- 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.