Stress-----Always refer to service load?

[RobertEIT]

We know that ACI requires to use LRFD to do design calcs. But now if somebody asks for the shear stress in a reinforced concrete beam, say if we were given 10k dead load + 20k live load, should we use service load 10k+20k to calculate the shear stress or should in the beam we use factored load 1.2*10k+1.6*20k to calculate the shear stress in the beam?

Thanks a lot for your help.

 

[COEngineeer]

You factor it to check shear also.

The only time you dont factor it is when you check deflection and when you look at the stress of the steel for crack control.

Never, but never question engineer's judgement

 

[frv]

Anyone who asks that question will liekly know what he/she is talking about; I suspect they'd most likely as for either service or strength (factored)-level loads.

In lieu of that, I would assume that they would be asking for factored shear stress, given that shear is not a serviceability issue, but a strength issue.

One caveat: if this is a forensic problem, then you'd liekly need to know the actual stresses in the beam (or an approximation of the stresses, anyway).

 

[swivel63]

factored

 

[Lion06]

When is the last time someone calc'd the shear STRESS in a RC beam? I alsoys calc the shear FORCE and shear STRENGTH.

 

[RobertEIT]

Well, The big problem is that "somebody" is a well known authorized organization. We were unable to ask what he want from us --- service load based stress or factored load based stress. We can only guess what he want from us. You know what I mean. If you were under this situation, which load will you use to calculate the stress?

Thanks a lot.

 

[swivel63]

i've seen it done before. as a quick-calc. from that, he would come up with stirrup spacing.

i use factored shear stresses in my cap and footer designs because i have the phi-vc memorized as opposed to the phi-Vc.

 

[swivel63]

i've also done it with link beams in the corewall buildings i've done.

 

[kslee1000]

Can you type the question entirely, not just "provide me with stress", there could be a clue to which the guy is looking for.

Otherwise, do both and let him pick. But it may make you seem not understand THE ISSUE he has been troubled by.

 

[RobertEIT]

Sorry, kslee, I can not post the exact question. It will violate the agreement we signed with that organization. By asking this conceptual question, I did not violate the agreement. It related to the event happened in April 24. I suppose many engineers in U.S know what event happened in U.S. that day.

 

[kslee1000]

If something related to failure, the working stress is more useful in investigation.

I will present this way:

1. Provide unfactored DL & LL, and any other load acting on the beam seperately.
2. List unfactored shear stress (over b*d) for each load case.

By doing so, he can do any combination he wants to, using any load factor he sees fit.

 

[JAE]

Ya - I'd do what kslee1000 is suggesting - give them stress both factored and unfactored and let them figure out which to use.

Just be clear in your correspondence in describing each.

 

[miecz]

I would provide service load stresses only, and break them out into dead load and live load. Factored stresses are useless, as there is nothing in the code to compare them to.