Wood vs Steel Deflection Criteria
Wood vs Steel Deflection Criteria
(OP)
In a bridge design manual developed to guide designers on the design of trail bridges (hiking, biking and snowmobiles) they have two deflection criteria. For wooden bridges 1/200 for steel bridges 1/500, yet the loading criteria are the same for both types. These bridges do not carry highway traffic or emergency vehicles. The maximum load is self weight, 30 psf over the whole deck + 10,000 lbs so, they really don't follow AASTHO guidlines either.
Can anyone provide the logic behind this? Why not the same criteria for both? Why should the steel bridge criteria be more restrictive than the wood bridge?
Can anyone provide the logic behind this? Why not the same criteria for both? Why should the steel bridge criteria be more restrictive than the wood bridge?





RE: Wood vs Steel Deflection Criteria
The logic behind this approach is primary very low Young Modulus for timber - so deflection for a timber bridge is usually large. Limiting the deflection to L/500 will lead to oversized members, as the displacements will govern the design.
Also, due to the excellent vibration-damping characteristic of the timber structures, larger deflection will be not so uncomfortable to the users.
It should be added that the deflection criteria is mostly a "comfort" requirement in the design process, and large deflection does not necessarily mean that the structure is unsafe.