A decade after 9/11, new innovations in skyscraper design

Defying expectations, high-rise design and construction has flourished around the world since the terrorist attacks of Sept. 11, 2001. Meet the next generation of high-performance skyscrapers.
Written by Reena Jana, Contributor
A graphic of a 9/11 plane-crash hole in one of the World Trade Center towers 

NEW YORK --Each day in lower Manhattan, rays of morning light creep over the sprawling roof of the Century 21 department store; along the chipped, centuries-old edges of the gravestones next to Trinity Church; and through the green metal bars of the entrance to the Cortlandt Street subway station. That station remains closed, though, a chilling reminder of the horrific events of September 11, 2001, when terrorists hijacked two Boeing 767 passenger jets and drove them into the sides of the two towers of the World Trade Center across the street.

Ten years after that tragic event, sun finds its way among the shadows. Light pours onto freshly hardened concrete, just-hung sheets of glass, and newly erected steel beams as construction progresses toward the completion of all 1,776 feet of a newer, taller tower at One World Trade Center.

The reborn site is where a skyscraper sometimes referred to as the "Freedom Tower" is taking shape, its completion date scheduled for 2013. At 78 floors high, it is the largest and most prominent of a new collection of New York City structures at Ground Zero intended to replace what was destroyed, memorialize what was lost, and most importantly, signal what will be.

A rendering of Manhattan's future skyline when the new One World Trade Center is completed

Above all, though, the massive civic project indicates that the next chapter in international skyscraper design has begun. As a new urban silhouette emerges in Manhattan, a renaissance in tall buildings is blooming across the globe. Today, it's hard to believe that such a design trend started at a time when some architects feared that the skyscraper era was coming to an end.

"After 9/11, asking whether the skyscraper was 'over' was a legitimate proposition, as it was clear that high-rises were terrorist targets," said Cathleen McGuigan, editor-in-chief of the trade publication Architectural Record. "In fact, the opposite happened."

In some ways, though, the pre-9/11 notion of what a skyscraper should be is over, as the new tall buildings that are on the rise are designed to be higher, stronger, safer, and smarter than those of the Twin Towers' epoch.

Since the 9/11 attacks, architects who specialize in designing high-rises for major cities have worked to conquer design challenges that simply did not exist before 2001. Unprecedented security concerns and new safety regulations and codes -- not to mention lingering terrorist fear in the minds of occupants -- have required a new approach to tall structures in the post-9/11 age.


Many fundamental changes to skyscraper design were the direct result of regulation enacted in the wake of the destruction of 1 and 2 World Trade Center. These largely involve the use of redundant building systems to prevent the "progressive collapse" of a structure, where a series of local failures spread until the entire structure is at risk.

Research conducted by the U.S. National Institute of Standards and Technology on the collapse of the Twin Towers (published in 2005 and available here as a .pdf) and 7 World Trade Center (published in 2008 and available here as a .pdf) outlined in extensive technical detail how the structures failed and offered 30 recommendations to improve the design of high-rises.

Among them: increased structural integrity through alternate load-bearing paths, fire resistance by design ("burnout without collapse"), better building evacuation routes and redundant systems for sprinklers, hoses, fire alarms and smoke management.

"Had the building been significantly more than one-third to one-half occupied, the casualties would likely have been far higher," the NIST report authors write of 1 WTC's design. "The exiting population would have exceeded the capacity of the stairwells to evacuate them in the time available."

Many of the NIST’s recommendations have been adopted, with others planned soon. In 2009, the International Code Council added 23 code changes to the International Building Code and the International Fire Code based on the NIST's report on the World Trade Center's destruction. Some of the changes included a requirement to increase the width of all stairways by 50 percent in new high-rises and strengthened criteria for placement and inspection of sprayed-on fire-resistant materials. State and local governments use these codes as a base for their own building and fire regulations.

Last year, the ICC approved 17 additional code changes based on the NIST’s investigation, including the improvement of emergency radio communications for first responders and elevator safety. The changes will be included in the 2012 International Building Code and International Fire Code.

A floorplan of One World Trade Center, with multiple exits shown

Many of these concerns are addressed in the design for the new tower at One World Trade Center. The building will feature extra-wide stairs and multiple exits, numerous backup emergency-lighting systems, and concrete protection for sprinklers.

Skidmore, Owings & Merrill, the building's architects, say they have exceeded the NIST's recommendations and New York City building codes, adding filters to protect against biological and chemical attacks; designating areas on every floor for occupants to gather in an emergency; and dedicating a staircase for use by emergency responders.


One way that architects at SOM and other firms are meeting new building codes that surfaced after the destruction of the original World Trade Center towers is through a borrowed-innovation approach, in which multi-national firms take their most inventive responses to strict safety and security regulations and import them into nations where regulations are less demanding, including the U.S. The approach can reduce the time needed for original research and development.

“Some architects have been translating from developing European safety standards and building codes in other parts of the world,” said Carl Galioto, managing principal at HOK and a contributor to the 2003 book Building Security: Handbook for Architectural Planning and Design.

“For instance, in Europe, there are standards for service elevators that firefighters can use in an emergency. This also exists in Chinese building codes," Galioto said. "Now architects are bringing those here, to the U.S.”

Protections around the entire perimeters of building sites and extra space for security-screening areas and turnstiles that record entries and exits are also much more prevalent in new construction since 9/11, he said.

The architects of SOM say they mined their extensive global portfolio for the design of the new One World Trade Center.

“We looked at our designs for foreign embassies and courthouses and brought that knowledge to bear on the new One World Trade Center’s tower,” SOM director Kenneth Lewis said in an e-mail. “We also looked at how buildings are constructed in other major cities and decided to push for a concrete core instead of an all-steel core. We examined how we provide a design that may bend but will not break.”

The firm designed the concrete core to encase all of the building’s safety systems, including the tower's communication antennae (to help ensure communication with first responders in a disaster), as well as exhaust and ventilation shafts and elevators.

For one of the most symbolic and highly anticipated skyscrapers in architectural history, the approach helps to guarantee SOM a design strategy that is both tested and efficient.

“These [building details] sound like subtleties, but they all come together in the design to provide a class A office space in the most demanding market in the world and meet the security and safety requirements this location demands,” Lewis said.

So far, the strategy is working. Earlier this year, publisher Conde Nast (Vogue, The New Yorker, Vanity Fair, Glamour) signed a $2 billion, 25-year lease to be an anchor tenant when the building opens in 2014--leaving behind a glassy high-rise at 4 Times Square, a prestigious address in Manhattan's Midtown business district.


In the immediate aftermath of 9/11, it may have seemed possible for security concerns to outweigh the cultural role that skyscrapers play as trophies of industry and milestones of human engineering. But today, supertall structures -- defined by New York’s Skyscraper Museum as buildings more than 1,250 feet tall, or the height of the roof of the Empire State Building -- remain in vogue both here and abroad.

The new One World Trade Center fits the bill, of course, but it's hardly the tallest, despite its long silhouette. According to two Global Supertall Surveys conducted by the Skyscraper Museum in 2007, 35 existing and planned buildings around the world surpass the 1,250-ft. mark. The most prominent of them is the 2,651 ft.-high Burj Khalifa in Dubai, a mixed-use tower designed by SOM that has held the record for tallest structure in the world since its completion in 2010.

Despite an international economic downturn, there are now 48 supertall structures that make the cut, according to this year’s Global Supertall Survey. Each new addition carries more safety and energy-efficient features than its predecessor, and top American architectural firms such as SOM, Kohn Pedersen Fox Associates, and Gensler are designing skyscrapers in Asia and the Middle East that are setting new standards for supertall structures in terms of durability, efficiency and aesthetics.

A comparison of the number of supertall buildings in 2007 (left) vs. in 2011 (right)

“The most difficult aspect of designing any significant building post-9/11 is three simple words: high performance design,” Lewis said. "High performance design is what drives our high-rise practice; energy efficiency, indoor quality/comfort, space efficiency/usability, safety and security are all about performance."

"Security is of the highest concern, a concern that must be balanced with making a place that people want to be in, live in and work in," he said. "Otherwise there is nothing worth securing or protecting from acts of God or man.”


Did the Sept. 11 attacks really spark changes in skyscraper design across the globe, or did the event merely serve to mark a new chapter in an ongoing evolution?

Skyscraper Museum director Carol Willis said historians, designers, and the public alike must be careful not to confuse causation and correlation.

“When we think of how world changed after 9/11, safety and security are the principle factors we think about,” Willis said. “But there are other types of security that architects and structural engineers are designing for as well today. Yes, they are addressing building security against terrorism…but around the world, you will see new designs and structural engineering for security against loss prevention and security against damage from earthquakes and from wind.”

In reality, little has changed in places with healthy high-rise markets, such as Hong Kong, Willis said. New tall building design in Asia and the Middle East may feature safety innovations that stabilize high floors and prevent them from swaying, but such details were a “natural progression” of skyscraper design and engineering that were likely to occur even if the Sept. 11 attacks did not, she said.

"One premise that people in the U.S. get hung up on is thinking of the rest of the world in the context of a ‘post-9/11’ era," Wills said.

There remains a ripple effect, however. Some new Asian skyscrapers were designed with security features that reflect post-9/11 concerns. For example, the 1,614-ft.-tall Shanghai World Financial Center, designed by Kohn Pedersen Fox and completed in 2008, requires visitors to pass by a security desk. But the feature is an exception outside of the U.S., Willis said, and likely implemented overseas only “because of American firms there.”


Whether terrorist target or industrial icon, the skyscraper continues to serve as the vehicle for cutting-edge technology, daring design, and inventive engineering. A decade after the Sept. 11 terrorist attacks that destroyed two prominent high-rises and thousands of lives, it's clear that architects and engineers are working fastidiously and creatively to ensure that the newest generation of supertall structures can reach higher than ever without sacrificing safety and efficiency.

In doing so, they continue to build on the century-old concept of the skyscraper as the world’s most visible and practical symbol of optimism, community and technical achievement.

“The skyscraper is an American invention," Architectural Record’s McGuigan said. "It has always been about the outward edge of building technologies since the 19th century, when the safety elevator was invented, and higher buildings became possible. It’s important to have this historical context."

"Skyscrapers have always been about expressing humankind’s desire to push forward. To make progress.”

Related on SmartPlanet:

Photo illustration based on original by NIST

Images of One World Trade Center renderings courtesy Skidmore, Owings & Merrill LLP

Image of Manhattan skyline with One World Trade Center courtesy Silverstein Properties, Inc./dbox studio

Image of Supertall buildings comparison courtesy Skyscraper Museum

This post was originally published on Smartplanet.com

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