Friday, August 15, 2008
Kevin Ryan knew nothing about how floor assemblies are tested by his own company Underwriters Laboratory. He rported that they tested the steel and it withstood 2000 deg for 3 and 4 hours. The UL tests floor and wall assemblies not the steel per se. The problem is that the long span floors used in the towers were never tested in their long span configuration of 60 feet. As any architect knows the longer span floors require either massive joists or beams uneconomical for high rise buildings or specially designed construction such as steel trusses. What most architects apparently don’t know is that lightweight, long span steel trusses fail at fire temperatures not yet compensated for in the codes. The standard furnace test can only handle 17 foot lengths of flooring. The furnace standards were set in the 30s 40s and 50s when about 15 feet was the standard span used in high rises. These older buildings used shorter spans, more robust columns and beams, stronger connections and better fireproofing then now. If a floor failed the pull-in (catenary) forces created by the short spans were easily handled by the strength of the rest of the structure. For this reason the codes allowed floors to have a shorter (3 hour) fire endurance rating than the columns or girders (4 hours). The 17 foot furnace test, currently still used, is meaningless for the longer spans. The main problem in the WTC flooring was due to the differential elongation (expansion) of the steel parts of the trusses. NIST’s studies found that the different expansion rates immediately deformed the steel parts, buckled the top chords and struts and disconnected the bond between the concrete slab and the joists. Greater thermal expansion of the bottom chords releases the tension and allows the cool top chord to sag until it acts as a cable in suspension creating pull-in forces on the columns. Contraction of sagging, long span steel flooring during the cool down faze after fire die down puts heavy pull-in loads on the connections. It is thought that Building 7 collapsed from this contraction in the beams disconnecting enough of them to affect column stability. The whole interior and core failed before the perimeter wall which came down as a unit at near free fall speed.