Allowable bending stress = Fy'
Allowable bending stress = Fy'
(OP)
I recently reviewed a simple-support steel beam design performed by an outside consultant. The consultant used ASD. Allowable stress he used to check a HP 10x42 was Fy' = 29.4ksi. Why would this have been done?
I have never seen yet in practice or in books of Fy' (value of when flanges become non-compact) be used as the allowable stress.
Can someone possibly elaborate on maybe why this was done. I don't have the opportunity to contact the consultant, hence why I'm asking here.
I have never seen yet in practice or in books of Fy' (value of when flanges become non-compact) be used as the allowable stress.
Can someone possibly elaborate on maybe why this was done. I don't have the opportunity to contact the consultant, hence why I'm asking here.






RE: Allowable bending stress = Fy'
RC
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Edmund Burke
RE: Allowable bending stress = Fy'
RE: Allowable bending stress = Fy'
RE: Allowable bending stress = Fy'
The beam was an HP 10x42. AISC 9th edition lists Fy' as 29.4 ksi. The steel was A36 steel.
He actually substituted Fy' = 29.4ksi for Fy and made the allowable stress = 0.66(29.4ksi).
The beam I believe was not laterally braced in any appreciable way.
RE: Allowable bending stress = Fy'
RE: Allowable bending stress = Fy'
The section would still need to be braced appropriately (which you didn't seem to think it was), and if it is there is nothing wrong with doing what he did. I've actually heard a QA/QC head at a large/prestigious firm that they teach their young engineers to do this. This was before the 13th edition was prevalent and the 9th edition was still widely used.
RE: Allowable bending stress = Fy'
RE: Allowable bending stress = Fy'
HP sections are often used by Bridge Contractors for temporary structures for at least two reasons, the first one is obvious, the second one is subtle:
1. Over time a Bridge Contractor will end up owning a lot of HP that he did not need, or were cutoffs.
2. Temporary structures will have to be removed (wrecked). The Contractor's objective is to do this with the least amount of damage to his temporary structural members (so they can be reused). Removal often means pulling one the members "sideways" (the y-axis). As you know, HP have significant y-axis section modulus because of the very wide flanges. An HP will successfully withstand y-axis loading that would functionally destroy a typical W section.
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RE: Allowable bending stress = Fy'
What he did didn't hurt anything and wouldn't but I don't think there was really any good reason to use Fy less than 36ksi.
RE: Allowable bending stress = Fy'
Additionally, to look at the true behavior I don't see this as much different than any other beam. If you have an A36 beam with Fy'=38ksi, it likely isn't truly compact since the actual yield stress is probably higher than the minimum of 36ksi. If the actual yield stress exceeded 38 ksi, then you wouldn't have a compact section, but you still assume the 36ksi and use it as a compact section and it is conservative. I see no difference between the two.