The staircases in the twin towers -- their number, location, and the weak walls around them -- emerged as critical factors in the deaths of many of those killed in the Sept. 11 terrorist attack on the World Trade Center, according to a federal safety report released yesterday. The findings will be used to shape federal recommendations for building-code changes across the country.
And after more than two years of intensive research, investigators uncovered what they said was an elementary shortcoming in the trade center towers: neither building had enough staircases to meet any of the major building codes in the country, including New York City's.
For nearly every man and woman on the upper floors of the towers, the lack of intact staircases meant that they could not get out after the planes struck. Clustered in the centers of the buildings, those staircases were encased in lightweight drywall that was immediately destroyed. Sturdier walls around staircases that were remote from each other "might have provided greater opportunities for escape," said the lead investigator, Shyam Sunder.
In a sobering lesson drawn from one of the day's great successes -- the escape of nearly everyone below the points of impact, about 14,000 people -- the report said that it had taken about twice as long to go down a single flight of stairs as had been projected by the current engineering standards for tall buildings. The buildings were only half full, investigators said, and if the attack had come at a time when they were filled to occupancy, the evacuation would not have been successful. Thousands more people were likely to have been trapped on the stairs, Mr. Sunder said.
The report, issued by the National Institute of Standards and Technology, also formally confirmed what had long been identified as a significant failure on that day: the leaders of New York's Police and Fire Departments did not coordinate their efforts that morning. The investigation suggested that many of the rescuers died because they simply did not know what was happening around them.
"A preponderance of evidence indicates that a lack of timely information sharing and inadequate communications capabilities likely contributed to the loss of emergency responder lives," the report stated. It cited an interview with an unnamed firefighter who told the investigators, "If communications were better, more firefighters would have lived."
The findings were included in a draft final report from the institute, a branch of the United States Commerce Department that was given authority by Congress in 2002 to investigate the towers' collapse, the evacuation and the emergency response.
The findings total some 10,000 pages, of which 3,400 were made public yesterday. The remainder will be released later in the spring, according to Mr. Sunder. The institute will make recommendations on improvements in the areas it studied.
Building-code changes are decided by local governments, generally using model codes developed by technical experts who work with the insurance and real estate industries.
In a presentation yesterday at a Times Square hotel, Mr. Sunder outlined the techniques used to project the sequence of events that led to the collapse of each tower. Although each building was hit by virtually identical planes, the south tower collapsed in 56 minutes and the north tower in 102 minutes.
A combination of common factors shaped the course of events, he said. The planes plunged through the exterior curtain of each building and fragmented as they passed through the building, with parts emerging on the other side. The impacts killed hundreds of people instantly. In the north tower, American Airlines Flight 11, moving at 443 miles per hour, took .685 seconds to pass through the building; in the south tower, United Airlines 175, hitting at 542 miles per hour, passed through in .58 seconds.
The impact changed tower structures in two critical ways, Mr. Sunder said. First, of the 47 columns in the core of each building, 9 were either severed or badly damaged in the north tower, and 11 in the south tower. Second, the impact dislodged the fireproofing that was sprayed on the floors and the columns. As the fires ignited by the jet fuel burned, the floors were weakened.
The floors played an important role in the structures, because they connected the exterior supports -- the pinstripe columns that gave the towers their distinctive appearance -- to the columns hidden in the cores of the buildings. As the unprotected floors were weakened by fire, the exterior columns bowed inwards, the investigators reported. In the north tower, a photograph showed they had moved 55 inches off center a few minutes before the collapse; in the south tower, they were 20 inches off center. As those columns became unstable, the building load shifted, but the instability was too great and the cascade of collapse began.
Much of the jet fuel burned outside the buildings in a fireball, but enough remained inside to ignite the office furnishings and building contents.
In its early phases, the investigation by the institute raised serious questions about the adequacy of the original fireproofing applied to the steel in the towers, and Mr. Sunder said those concerns remained. But, he said, in the areas where the fires were most severe, the amount of fireproofing that originally had been applied was "moot" because whatever had been there was knocked off by the planes.
Hundreds of people were trapped above the impact, on floors where there was no immediate damage. This made escape routes an important part of the agency's study.
During the design of the trade center, the Port Authority of New York and New Jersey had decided to use a new version of the city building code that did not require as many staircases as the earlier edition. Instead of six staircases, including a specially reinforced fire escape, the trade center had three stairs in each tower. The investigators determined, though, that even the liberalized code required a fourth staircase, to accommodate the more than 1,000 people expected in the restaurant at the top of the north tower and at the observation deck atop the south tower.
As an interstate agency, the Port Authority is not bound by local building codes, but it had publicly pledged to "meet or exceed" the city code in building the trade center. However, the institute investigators determined that the Port Authority had not supplied enough staircases.
"Once you go over 1,000 people on a floor, you need to have a fourth stairway," said Richard W. Bukowski, a senior engineer with the institute. A spokesman for the Port Authority said its engineers believe that the findings are mistaken. New York City building officials who reviewed the trade center plans, both in the 1960's and after the 1993 terrorist bombing, had not raised any questions about the missing staircase.
Glenn Corbett, a professor at John Jay College of Criminal Justice and an adviser to the institute investigation, said he had asked about the exits for the restaurant and the observation deck. "Imagine what a staircase in the right spot might have done for people that day," Mr. Corbett said.
One of the documents included in yesterday's report showed that the Port Authority was eager to cut down on the amount of space devoted to stairs.
"The tower core should be redesigned to eliminate the fire towers and to take advantage of the more lenient provisions regarding exit stairs," wrote Malcolm P. Levy, the Port Authority's chief planning engineer, who is now deceased.
In the impact area of the north tower, the three staircases were about 70 feet apart and were destroyed immediately.
In the south tower, the plane hit on floors where the three staircases were about 200 feet apart, and one of them survived at least partially intact.
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