the technical one! Retrofitting a local building component at level X will not increase the stability of level Y of a skyscraper, nor affect the safety of the building next door, nor guard the city's transportation system. The big picture must be considered in reducing the overall vulnerability of the Mega-City.

What we suggest is a built-in redundancy in design and operation of the Mega-City in emergency situations, similar to a second or third airbag installed into a car which would inflate in progression during a particularly bad accident.

The keyword is "built-in redundancy," since a good structural design considers redundancy in structural components and systems. This structural redundancy enabled the WTC towers to withstand direct impacts by the planes and the subsequent explosions. Given the new physical and social reality, we suggest that this principle of redundancy should be extended to the designs of other systems with slower progressive failure, such as fireproofing, evacuation planning, fire fighter operations, and so on. This is a reasonable challenge given recent advances in materials sciences, and structural design of large scale systems. Furthermore, we suggest that these technical elements as well as the elements of social science be integrated into the design process.

Recent advances in materials sciences make it possible to design construction materials for specific performances. A new generation of fine tuned cement-based or ceramic composite materials could be employed in innovative ways with multiplicity on critical structural components, providing redundancy of fire resistance, fireproofing, etc. This would increase the time of dimensional stability of the structural components, and allow more time to evacuate a building.

On a floor level, energy-absorbing structural systems could increase the dissipation of impact energy. Such self-absorbing materials are currently in development for the next generation of crashworthy cars and boats. These lightweight materials contain an engineered system of cavities, and their large deformability increases the energy absorption capacity of the structure. These developments could be extended to large-scale material systems, and employed as in-fill material of the core column, space separation walls, etc, would offer a redundancy of the energy absorption mechanisms.

The next generation of skyscrapers will consider built-in redundant evacuation schemes in their design and operation, similar to innovative tunnel structures such as the 30-km-long "Chunnel" connecting France and England. During a 10-hour-long blaze in the Chunnel in 1996, everyone trapped underground escaped safely through an emergency tunnel. A similar emergency tunnel could be built vertically into the next generation of skyscrapers with appropriately designed redundant elevation systems. In connection with vertical firebreaks to prevent the spread of fire and smoke in vertically erected systems, such a built-in evacuation scheme could provide redundancy for evacuation management.

An efficient evacuation management system must include protection for the firefighters. As it develops, wireless and wearable information technology could provide firefighters with the necessary information about structural stability, fire location, obstacles, escape routes, and so on. The technology is currently being tested for ground military operations and aerospace applications.

Do we need to retrofit our Mega-Cities for the new physical and social reality created by the Sept. 11 events? Clearly, this is a political decision, not a technical one!

Technically speaking, we can increase the impact resistance of skyscrapers by multiple structural means. We can improve the fire protection by engineered protective systems, enhance the communication facilities, redesign evacuation, and so on.

by Prof. Eduardo Kausel

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.

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.

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.

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.

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.

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.

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.

Technology Review's web page includes an extremely interesting story interviewing more MIT engineers and architects on the collapse: http://www.technologyreview.com/web/special/roush.asp