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Response to John Jones ? on Allowable Stress Increases by WTCA

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To: John Jones

Please see comments below.


13 Message:0013 13


To: <seaint(--nospam--at)>

Subject: RE: factory-made roof trusses


This reminds me of something I have been curious about on truss submittals. In their computer output(Alpine) there is usually a little box labeled something like allowed stress increase and in my area (Central Alabama) the box is usually labeled 1.15. The code does allow a repetitive stress increase of 15% for bending members provided you meet a few requirements. This is not clearly spelled out the way the little box is described on the output. My interpretation of the output is that everything is bumped by 15%therefore I always write in 1.00 and mark the shop drawings "Make corrections noted" Has anyone run into this before and do you know if the program is, in fact, only bumping the Fb by 15%.

The allowable stress increases are taken as defined in the National Design Specification for Wood Construction NDS published by the American Forest and Paper Association. A good contact there is John Showalter, Jr. at 202-463-2771 or Buddy_Showalter(--nospam--at) for questions on wood design. I am actually surprised that allowable stress increase in Alabama is only 1.15, which is for snow load duration. It really should be higher. If you have additional questions please ask us. There is a good deal of confusion on these issues if you are not used to doing design in wood.

Respectfully Submitted

Kirk Grundahl, P.E.
Executive Director of WTCA