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Re: Shape Factors and AISC ASD

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>>> <Stanley_P_Johnson(--nospam--at)> 02/06/01 04:41PM >>>
For weak axis bending of I-shaped members, the commentary mentions that one
reason for an allowable stress of 0.75*fy is the favorable shape factor for
I-shaped members bent about their weak axis.  My question is, "What is a
shape factor and what influences a shape factor?"

The shape factor is the ratio of the plastic section modulus, Z, to the elastic section modulus, S.  It is a measure of a section's post-yield capacity.  The average W section has a shape factor of about 1.15 in bending about the strong axis and 1.55 in the weak axis direction.  Because there is significant post-yield reserve capacity for the section bent about the weak axis, a higher allowable is warranted (assuming that local buckling of the flanges is not a limiting factor).

Jason Emoto