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Re: existing wood floor joist bending stress

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Roger is right. The behavioral equations changed in 1991 when the design
values changed. Here's a link to more background information on In-grade
testing and the changes that occurred in 1991:

The last paragraph states: "Note that concurrent with development of new
design values in the 1991 NDS, behavioral equations for column, beam, and
beam-column design also changed as a result of the In-Grade Testing program.
Therefore, an advisory was issued with the 1991 NDS indicating that new
design values were to be used simultaneously with new design equations and
pre-1991 design values be used with pre-1991 design equations."

Wrt evaluation of existing structures, I'm not aware of a "national
standard" that governs how it should be done. Each jurisdiction has it's own
requirements. Evaluation, Maintenance and Upgrading of Wood Structures
(ASCE) has a write-up on different building rehab regulations, but it's
dated. My suggestion to Joseph Eribarne was to evaluate it based on the
original stresses, then evaluate it based on current stresses, then use
engineering judgement based on knowledge of safety factors both on the loads
side and the resistance side.

Which reminds me, Joseph E., I forgot to suggest that you take a look at the
design using LRFD. If there are multiple transient live loads, then
typically the load factoring alone will provide some efficiencies in your

Here's a link to a few more resources on evaluation of exising structures
for those of you interested in exploring this further:



John "Buddy" Showalter, P.E. 
Director, Technical Media 
AF&PA/American Wood Council 
1111 19th Street, NW, Suite 800 
Washington, DC 20036 
P: 202-463-2769 
F: 202-463-2791 

The American Wood Council (AWC) is the wood products division of the
American Forest & Paper Association (AF&PA). AWC develops internationally
recognized standards for wood design and construction. Its efforts with
building codes and standards, engineering and research, and technology
transfer ensure proper application for engineered and traditional wood

The guidance provided herein is not a formal interpretation of any AF&PA
standard.  Interpretations of AF&PA standards are only available through a
formal process outlined in AF&PA's standards development procedures.


From: "Joseph R. Grill" <jrgrill(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: Re: existing wood floor joist bending stress

It would be nice if AWC would reply to the list or if Joseph will let us =
know what he might find out. What I "think" I remember from a seminar = is
that the wood hasn't changed it is the way it is tested and that the = new
values should be used no matter the time frame. So, I would be very =
interested in some explanation, again, from somebody in authority. Joseph

----- Original Message -----=20
From: Roger Davis=20
To: seaint(--nospam--at)
Sent: Wednesday, September 06, 2006 3:09 PM
Subject: Re: existing wood floor joist bending stress

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, =
less that the original design. However, the present day code = indicactes
that the allowable bending stress for this wood is now 1006 psi. The =
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
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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