Fire fighters have wondered if the roof support system in tilt-up buildings burned down, would the walls collapse? They didn't in March 1986 when a severe fire in a Phoenix tilt-up building consumed the entire wood roof system. The building, a U.S. postal annex built around 1970 was constructed of 5.5-inch-thick tilt-up concrete walls with a wood roof system. The plywood roof deck was connected to a ledger consisting of a 6 by 8.2 steel channel with a wood nailer attached. After the fire, all that remained of the roof system was the middle one-third cross section of the laminated girders. The walls deflected out 6 to 8 inches on the north, west, and south sides of the building. Aggregate popouts and large panel deflections suggested a very hot fire.
Investigators studied effects of the fire by taking cores from the walls and pilasters 20 days after the fire. Inspection of the cores after compression testing showed the failure mode to be vertical splitting, indicating the fire had severely damaged the concrete. Compressive strengths of the cores ranged from 2145 psi to 2778 psi. All the information from the site inspections and laboratory tests led investigators to conclude that the tilt-up concrete walls were damaged beyond repair. But despite the large deflections in the walls and the extensive damage to the concrete, the walls did not collapse. For fire fighters, this means that in properly designed and erected tilt-up buildings the walls are likely to remain standing during a fire.
