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RE: Acceptable Level of Overstress

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Gerard:

Also, I certainly do not exceed 5% overstress (Usually no more than 2% to be honest) on new members,

I don't see why 5% would be unreasonable for a new beam.  On GLB I might be a little more circumspect because I don't know how they come up with the allowable stresses.  On solid sawn (visual graded), I have no problem at all.  The NDS values are derived in a manner that if you design to 100% you have a safety factor of about 2.5.  This is the safety factor on the value saying only 5% will fail.  If I am off target on this, feel free to correct me.

In terms of how much overstress to allow, I say it's purely up to the engineer and how comfortable they are.  Obviously, the standard accepted values for DESIGN are the ones noted in the NDS.  Values higher than that are subject to the Engineer.

Seth

 but on existing, I have had bosses/owners to push it to 15% overstress (when I was younger). Now that my stamp is on the drawings, I get a little nervous when I approach 10%. I don?t have a problem up to about 7-8%
 
Thanks,
-gm
 
-----Original Message-----
From: Seth W. Cutler [mailto:seth(--nospam--at)rlmorrisonengr.com]
Sent: Thursday, December 04, 2003 8:56 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Acceptable Level of Overstress
 
Gerard:

When we check existing wood beams we use the allowable stresses at the time the building was designed/built.  Our office standard is 5% overstress due to the variability in the material and how likely the member will see the full load.  If it's 6% or higher then we might come in and sister on a member at mid-span to bump up the section.  There are other solutions too if it's exposed.

I'm assuming it is bending stress that is controlling.  Make sure you are using all allowable stress increases.  If it's shear stress then you could use the newer NDS which allows a higher shear stress.  The reason for this was discussed in previous threads.

Seth

At 03:22 PM 12/3/2003, you 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
California Licensed Structural and Civil Engineer
 
Madden Engineering
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