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Acceptable Level of Overstress

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Basically overstress is what you are comfortable with.  When designing a new 
structure, I will generally limit overstress to 3, 4, or 5 percent, and 
look very hard at 6, 7, or 8 percent overstress.

For an existing building that has been standing for years and years (say, 20 
or more years), and has not experienced distress/failure and does not have 
any visible sign of distress, I would probably be comfortable with 10, 11, or 
12 percent overstress.

HOWEVER, if wood members were designed before the 91 code and used the old 
allowable stresses, a 5 percent overstress may well be a 50 percent 
overstress using the current allowable stresses.  Remember, the stresses were 
changed based on testing RANDOMLY selected full-size, in-grade lumber.  
Randomly selected means that they did not separate "new growth" lumber from 
"old growth" lumber for the testing.


A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Gerard Madden wrote:

. > What is the general practice on acceptable level of overstress when
. > checking an existing member for capacity?
. > 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???
. > Tia,
. > -gm
. > Gerard Madden, SE, PE

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