The Future of High-Rise Building Design

Speaking In Code. National organizations consider fundamental changes to how high-rises are designed, built and operated.

Written by Mike Lobash.

The collapse of the World Trade Center towers triggered an investigation unprecedented in cost and scope. Nearly 200 investigators spent more than $40 million and three years to determine why the towers collapsed and to issue recommendations on how to make high-rises safer.

Now, with 43 reports constituting more than 10,000 pages filed, the question has changed. No longer is the question, “Can high-rises be safer?” Rather, it is, “How safe should high-rises be?”

“The bottom line is no one can tell us how safe is safe,” says Gerald Jones, co-chairman of a National Institute of Building Sciences (NIBS) committee reviewing recommendations in the federal government’s final report on the collapse of two of the three World Trade Center towers that were destroyed in the attacks. “Everything is a compromise between safety and economics.”

Code organizations will likely spend the next several years discussing 30 recommendations contained in a National Institute of Standards and Technology (NIST) report aimed at improving the safety of buildings taller than 120 feet. At issue in the discussion will be whether the recommendations — taken individually — should be made part of the model building codes and national standards that govern how buildings are designed and constructed.

The recommendations are broken into eight subject areas. They are:

    * Increased structural integrity.
    * Enhanced fire resistance of structures.
    * New methods for fire resistant design of structures.
    * Improved active fire protection.
    * Improved building evacuation.
    * Improved emergency response.
    * Improved procedures and practices.
    * Continuing education and training.

As the recommendations are reviewed, some are likely to become model building codes with little debate. And a few recommendations, particularly those regarding education and training of emergency responders and of architects and engineers, require little or no action by code and standards organizations. One recommendation suggesting that academic, professional short courses and Web-based training materials be offered in the use of computational fire dynamics and thermodynamics, for example, affects professional associations and doesn’t require code changes.
Fundamental Shift

Other recommendations will prove to be more controversial, with the amount of debate surrounding the adoption of each being proportionate to the cost involved with implementing it. Perhaps none of the recommendations are more controversial than the ones that aim to change the way high-rises are evacuated. One recommendation calls for full building evacuations during building-specific and large-scale emergencies, such as widespread power outages, fires, explosions, terrorist attacks and unforeseen natural disasters. Another would require high-rises to contain more remote exits and two sets of structurally hardened elevators. During emergencies, one set of elevators would be used by occupants and the second set by emergency personnel. The exact number of exits and elevators required would depend upon building size and the number of occupants.

Those recommendations constitute a sea change in the approach to the way high-rises are designed to accommodate evacuations. By and large, high-rises are designed to handle the evacuation of a single floor. If necessary, multiple floors are evacuated one at a time in what’s known as a staged evacuation.

That was the design approach at the World Trade Center towers, says Shyam Sunder, deputy director of NIST’s Fire Research Laboratory and lead investigator on the collapse of the towers. Stairwells were designed to be wide enough and positioned so that 390 occupants — the maximum on any one floor — could be evacuated at one time.

On Sept. 11, 99 percent of the people located below the floors of impact were able to escape the World Trade Center towers before they collapsed, even though the building wasn’t designed to accommodate full building evacuations, according to the final NIST report on the collapse of the towers. The story would have been different if the buildings were fully occupied.
Beyond Terrorism

Even so, Sunder says the recommendations in the NIST report were not based on the ability of a high-rise to withstand the impact of an airliner or to evacuate occupants after such a terrorist attack. Rather, they were developed to aid the evacuation of buildings for any emergency situation, including earthquakes, widespread power outages, tornadoes and other events.

“The need to evacuate a high-rise might need to happen even without a terrorist attack,” Sunder says. “If you look at the nominal life of a building as 100 years, there are likely emergencies that require full evacuation of buildings.”

If the recommendations will make high-rises safer in case of events other than terrorist attacks, that would help justify their being adopted in model codes. Some perceive the NIST report as being narrowly focused on preventing the catastrophic results of Sept. 11.

Samir Mokashi of the engineering firm IDC says immediately following the terrorist attacks, everybody thought that hardening buildings to withstand the impact of airliners was a potential design goal. But now that the event hasn’t been repeated in more than four years, support for that idea has waned.

“Is a plane going into a building something we need to protect against?” he asks.

Phil DiNenno, one of eight members of the National Construction Safety Team Advisory Committee, which advised NIST in its investigation, says of the 30 recommendations issued by NIST, one requiring additional remote exits might have changed the outcome of the attacks on Sept. 11. The addition of those exits — stairwells on the corners and on the perimeter of the building — might have allowed more occupants located above the floor of impact to evacuate.

“But that is singularly an expensive change,” says DiNenno, who is also president of Hughes Associates, a fire protection engineering firm.

The expense comes not only in the design and construction of additional exits, but in the loss of rentable real estate, most notably, the amount of premier daylit space landlords could lease to tenants along a building’s perimeter.

As the recommendations work their way through the code adoption process, expect debate among code-making bodies, building owners — who will ultimately fund the cost of the new codes through increased high-rise design and construction expenses — and building component and material manufacturers whose businesses stand to benefit or suffer from the adoption of certain recommendations as codes.
Seeking Help

NIST has already contracted with NIBS to work on turning the recommendations in the final WTC report into code language that can be adopted by model code groups, including the International Code Council and the National Fire Protection Association (NFPA). Jones’ committee, aptly called the Committee for Translating the NIST WTC Investigation into Model Building Codes, is moving quickly to get at least some changes incorporated into the 2007 supplement to the International Building Code

. The deadline for submitting code change proposals for consideration in the next code cycle is March 24, with final action scheduled for May 2007.

“We’re going to do our best to get at least some of the recommendations adopted in the current code process,” Jones says.

Some of the measures recommended by NIST began working their way into codes and standards even before the agency completed its investigation. For example, NFPA included a provision in its 2003 edition to require wider stairwells for buildings with more than 2,000 occupants. In the 2006 code cycle, there were no dramatic code changes based on the NIST report, says Gary Keith, vice president of building and life safety for NFPA.

For the 2009 code cycle, which will be available for public comment in 2007, Keith says the association’s High-Rise Safety Advisory Committee will review the recommendations and monitor proposals for code changes.

“The questions will be whether anyone looking at the report sees recommendations that are worthy of code changes,” he says.

Shortly after NIST’s final report was issued, BOMA International, representing building owners and managers, issued a statement calling for further study, including a risk-benefit analysis, into some of the recommendations. One comment took issue with a NIST recommendation calling for the development of national standards and codes to prevent the progressive collapse of buildings.

“The big issue here would be to what extent it would be required to prevent progressive collapse?” wrote BOMA’s David Johnston, a member of the organization’s advocacy staff, in submitting written comments to NIST. “How many column failures (specific number, a percentage of a floor, etc…) would need to be able to be withstood? ‘Progressive collapse should be prevented’ is a pretty vague goal without a better explanation of what factors must be considered.”

Another recommendation that has drawn early criticism from BOMA is one requiring the fire resistance performance of structures. NIST recommends that uncontrolled building fires result in burnout without a structure suffering either partial or total collapse.

“The effort is a bit unrealistic and unwarranted,” BOMA wrote. “NIST assumes the complete loss of the sprinkler system, and no intervention by fire departments.”

As the process of reviewing the recommendations moves forward, code-making bodies will likely hear those types of comments as well. While the arguments might not be geared at questioning whether a certain code will make buildings safer, they will be pointing out that the cost of implementing a code might not be worth the expense.

“The real question as the process moves forward is how much will a change cost,” DiNenno says. “That’s the real world where code is written.”