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How safe are our skyscrapers?: The World Trade Center collapse

The authors are professors of civil and environmental engineering at MIT.

The terrorist act on New York's World Trade Center Towers was the first attack on a mega-city in the 21st century. We are all shocked at the enormity of this horrible act, but we need to look ahead. How do we reduce the vulnerability of our society to a terrorist attack?

One way would be to simply avoid them through anticipation and prevention. The 11th of September, however, proved the limits of that approach.

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 built into a car, which would inflate in progression. This redundancy would not only extend to structural engineering of buildings, but also to other key systems such as transportation.

The Collapse

As the WTC towers sunk to ground zero and below, the logic of a world collapsed. A building designed to rocket toward the sky, imploded into the ground. Ever since, structural engineers all over the country and beyond have been seeking explanations for how the towers failed.

Their collapse highlights that when designing buildings one can only anticipate the worst-case scenario known at the time of construction. If this scenario is known, collapse can be prevented by innovative engineering design of materials and structures. The WTC towers were indeed designed to withstand the impact of a large commercial aircraft. They were not, however, designed to withstand the prolonged effect of fire resulting from a bomb in the guise of a fully fueled aircraft.

Some 60 tons or more of jet fuel could have easily caused sustained high temperatures of 1,500 F and higher. Under these conditions, structural steel looses rigidity and strength. The resulting failure of the 2-3 floor system at the site of impact sent the 30 to 25 floors above free-falling onto the 80 to 85 floor structure below. The enormous energy released by this collapse was too large to be absorbed by the structure below. That impact may have ultimately caused the explosive buckling, floor after floor, of the WTC towers. Similar to a car crash in a wall, the towers crashed into the ground with an almost free-fall velocity.

We do not want to contribute to speculations over whether the collapse mechanisms could have been avoided or modified by another design. We know that the towers were ingeniously designed for the physical and social reality prior to September 11. Even on that day, which changed this reality, the towers did not significantly tilt throughout their failure and they did not fall over. This no doubt avoided an even greater catastrophe.

We need to look ahead and face a new reality.

Are Skyscrapers Still Safe?

Let's say it clearly and loud: All skyscrapers in the country and beyond are as safe as they were on the morning of September 11, 2001--technically speaking. The technical reality did not change, nor did materials or structures change their technical performance. What changed is the physical reality and the social reality, and of course the context in which the definition of vulnerability must now be expanded.

Technically speaking, structural engineers can design skyscrapers for a full-fueled aircraft attack at level X of any building. But is this the answer? There is a social and political context in addition to the technical one. And retrofitting of a local building component at level X will not increase the stability of level Y of a skyscraper, nor the safety of the buildings beside it, nor the city's transportation system. The big picture needs to be considered in reducing the vulnerability of the Mega-City at multiple scales.

New Technology for Design of Skyscrapers and Other Critical Structures?

The key is built-in redundancy in all system components of a complex system such as a skyscraper. A good structural design considers redundancy in structural components and systems. It appears that this structural redundancy made the WTC towers initially withstand both the impact of the aircrafts and the following explosion.

We suggest that this principle of redundancy should now be extended to designing systems with slower progressive failure, to fireproofing, to evacuation planning, to fire-fighter operations, and so on. This is a reasonable challenge given recent advances in materials science and the structural design of large-scale systems. We also suggest that elements of social science be integrated into the design process.

Recent advances in materials science and engineering make it possible to design construction materials for specific performances. Under high temperatures, a new generation of cement-based or ceramic composite materials could be employed in innovative ways on critical structural components, providing redundancy of fire resistance, fireproofing, etc. This would increase the time of dimensional stability of the structural components, thus increasing the time for evacuation.

For each floor, we envision the use of energy-absorbing structural systems to increase the dissipation of impact energy. Self-absorbing materials currently in development for the next generation of crashworthy automotive and marine vehicles could be employed inside key units such as core columns and space separation walls. These materials are lightweight, containing an engineered system of cavities. Their large deformability increases the energy absorption capacity of the structure.

The next generation of skyscrapers must also consider built-in evacuation schemes in their design and operation. By this we mean redundancy in evacuation of the buildings, similar to that for innovative tunnel structures such as the Channel tunnel connecting England and France. During the "Chunnel" fire in 1996, which lasted for some 10 hours, casualties were avoided thanks to a fire-proof 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, which would prevent the spread of fire and smoke, such a scheme could provide redundancy for evacuation.

Efficient evacuation will, first and foremost, include protections for firefighters. Wireless and wearable information technology, for example, could provide firefighters with information about structural stability, fire location, obstacles, escape routes, and so on. This technology is already available. It is currently being tested for ground military operations and aerospace applications.

Can We Retrofit?

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

Technically speaking, we can retrofit the impact resistance of skyscrapers by multiple means. For example, we can improve the fire protection by engineered protective systems. We can also improve the communications facilities, and redesign evacuation plans.

But the key issue remains redundancy at all scales of the critical components that form the backbone of our Mega-Cities, and society at large. Redundancy is required in structures and evacuation, but also in the national transportation network. The events of September 11 highlighted the lack of such redundancies; air traffic was frozen nationwide.

Redundancy of the transportation network would require an enrichment of the current transportation matrix. Adding high-speed trains for commuting between mega-cities, for example, would reduce dependency on one single mode of transportation.

Redundancy in other dimensions of society can also be engineered, including economic activity. Given the new physical and social reality created by the September 11 events, it will be up to the societal and political leadership to decide when and how the technology of redundancy will be implemented in our mega-cities. Redundancy appears to us as the most efficient way to reduce the vulnerability of the mega-city to man-made and natural disasters in the changing environment of the 21st Century.

As society, we must remain intact and on the move adjusting to the changing conditions. The best we can do is to respond to such tragic events in a decisive and swift way so that they will not occur again, and their impact on our mega-city will be minimized.

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