The collapse of the historic World Trade Center on 9/11 serves as a stark and stunning reminder to architects and engineers that the modern skyscraper must change dramatically to withstand any future terrorist attacks.
Architects in the post-9/11 world are reaching higher into the skies with the construction of modern skyscrapers that are “terrorist resistant,” including the use of unique, six-inch concrete, bomb-resistant glass, and super-strong steel cores.
The newer skyscrapers are being built as giant glass fortresses, with a sleek open feel compared to the old concrete high rise with block shaped designs.
As America's buildings grow taller, they are being constructed using a unique six-inch concrete in certain areas in the structure that can withstand major blows.
America’s architects are also installing a web-like glass support system in the windows that will "catch an explosion like a baseball mitt.”
Today’s modern skyscrapers are now packed with a laundry list of safety features, including widened stairways that allow for quick evacuation, elevators that function during an emergency, better bonding for the fireproofing materials sprayed onto walls, additional water supplies and enhanced sprinklers, and improved communications and monitoring systems.
“Changes such as equipping a skyscraper with fire-service elevators may seem mundane, but they’re actually 'revolutionary' for the building trade,” said Shyam Sunder, the leader for the federal and fire safety investigation of the World Trade Center disaster, in the aftermath of the terrorist attacks of Sept. 11, 2001.
Elevators now have strengthened shafts, waterproofing and access to electricity even when power to the building is shut off. They let firefighters arrive to the top floors of a building quickly.
The skyscraper improvements also include planters, posts and other landscaping features around the building perimeter so that pedestrians can flow through it easily, while vehicles can't.
This tactic thwarts car bombers, while potential terrorists will encounter thickened, blast-resistant walls. Other lines of defense include bio-terrorism filtering systems in air conditioners that kick on when a biological or chemical attack is detected.
Many high-rises are also tougher to enter. In the immediate aftermath of the attack, Willis Tower installed airport-style security, complete with officers searching bags.
However, experts say the idea of making skyscrapers “terrorist-resistant” is different than making it “terrorist-proof.” A building designed to weather the impact of a jetliner, like the ones used on 9/11, does not currently exist.
"It is much cheaper and more cost-effective to keep airplanes away from buildings, and terrorists away from airplanes," Sunder said. "We don't design buildings for airplane impact and, if we did, it would be cost-prohibitive. And our cities would become fortresses."
The crowning One World Trade Center is currently under construction at ground zero and is expected to be completed by 2013, with the other towers following.
The world famous skyscraper was destroyed during the 9/11 terrorist attacks with nearly 3,000 lives lost.
The 1,776-foot-tall skyscraper is using everything investigators learned from the collapse of the World Trade Center’s twin towers on 9/11.
David Childs is the architect of "One World Trade Center," the building that will replace the two that fell.
He said the first 19 stories of the new building are built like a bunker to protect the structure from the force of a car bomb. Eventually, they will be covered over with bomb-resistant glass.
To prevent the pancaking that happened at the World Trade Center, when one floor fell onto another, the city requires high rises today to be built to prevent a "progressive collapse."
"Skyscrapers have become real-life towering infernos on several occasions over the years, said Miles O'Brien, a PBS science correspondent. "And none of them came even close to crumbling. The steel cores of the Twin Towers were covered in drywall, rather than concrete, no match for the impact of the airliners."
O'Brein added, "Ten years after 9/11, One World Trade Center seems to prove someone is changing the way skyscrapers are built all over the world. Anything less would be an insult to the 3,000 people who died here. And, if it is a success, it may be a model for how tall buildings are built everywhere."
Chicago, for example, adopted an ordinance that requires high-rises to have an emergency evacuation plan on file with the city. And the tallest buildings must provide the fire department with their floor plans so crews know the exact layout of the buildings when they walk in.
The United States is home to some of the world's tallest skyscrapers; eleven American buildings have held the title of tallest building in the world, and every titleholder from 1890 to 1998 was in the United States.
There are nearly 6,000 skyscrapers is New York alone and about 1,100 in Chicago.
Beyond New York City, there is hi-rise construction going on in other parts of the United States. But there is a high-rise real building boom in Asia, particularly China and South Korea, as well as in the Middle East, particularly Dubai.
"All those projects too are being influenced by the lessons learned since the influence of 9/11," Sunder said.
Looking out further into the future, skyscraper safety will continue to improve. For example, the National Institute of Standards and Technology is building a $25 million fire research lab to study how structures respond to fire under load.
A federal investigation of the World Trade Center’s collapse found that dislodged fireproofing material and the resulting multistory fire brought the buildings down, not the airplane strikes themselves. The findings resulted in a laundry list of recommendations to update the international building code.