NIOSH Publication No. 2005-132:
Preventing Injuries and Deaths of Fire Fighters Due to Truss System Failures
All-steel trusses present their own hazards when exposed to fire. The mass and surface area of steel truss components are factors that determine time to failure. A heavy, thick section of steel has greater resistance to fire than a lightweight section of the same length because of the increased mass. A large, solid steel truss can absorb heat and take longer to reach its failure temperature, whereas a lightweight steel truss such as an open-web bar joist will be heated to its failure temperature much faster.
Once the failure temperature is reached, heavy steel trusses and lightweight metal trusses will react to the fire and fail in a similar manner. A steel member fails at the internal temperature of the steel and not at the ambient air temperature. This temperature is often referred to as the critical temperature of the steel member.
Findings reported by the National Engineered Lightweight Construction Fire Research Project indicate that unprotected lightweight steel C-joists fail within 4 to 6 minutes of exposure to fire [Grundahl 1992]. Testing conducted by the U.S. Bureau of Standards (now known as the National Institute of Standards and Technology, or NIST) showed that unprotected steel open-web bar joists reached 1,200: F in 6 to 8 minutes [Brannigan 1999]. Table D-1 illustrates that steel retains only 25% of its original strength at 1,200: F and retains only half its original strength at approximately 900 :F. Building design calculations are based on original strength at normal temperatures. At elevated temperatures, steel may retain no excess strength.
...steel loses strength when exposed to temperatures commonly found in structural fires. Steel has a high thermal conductivity, which means it can transfer heat away from a localized source and act as a heat sink. As long as the flame impingement is localized, the steel can transfer heat to other regions of the member-and thus the time to reach the critical temperature is delayed. If an intense fire is evenly distributed along the steel member, the critical temperature may be reached very quickly. Steel also has a high coefficient of expansion that results in the expansion of steel members as they are heated. As an example, a 50-foot-long steel beam heated uniformly over its length from 720 to 9720 F will expand in length by 3.9 inches. The same beam uniformly heated to 8000 F would expand by 3.2 inches; if heated to 1,2000 F, the beam would expand by 4.9 inches [Grundahl 1991; Cotes 1997].
