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Re: existing wood floor joist bending stress

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Grading standards changed with publication of 1991 NDS. Wood graded by the pre-1991 standard has to be designed by the NDS rules in effect at that time. Wood graded by the post 1991 standard has to be designed by the post 1991 rules. You can confirm that with AWC Help Desk.
Roger Davis
SDS Architects, Inc.

Joseph Eribarne <jeribarne(--nospam--at)> wrote:
I am involved with a structure that was constructed in 1988. 2x12 floor
joists were designed for a tile flooring weighing 30psf. The allowable
bending stress for a repetitive Doulas Fir Larch joist was 1450 psi in the
1988 UBC. A flooring weighing 3 psf was applied (to save money).

The client would now like to apply a flooring weighing 10 psf, substantially
less that the original design. However, the present day code indicactes
that the allowable bending stress for this wood is now 1006 psi. The design
bending stress in the joist with the 10psf flooring is 1200 psi. What is
the allowable bending stress for the joist in the existing structure? Is it
the origial 1450 psi, resulting in a design stress of 83% of capcity or is
it 1006 psi, resulting in a 19% overstress, or is it something else?

Thank you for your attention.

Joseph Eribarne

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Roger Davis
SDS Architects, Inc.

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