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Q.Is it true that a wood beam is safer than a steel beam in a fire? I’ve heard that metal twists and deflects in the presence of heat, while a wood beam will withstand the heat and a great deal of fire before burning through enough to collapse.

A.Brad Douglas, director of engineering at the American Forest & Paper Association, responds: Large solid-sawn and glulam timbers provide a substantial degree of fire endurance. The superior fire performance of large timbers can be attributed to the charring effect of wood. As wood members are exposed to fire, an insulating char layer is formed that protects the core. Thus, beams and columns can be designed so that a sufficient cross-section of wood remains to sustain the design loads for the required duration of fire exposure.

A fire test conducted in 1961 at the Southwest Research Institute compared the fire endurance of a 7x21-inch glulam timber with a W16x40 steel beam. Both beams spanned approximately 43.5 feet and were loaded to full design load (approximately 12,450 lb.). After about 30 minutes, the steel beam deflected more than 35 inches and collapsed into the test furnace, ending the test. The wood beam deflected 2 1/4 inches with more than 75% of the original wood section undamaged. Calculation procedures provided in a new publication available from the American Wood Council, entitled Technical Report 10: Calculating the Fire Resistance of Exposed Wood Members, estimates that the failure time of the 7x21-inch wood beam would have exceeded 65 minutes if the test had not ended at 30 minutes.

For additional information on the fire performance of wood, contact the American Wood Council at 202/463-4713 or www.awc.org.