- 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.