THE WOODEN I-BEAM JOIST
The wooden beam sawn out of a tree trunk is inefficient in terms of weight and cost. Recently the sawn beam has been replaced by wooden I-beams (composite wood joists).
Look at the end of a steel I-beam. Since the steel is extruded through a die, the designer can choose the most efficient shape. Both the top and the bottom flanges are wide to cope with compressive and tensile loads. The web, which separates the top from the bottom, is thin, just sufficient to keep the top and bottom apart.
A sawn wooden beam can be thought of as “containing” an I-beam with “surplus” wood along the sides. The surplus wood is what makes it possible for firefighters to stand and operate on a burning structure. This “fat” has been the basis of interior firefighter tactics. As long as only the “fat” or surplus wood is burning, a firefighter is relatively safe. Until it burns away, the beam is structurally sound. The gradual sagging of sawn beams due to the loss of the exterior wood often gave warning of impending collapse.
Wooden I-beams are another method of making buildings lighter. The beams consist of solid or laminated 2X4 for the top and bottom chords and a plywood of oriented strand board web between them. They are manufactured in lengths up to 60 feet. Contrasted with the sawn beam, the “fat” or surplus (structurally unnecessary wood along the sides) is no longer present.
The web of the I-beam is often penetrated by utilities, and in some cases sizable holes are cut into it so that any fire gets a grip on both sides of the 1-beam at the same time, guaranteeing early failure.
Often, cut-off ends of I-beams are used as “tirestopping.” Be alert to them.
—from Building Construction for the Fire Service, third edition, Francis L. Brannigan (NFPA, 1992) pp. 551552.
