Strength design versus Allowable stress deisgn in IBC

[clarke1973]

I have received a design from a contractor to review which is for a single storey steel framed warehouse (portal framed building).
He has used the allowable stress design load combinations (IBC section 1605.3.1) for the analysis and member designs. Generally for strength checks I always use the ultimate state combinations (1605.2.1) regardless of the building type, and use the basic load combinations for serviceability checks.

Is there anything in the code that will permit strength checks to be done using the allowable stress design method. Can anyone offer any advice on this?
Thanks

 

[ajh1]

Either ASD or LRFD is perfectly acceptable for all designs. Generally you should not use a mix of both.

 

[TehMightyEngineer]

Yeah, don't mix both; that's asking for trouble. There's nothing wrong with using ASD with the IBC. If anything using ASD will be slightly more conservative.

About the only thing I'd be clear on is he should be technically using the allowable strength design (not stress). If he's using the 9th edition ASD (allowable stress design) then that wouldn't be acceptable.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[JAE]

If he's using the 9th edition ASD (allowable stress design) then that wouldn't be acceptable.

Well that is probably true in most all cases.

But if a city or jurisdiction has not updated their code in many years and the current, applicable code refers to the 9th edition then that is the legal, adopted and required specification to use.



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[TehMightyEngineer]

JAE, you're of course correct; I should have said "most likely wouldn't be acceptable".

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[XR250]

HMM, I still use the 9th edition, but most of my steel is governed by serviceability and I never design to the gnat's ass.
Are the changes significant?

 

[TehMightyEngineer]

In short; yes. Are your serviceability controlled designs going to fail any time soon; no. Should you switch if your AHJ requires anything beyond 9th edition; definitely.

You do mostly low-rise right XR250? If you're just sizing small beams and posts then you're not going to see much change. It's when you get into the second order effects for large structures and such things that the changes start becoming more critical.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[XR250]

Yup, mostly low-rise and residential so I ain't gonna worry about.
Thanks for the info

 

[Nor Cal SE]

One item from the original post has not yet been addressed. Even when using LRFD design, my thought is that serviceability (deflection) checks should always use unfactored loads, because ultimately I want to see the "real expected" deflection, and compare that deflection to the "L/___" criteria dictated either by code, by best practice given the circumstances/materials, or by client request, whichever is the most stringent. I don't want that "L/___" number to be based on amplified loads, because that will give me a skewed result.

I do a fair amount of LRFD design, but when laying out my calcs, I simply keep loads in unfactored form and then apply the load factors on the same line where I calculate moment and shear.