Hazards of “Twin-T’s”

Recently MDFR units responded to a fire in a one story commercial building. The fire building was a one story single bay warehouse with concrete block walls and a roof consisting of precast Twin-Ts. This fire presented firefighters with some life-threatening hazards; hazards that they avoided by attacking the fire with a defensive, exterior attack (Photo #1).

photo#1

FIRE RESISTIVE VS NONCOMBUSTIBLE

There is a vast difference between fire resistive and noncombustible construction. A building classified by the building code as fire resistive or “Type I” construction has floors, walls and a roof that can withstand collapse and penetration by fire for as much as 2-4 hours. Fire resistive structural assemblies are tested in a large furnace by Underwriters Laboratories. Floor and roof assemblies are installed at the top of the test furnace; walls are installed as a side to the furnace. Although performance in tests does not guarantee that fire resistive structural
assemblies will perform similarly under actual fire conditions, it is highly unlikely that they will collapse. Fire resistive buildings generally have concrete structural members. When steel is used in fire resistive construction it must be protected from fire by encasing it in concrete or enclosing it with a fire-rated barrier. It is critical that all firefighters understand the following fact: NOT ALL CONCRETE ROOF STRUCTURAL ASSEMBLIES ARE FIRE RESISTIVE.

Cast in place reinforced concrete slabs have always proven to be the safest under heavy fire conditions. Precast concrete, on the other hand, will not withstand significant fire exposure. Precast Twin-Ts, for example, are deceivingly dangerous because of their appearance; many firefighters assume that such massive concrete members must be fire resistive but, in reality, Twin-Ts are far from it. Our Department has learned the hard way that Twin-Ts can collapse