You may say it’s highly unreasonable to believe that 4 steel constructed high-rise buildings collapsed from fire in one day? Well they all had one thing in common. Long Span steel composite floors. Long span steel beams have a magnified response to heat. They expand a longer distance than short span beams and they still have their full strength in the beginning of the expansion. NIST computer studies show that this strength while elongating can shear off the bolts connecting the beams to the columns or girders as the restrained beams expand. This strength can also crack the concrete slab at the shear studs and buckle the beam itself as the compression builds up during expansion.

A buckled or bowing (bowing occurs when the bottom flange of a steel beam expands faster than the top flange) long span (typically 40 to 50 feet) beam can impart large tension forces on the connections especially when the deflected beam begins to shrink as it cools. The fireproofing insulation thickness schedules in the Building Code were developed for the short span floors which were used in the older buildings and this insulation defended against weakness in the steel beams which occurs at higher temperatures 1100deg. F (about 600 deg. C). Low temperature expansion effects occur at temperatures below 400 deg. C and have not yet been compensated for in the codes. This deficiency in long span, high-rise office buildings is a new discovery discovered by the study of the collapse of the Twin Towers and Buildings 5 and 7 and was first illuminated by engineering computer studies. WTC 7 met all New York City codes and was not hit by any planes and was destroyed by a fire in ordinary combustible furnishings.

Another important characteristic of large open areas with combustible furnishings is that a fire can spread over the whole area and release a large quantity of heat if not extinguished immediately as by a working water spray system (sprinkler). The water spray systems in both towers were damaged by the forces of plane impacts and in Building 7 the water spray and standpipe water supply were cut off by the tower’s collapse impacts damaging the water mains in the streets.

The other thing about long span floors is that the builders relocate or remove the interior columns to get the open office areas that they wanted in the first place. When a column or columns fail under such large open area circumstances it or they may not be able to redistribute the floor loads to other columns and the collapse is likely to progress upwards. If the building is not protected against progressive collapse, global (total) collapse can ensue. Building 7 collapsed because one key interior column failed after the long span floors failed around it

Arthur Scheuerman
Ret. Battalion Chief, FDNY