FIRE TESTS on the floor truss design used on the World Trade Center's (WTC) twin towers last week cast doubts on the original fireproofing specification They showed that the thickness of fire protection in the original design would have given only half the specified two hour fire resistance.
The tests were the last two of a series of four carried out by the US Department of Commerce's National Institute of Standards & Technology (NIST).
They recreated the type of fire test that could have been carried out on the WTC trusses in the late 1960s in accordance with the standards of the time, but of which there is no record.
One test was carried out with 19mm thick cementitious fire protection coating, equivalent to that found after the towers collapsed in 2001. The second used the thickness of 13mm specified by the towers' designer Leslie Robertson. Both trusses were placed in a furnace which was to be heated to 1,200infinityC.
They failed at around 1,000infinityC.
At the time the New York City building code was not mandatory for the WTC, but the towers were claimed to have complied with it. This code required two hours' fire resistance for the floors as determined by the ASTM E119 specification.
Last week's test in accordance with the 1968 edition of ASTM E119 was carried out on 5m lengths of truss coated with 13mm of fire protection. They showed the resulting fire rating was only one hour. The fire rating of the previous test with 19mm protection gave a much better two hour rating.
NIST spokesman Michael Newman said the tests did not explain what happened after the aircraft crashes on 11 September 2001 but gave a better understanding of the relationship between standard fire testing and performance in real fires.
'Earlier full scale tests in Canada suggest that E119 may need revision. It seems that it is very difficult, especially on unusual buildings, to scale up the performance in standard fire tests to much larger elements.'
Trusses on the WTC were up to 18m span and their degree of end restraint was difficult to determine. The Canadian full scale tests gave fire resistance periods as low as 1.5 hours even with 19mm protection.