Towers Stood Strong
Withstood impact, but failed in fires

By Graham Rayman and Bryn Nelson

May 1, 2002

The design of the Twin Towers amounted to a double-edged sword, according to excerpts from a federally funded engineering study obtained by Newsday.

The ability of the World Trade Center buildings to withstand immediate collapse Sept. 11 spoke to their design and construction characteristics, the panel says, but so did their vulnerability to the hijacked jets' impacts and fires.

The study, to be released today during a congressional hearing, concludes the towers behaved "remarkably," and states that probers did not uncover any substandard structural features.

The study team recommends that funds should be used to improve aircraft security, rather than hardening buildings against airplane attack. "Buildings are not designed to withstand any event that could ever conceivably occur," the report states.

While floors gave way first, the collapses in each tower took different paths. In the south tower, the exterior support columns failed first. In the north tower, the core structure failed first.

The report notes that design features may have played a role in the collapse and the inability of some occupants to get out. These include the steel trusses - 60-foot webbed bars that supported the floors, which failed Sept. 11 - the grouping of the three exit stairways in the core, the lack of impact-resistant enclosures around stairwells and the effectiveness of fireproofing.

The study concludes that the buildings could have remained standing following the plane impacts alone, if not for the enormous fires that ensued.

The heat produced by the fires - comparable to that produced by a power station - eventually caused the structural steel to fail, the report finds.

The jet fuel acted as the spark to ignite the fires, the report finds. A substantial portion of the fuel was immediately consumed by a fireball; the rest burned off in a few minutes or dropped through elevator shafts and ducts, starting more fires.

The plane impacts left the damaged areas without protection against fire, the report says. They disabled the sprinkler system; slashed through standpipes that supplied water to fire hoses; dislodged fireproofing and weakened the structural steel lattice.

The report, to be be discussed in Washington at a hearing of the House Science Committee, repeatedly qualifies its findings, saying substantial additional study is needed. Critics have asserted that the report overlooks important areas of research.

The National Institutes of Standards and Technology is expected to issue a plan today for a more detailed, two-year $16 million study of the collapses, including a request for probers to be given subpeona power. In addition, new legislation is to be proposed by the House panel to create a formal protocol for investigation of major collapses.

The study, sponsored by the Federal Emergency Management Agency and the American Society of Civil Engineers, includes some general recommendations. Among them, it says the longstanding fire resistance ratings should be revised to more closely follow the behavior of actual fires.

The collapse of 7 World Trade Center - an office building that housed the city's emergency command center and was struck with debris from the towers and burned for seven hours - likely began on the fifth to seventh floors with failure of a key structural piece, the report concludes. While the skins of the Twin Towers peeled outward from the top, Building 7 fell straight down, suggesting an internal collapse.