||05-25-2010 01:16 PM
Steel beams are not listed in the format you described, which I assume is depth x width. The closest shape I have in my tables is a W4x13, which is 3.83 inches deep x 4.16 inches wide, with a moment of inertia about the XX axis of 11.3 in^4.
As mentioned by Yoyzit, the relative stiffness of the beam is a function of the modulus of elasticity of the steel, which is approximately 29 million, versus about 1.8 million for wood (depends on species). The relative STRENGTH of a steel I beam versus wood is a function of the yield strength of the steel, which is typically a minimum of 36 million psi for structural steel, but may also be 60 million psi.
So to determine if the I beam has greater or lesser capacity than the wood, you need to know the yield strength of the wood, the exact dimensions of the wood, and the yield strength of the steel.
All that said, are you sure you even want to consider replacing existing joists that are sagging with steel? That would be a large job, and unless there is a real structural problem, may be an unncessary expense.