Joist-Rafter versus Lightweight Wood Truss

Issue 7 and Volume 141.

Joist-Rafter versus Lightweight Wood Truss FEATURES BUILDING CONSTRUCTION The strength of these assemblie comparable—until fire strikes. Photos by John Mittendorf. A recent article on lightweight, metal-plate-connected wood truss assemblies (and other newer building components, such as wooden I-beams) focused on concerns of fire service personnel about the performance of this construction when it’s exposed to fire. The article relied on laboratory tests to validate a comparative fire performance equivalency between two-by-four-inch wood truss structural members and conventional joist-rafter assemblies. This prompted a response from Francis L. Brannigan (Fire Engineering, June 1988) which marked the dangers and inaccuracies in using some widely accepted test methods to develop fireground operational procedures and shape general perceptions of these structural assemblies when Sexposed to fire of significant proportions. Using Frank Brannigan’s article as a springboard, let’s expand on the basic differences between lightweight, metal-plate-connected trusses and conventional joistrafter assemblies as commonly used in modern…

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