The 9/11 attacks made us nervous about erecting tall buildings but actually we need them more than ever - after all, they offer a more sustainable solution than urban sprawl

The events of 9/11 were a terrible tragedy and will always be a part of the American, and even global, psyche. But it’s important to remember that while their height made them easier to identify and hit, the World Trade Center towers weren’t necessarily targeted because they were tall buildings. They were primarily targeted because of what they represented - financial power and America’s capitalist system. The Pentagon was also attacked, and it’s only five floors.

We cannot stop building important buildings and particularly tall buildings because we’re afraid someone is going to attempt to bring them down

We cannot stop building important buildings and particularly tall buildings because we’re afraid someone is going to attempt to bring them down. Instead, we have to make our buildings as safe as we can without compromising their significance, their beauty, or the quality of life that goes
on inside and around them. The bottom line is that a building’s design shouldn’t be the first barrier of defence against an attack
from a jet, from unnatural forces. The responsibility for defending against such an attack lies elsewhere.

The structural systems of the World Trade Center towers were a creative and unique solution for such tall buildings, which were designed to rise to their great height in a simple and elegant fashion. A major goal of the structural design was to reduce weight in the building, lightening the structure. A steel frame and Sheetrock core wall, while achieving this goal, proved to be the Achilles’ heel when the building was attacked, allowing the engines of the plane to penetrate the core with its resultant damage.

Additionally, when the planes struck the buildings, the spray-on fire proofing came loose, exposing the lightweight steel to the 2,000ºC temperatures of the jet fuel. A 2-4ft reinforced-concrete core could have withstood the impact and more likely stopped the plane engine, particularly if the exterior structure slowed the jet down sufficiently. Sheetrock couldn’t stop it, even if the jet had been slowed down.

However, what we can learn from this is how future skyscrapers can be designed better in the US by looking at the way buildings are being built in other parts of the world, such as Asia, South America, the Middle East, and in London. The building and fire codes in Asia, where we’re designing a number of tall buildings, are more conservative than they are in the US. These building codes require a reinforced-concrete core, refuge floors located every 13 floors, pressurised vestibules leading to the fire stairs and special elevators for firefighters. In Europe, firemen’s lifts allow the firemen to reach the top of the building quickly, which facilitates easier evacuation for those in need, handicapped people and so on, rather than walking up the stairs to rescue people while people are rushing down to exit the building, as in the World Trade Center.

Lingering concern about the tall building is really dependent upon where you are in the world. In New York, we’re especially sensitive, but it is now 10 years after 9/11 and the concern here has been greatly reduced, with the exception of those who were directly affected. And obviously concern will lessen as time goes on. The reason we haven’t built tall in NYC in recent years is simply because there hasn’t been a market demand. Even before 11 September, there was no need in America to build a new super-tall skyscraper, 80 storeys or higher. The cost is so high - the weak economy didn’t call for it and the real estate market didn’t call for it either.

To replace the World Trade Center towers with 20 10-storey buildings means much more land would be used (over 20 times, including streets), allowing for less open public space

However, I am confident that the tall building is here to stay. The proof is that we’re still building them. In places like New York, Chicago, London, Hong Kong and Shanghai, the skyscraper reflects land value, density, and at times, ego. When you have large populations, without much land to build on, you’ve got to build tall. A city can’t stay vital and deal with growth and the future if tall buildings are eliminated from their potential vocabulary. To replace the World Trade Center towers with 20 10-storey buildings means much more land would be used (over 20 times, including streets), allowing for less open public space.

As a result of rapid world population growth and migration into cities, there is an escalating need for sustainable solutions. KPF believes that increasing density at city centres is more effective in preserving land resources and reducing energy usage, than the alternative of urban sprawl.

Fundamentally altering the way tall buildings are seen today, our work speaks to the promise of the tall building as a sustainable paradigm, in which individual buildings form part of a larger ecosystem of vertical centres linked by horizontal networks of public transportation (even connecting at upper levels with walkways). Rather than objects in isolation, transit-integrated tall buildings represent a sustainable model for future high-rise development.

Gene Kohn is chairman of architects Kohn Pedersen Fox Associates