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Re: Acceptable Level of Overstress

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> I have an existing wood beam that is 6% overstressed with new loads. I
> am prepared to deem that "Acceptable". I was always taught that 10% or
> less overstressed is okay. I have a plan checker wanting to cut the
> limit at 5%. Is this somewhere in the code???

First, how old is this wood beam, and are you using the correct allowable
stresses?  Older wood generally had higher allowable stresses, because
stronger wood was more common (modern wood is grown as fast as possible to
generate more revenue and is "weaker").

Second, inspect the beam and see if you think higher stresses might be
justified (tight grain, no knots, splits, or checks, etc.).  If reinforcing
is expensive, and the building official stands firm, then consider having a
testing company evaluate the strength of the beam.

If you have checked these criteria and you're still overstressed, I don't
know of any code provision allowing a set amount of overstress.  Basically
it's your engineering judgement as what constitutes protecting public
safety.  Is a factor of safety of 1.90 acceptable when the code says use
2.00?  If you think the beam is strong enough or if the maximum load
combination is unlikely to occur, then use it.

Keep in mind that the plan checker is also responsible for maintaining
public safety.  If he says that 5% overstress is his hard limit, then you
pretty much have to do what he says.

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

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