Allowable bending stress = Fy'

[btomcik]

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.

 

[PEinVA]

If the calculations follow the typical design method and the only difference is Fy' is it an issue?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[MiketheEngineer]

Was he using 50 KSI steel? Was it laterally braced??

 

[JStephen]

Did he actually specify that he was using Fy'? Or did he use a stress that just happened to be that number?

 

[btomcik]

I need to clarify a couple things.

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.

 

[NS4U]

Something to do with the HP being non-compact?

 

[Lion06]

If I remember correctly, Fy' is the value of Fy for which anything less than that the section can be considered compact (i.e. Fy<Fy' means a compact section).
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.

 

[JStephen]

I suppose the idea is that if the yield stress was 29.4 ksi, then the member would be compact and would be okay for that stress, and that therefore it ought to be good for that same stress even if the yield is higher. But I'm not sure why you'd do it that way. F1-3 would give a higher allowable stress.

 

[SlideRuleEra]

Suggest that you ask the Engineer if the HP is "used". Lowering Fy' may be his way addressing the possibility that the member is not in "new" condition. See my comments in your post: thread507-227886

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[btomcik]

I'm pretty sure the HP was "used." Still, in my opinion there would be no reason to take Fy anything less than 36ksi. The determination of the allowable stress for whatever situation (bending, compression, shear, tension) as identified in the specs of the AISC manual takes into account variations in material quality, construction tolerances, etc.

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.

 

[Lion06]

The reason was to get away from calc'ing an allowable bending stress and just using the generic 0.66Fy. I'm not saying that calc'ing the actual allowable bending stress is difficult, but there's not much that is easier than 0.66*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.