After an exhaustive, three-year study of the collapse of the World Trade Center, a federal panel will call for major changes in the planning, construction and operation of skyscrapers to help people survive not only terrorist attacks but also accidental or natural calamities, according to officials and draft documents.
The recommendations, to be made public tomorrow, include a call for a fundamental change in evacuation strategies for tall buildings: that everyone should have a way out in an emergency, replacing the current standard of providing evacuation capacity for a few floors near a fire or emergency. The panel also called for sturdier elevators and stairways, and found that current standards for testing fireproofing of steel for tall buildings are flawed.
Taken together, the recommendations, by the National Institute of Standards and Technology, are likely to open an intense national debate over the costs of such changes and whether lessons for other skyscrapers can reasonably be drawn from the extraordinary events of Sept. 11.
The agency's proposals are not binding, but are meant to influence the policies of cities and states across the country. Many of them have become public in draft form during the three-year inquiry and have prompted fierce lobbying or objections from prominent engineers, building industry professionals, and the Port Authority of New York and New Jersey, which built the trade center. While the agency has revised certain aspects of its findings on what precisely happened at the trade center, the package of recommendations makes it clear that the agency has essentially held firm on its emphatic and demanding safety agenda for the next generation of tall buildings in America.
S. Shyam Sunder, the engineer who oversaw the inquiry for the agency, said the investigators worked to identify issues of ''safety for the vast majority of buildings'' in fires, earthquakes, power losses and sudden hurricanes. The costs of the changes are unknown, but structural engineers suggested they would add 2 to 5 percent to development costs of ordinary buildings.
The study disclosed that critical design benchmarks and code standards used in the construction of the trade center -- the time it takes to walk down stairs, the distance separating stairways, and the fire-resistance tests -- turned out to have little relationship to the experiences or needs of people inside the towers. These findings, Dr. Sunder said, have broad application to buildings everywhere.
The investigation also found that most building codes do not recognize that people on high floors are isolated and easily cut off from help during an emergency.
The inquiry, conducted by more than 200 technical experts and contractors working for the agency, amounts to a 10,000-page autopsy of the trade center collapse. The report includes 25 pages of recommendations, which will be released for the first time as a full set in New York tomorrow.
''The whole purpose of the investigation was to make building occupants and first responders safer in future disasters and to learn whatever we could from what happened on 9/11,'' Dr. Sunder said. ''The recommendations will be reasonable and achievable.''
In the United States, building codes are generally adopted by local and state governments that use models developed by private groups like the National Fire Protection Association, established by the insurance industry, and the International Building Code Council, an organization of government construction regulators. Those two groups have set up committees to evaluate the recommendations.
''Will all the recommendations be accommodated verbatim in building codes? I think the answer is no,'' said Mohammed M. Ettouney, a principal at Weidlinger Associates, a New York-based structural engineering firm that is doing the security-related design work on the Freedom Tower planned for ground zero. ''But it will act as a lightning rod for a debate that will now really get under way.''
The trade center towers, where 2,749 people died in the Sept. 11 attack, were only one-third occupied that morning. If the buildings had been full, it is likely that 12,000 more people would have died because of limited evacuation capacity, the investigation found.
Already, a proposal for wider exits -- making it possible for people to leave faster but reducing the amount of rentable space -- has been rejected by one major code-writing organization.
Others have suggested that it is folly to think different rules might have forestalled the collapses.
''They are leading the public down the wrong path,'' said Jon Magnusson, whose Seattle-based structural engineering firm, Magnusson Klemencic Associates, is the descendant of the company that designed the twin towers. ''They are saying we are going to fix the codes in order to deal with Sept. 11th. The physics say that you can't do that.''
Dr. Sunder says that is a mistaken reading of the investigation. The agency, he said, does not suggest that buildings should be able to stand up to airplane impacts. ''It is more cost effective to keep terrorists away from airplanes, and airplanes away from buildings,'' he said.
The trade center was built by the Port Authority, which is not subject to any building codes. Despite promises by the Port Authority to ''meet or exceed'' the New York City code, the federal investigation found that the trade center had fewer exit staircases than required and that the Port Authority never tested the fire resistance of the floors. It also found no evidence that a rigorous engineering study supported the authority's repeated public assertion that the towers could stand up to the impact of a fully loaded commercial airliner.
In recent presentations, Dr. Sunder suggested that agencies that are exempt from building codes, such as the Port Authority, should have an independent party certify their compliance with codes, rather than simply deciding for themselves.
The three-year, $16 million federal investigation was broken into two primary parts. Using computers to reconstruct the attack, engineers found that when the towers were struck, they redistributed load to surviving columns. Once the fire weakened those remaining, extremely stressed columns, whose fireproofing had been knocked off by the planes, the structures collapsed, the report says.
That research found no flaw in the design of the towers that was a critical factor in the collapse, Dr. Sunder said.
As the computer reconstruction of the towers proceeded, others worked on a second inquiry: identifying weaknesses in building codes.
For example, investigators determined that if the towers had been fully occupied, it would have taken about four hours for survivors to exit, more than twice the time either tower stood and twice as long as planners had estimated. That led to the call for changes in evacuation planning.
At least some elevators in tall buildings should be built with more robust shaft walls and with electrical systems that will not fail if exposed to water, the report says, so that they can be used to evacuate people who cannot descend long distances and to take firefighters to high floors.
The investigation also raised hard questions about the usefulness of a century-old furnace test that measures the fire resistance of structural components. Last summer, the National Institute of Standards and Technology arranged a furnace test of a 17-foot piece of steel and concrete floor, the standard requirement at the time that the towers were erected. The floor passed the test. However, the tower floors were built not with 17-foot lengths of floor, but with 35- and 60-foot lengths. When a 35-foot length was tested in the furnace, the floor failed the fire-rating requirement.
The recommendations also say that tall buildings should be designed to prevent ''progressive collapse,'' avoiding a cascade of failures that can bring down a tower in seconds.
The study found that sprinklers, which can replace or reduce other fire-protection systems, should have a redundant water supply or power backups, to avoid being knocked out with one blow. Requirements for how well spray-on fireproofing should adhere to the steel columns also must be clarified, Dr. Sunder said.
The debate over integrating the proposals into building codes and practices will undoubtedly be intense. Mr. Magnusson serves on the special eight-member committee set up by the National Fire Protection Association, along with Sally Regenhard, chairwoman of the Skyscraper Safety Campaign, who is one of the nation's most vocal advocates for tougher building codes. She and Monica Gabrielle, a cofounder of the skyscraper campaign, lobbied Congress to finance the agency's investigation and have demanded that the institute not dilute its findings.
''We have to restore the public's perception of safety in skyscrapers,'' said Ms. Regenhard, whose son, a probationary firefighter, was killed in the attack.
The International Building Code Council moved last year to require that towers taller than about 40 stories have three hours' worth of fireproofing on structural elements, instead of two hours, but rejected proposals that would require wider stairwells and reinforced concrete or masonry walls in buildings over 25 stories.
The National Fire Protection Association, meanwhile, is expected to act in August to require stairwells that serve 2,000 or more people to be a foot wider than currently mandated, an official at the organization said.