NORTHBROOK, Ill. (AP) -- Federal officials re-created a floor system of the World Trade Center and conducted a fire test on it Wednesday as part of an investigation into the towers' collapse on Sept. 11, but results didn't immediately provide clear answers.
A 17-by-14-foot slab of concrete was placed in a furnace, where flames soon licked at the steel trusses, fireproofing material and metal decking underneath the floor.
Images from inside the furnace of buckling trusses and cracking concrete were beamed onto a video screen for journalists, engineers and family members of victims invited to view the test, conducted by the National Institute of Standards and Technology at Underwriters Laboratories in suburban Chicago.
The floor system failed after about an hour and 15 minutes because of excessive temperatures on the trusses and concrete. A test last week of the floor system as it was actually installed in the towers passed the 2-hour fire rating required by New York City code in the 1960s, when the trade center was built.
The difference was the thickness of spray-on fireproofing. The floor system in last week's test had 3/4 of an inch of fireproofing, which the NIST said was the average amount installed in the towers. Wednesday's floor system had 1/2-inch-thick fireproofing _ what trade center plans called for, though more was used.
``Certainly a quarter of an inch of more insulation would have a big effect on slowing down the heating of the metal,'' said investigator Bill Grosshandler.
The agency's investigation is an effort to determine why the towers collapsed and use those lessons to recommend improvements in building codes, evacuation procedures and other standards.
The floor system of the trade center _ with its columns at the core and exterior _ is of interest because of two hypotheses on the cause of the collapse.
Both say the hijacked jetliners that slammed into the towers caused severe damage to columns and floor joints. They vary on whether the fire-weakened columns failed and alone brought down the buildings, or whether floor trusses sagged in the intense heat, pulling the columns inward to collapse.
Grosshandler said test results need further analysis. The agency's draft report is due in December.
Monica Gabrielle, whose husband died Sept. 11, said she thinks there should be a nationwide building code _ and no structures housing people should be exempt. ``We need to put the sanctity of life and limb way above economic interests,'' she said.
