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Re: Acceptable Level of Overstress

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The section I mentioned is in 2003 IBC, but it is the same language that you are referring to. And, I fully agree with your interpretation. My assumption was that the existing member is fully stressed. 

Ben Yousefi, SE
Santa Monica, CA

>>> jkilgore(--nospam--at)leok.com 12/04/03 10:10AM >>>


> There is no specific provision in the UBC for this.
> But section 3403.2 of the IBC specifically mentions
> 5% as acceptable overstress of excising members.
> Section 15.8.4.2 of the NFPA 5000 also allows 5%
> overstress.

Is this the 2003 IBC?  In the 2000 edition, 3403.2 concerns fire escapes.
Section 3402.2 mentions a 5% number, but it states: "Additions or
alterations to an existing structure shall not increase the force in any
structural element by more than 5%, unless the increased forces on the
element are still in compliance with the code for new structures...".

I interpret 3402.2 to mean that you can basically add 5% load without
triggering a structural analysis.  If the member under consideration is
already at 100% capacity, then yes, you have a code-allowed 5%
overstress.

I do not interpret this as carte blanche to automatically assume the
existing member is 5% stronger than a new member of similar
construction.

If the existing structure is in good condition, then 5% over is reasonable.
At least I hope so, because I've used this value before.
Factors of safety for steel were higher in the past, concrete usually gets
stronger with age, and code-allowed stresses for older wood are higher
in general.  But I've seen way too many rusted steel, deteriorated 
concrete, and rotten wood beams to automatically assume that they're
capable of code-allowed stresses, much less 5% over, without a personal 
inspection.

----
Jason W. Kilgore
Project Engineer
Leigh & O'Kane, L.L.C.
Kansas City, Missouri



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