Ten years after the world watched two of its tallest buildings collapse on a sunny Tuesday morning in New York, the world is still building skyscrapers. In fact, it is building more than ever.
But the question of whether these skyward behemoths are truly safe if the unthinkable happens again is one that, forgivably, will always cross the mind of a person looking out the window near the top of one.
Experts say that new buildings are safer than ever, but there are no guarantees a structure can withstand another 9-11 type attack. Today's passenger planes are just too big, too fast and as we saw on Sept. 11, their impact on hitting a building is just too unpredictable.
"The best we can do is design buildings that can stand long enough for people to (get out)," says New York architect Barbara A. Nadel, the editor of "Building Security: Handbook for Architectural Planning and Design," a book on post-9-11 construction and design.
The number one lesson learned from an architectural standpoint from the terror attacks is to "think about the ways to get people out of the building," she says.
Mark Green, a civil engineering professor at Queen's University who focuses on the effects of fire on the structural performance of buildings, says some research suggests nothing can be done to save some skyscrapers that are overcome with a massive fire similar to those on Sept. 11.
"Certain tall buildings may collapse just because of fire over multiple stories, let alone any structural damage to the building," he said.
As a result, the emphasis has been placed on worst-case design -- getting everyone out, rather than hoping the building stays up.
The International Code Council significantly upgraded its code after 9-11. Its code for skyscrapers now includes glow-in-the-dark marking in stairways, a third or fourth stairway -- depending on the height of the building -- more fireproofing and numerous other upgrades.
One World Trade Center, the 102-floor building that is being built at Ground Zero, has taken extraordinary steps towards safety. The building's life-safety systems -- emergency stairs, communication cables, ventilations shafts and elevators -- all will be encased in a core wall that is almost a metre thick.
Firefighters will have their own dedicated stairway and perhaps most importantly, enhanced elevators housed in the protected core of the building should be able to serve all floors in case of an emergency.
The overwhelming majority of the more the 2,700 who died in the World Trade Center were those who were trapped above the impact zones of the planes. They had no way out. Elevators were down, stairways were impassable.
The new measures in One World Trade Center would give people above the impact zones a fighting chance to get out.
A new way of thinking
Before 9-11, the concept of a massive fire almost instantly encompassing several of the top floors of a skyscraper was unthinkable, Green said.
"It broke all the assumptions of fire design," he said of the attack.
The World Trade Center towers, like most buildings of the era, were built with fire redundancies that were expected to limit a fire to a single floor.
The National Institute for Federal Standards and Technology (NIST), an agency of the U.S. Department of Commerce, concluded there were two principal reasons for the collapse of the Twin Towers.
First, the impact of the Boeing 767s flying at about 750 km/h severed and damaged support columns, blew off fireproofing insulation, severed the sprinkler systems and dispersed tens of thousands of litres of jet fuel over several floors.
Secondly, the high temperature of the subsequent fires weakened floors and columns with the dislodged fire proofing, leading to the perimeter columns bowing inward and the eventual "pancaking" of the Twin Towers.
This has become known as a "progressive collapse" and it one of the fundamental aspects now taken into account by skyscraper architects. How does one keep the structure intact if multiple redundancies have collapsed?
Green says one of the major lessons of 9-11 is that designers and engineers need to "understand better what happens in fire from the behaviour of the whole structure" of a building rather than how an individual part of a building (say a column or a beam) reacts to fire.
"Typically, the design had been looking element by element in terms of fire performance . . . (but) the World Trade Center collapsed because of interaction between the floor system and the column system," he said.
"It's more than each element on its own, it's how the system works together."
NIST is building a new lab that can test larger structures rather than individual elements, but Green says because fires are so unpredictable, scientific modelling cannot be performed the same way as it can for how a building will react to an earthquake or wind, for example.
"No (fire) lab can simulate a tall building because it is not something we've been effectively able to scale down," he said.
As for older buildings, Green says there is a greater emphasis on inspection of fireproofing and code, but one prominent architect says nothing has been done to prevent another 9-11 type attack on a current building.
"I don't know of any buildings that have gone through a structural retrofit for the purpose of withstanding a major attack like 9-11," Adrian Smit, designer of the Trump International Hotel and Tower in Chicago and the Burj Khalifa in Dubai, now the world's tallest building, told The Associated Press.
New York is the only municipality to actually upgrade its building codes in response to the attacks, Nadel said.
But Nadel notes that the safety of a building is not just an architectural or engineering issue.
"A really comprehensive security plan integrates design, technology and operation," she said.
Building owner and landlords of skyscrapers need well-developed -- and practiced -- evacuation plans and security those goes beyond taking names at the front desk. And of course, the government's various intelligence services and airport security are responsible for safety in the skies.
Safety and security has always been built from the ashes of disaster, and since 9-11 lessons have been learned, changes have been made. The hope is they won't have to be tested.
