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Q.A while ago you ran an article recommending that you limit deflection of built-up wood girders to L/600 so that the floor joists wouldn’t vibrate excessively (see Practical Engineering, 8/97). My question: Does this also apply to steel girders, or is it mainly a wood-related issue, owing to the inherently greater flexibility of wood beams? In another words, would a steel beam sized for L/360 deflection be less bouncy than a wood beam sized for, say, L/480? Or should I absolutely make sure that all my steel beams are size for the L/600 limit?

A.Authors Frank Woeste, P.E., and Dan Dolan, P.E., respond: The forces that induce floor vibration and the reaction to it are blind to the beam or girder material. Each girder material, whether wood or steel, has an elasticity, E, and that design value is used to predict deflection. The designer sets limits on live-load deflection. A wood beam and a steel beam both designed to exactly meet the L/360 deflection limit should perform the same, assuming the weight of the girders themselves has been accounted for and that both are fastened securely to their support points.

Considering the price of steel (and the cost to repair a bouncy floor), you should upgrade all girders to meet a live-load deflection limit of L/600 if vibration is a sensitive issue for your clients.