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Re: Critical section for shear in wood beams

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Yes, wood and concrete are materials that markedly differ from each other,
and both differ from masonry, metals, and plastics. While principles of
mechanics have a universality of application, I remember my college
instructors always cautioning that the principles being taught right then
applied only to materials that are homogeneous and isotropic. Sawn lumber
and concrete are dramatically non-isotropic as to their strength properties,
and lumber is likewise in regard to stiffness properties.

Thus wood and concrete are inherently "inconsistent" with each other, and it
should not be troubling that design code provisions that deal with these
detail differences also appear inconsistent. 

Other relevant code provisions relating to calculating controlling
horizontal shear stress for short, deep wood beams, and/or where
concentrated loads occur somewhat more than the beam's depth from a support
may be found in the 1997 edition of ANSI/AFandPA's National Design Spec at
section 4.4.2. Versions of these have been in the NDS and UBC Standards for
many years. In general, a reduction in design horizontal shear (shear
parallel to grain) is offered where there is substantial cross-grain
compression acting simultaneously on the zone of high shear stress. 

Perhaps the "clamping" effect of the cross-grain compression near the
support acts to improve the strength of the wood beam against internal
slippage failure parallel to grain. Shear friction in concrete corbels,
etc., has a similar explanation in ACI 318 Commentary. And prestressed
concrete seems to enjoy an improved crosswise shear strength due to the
added lengthwise compression "clamping", compared to non-prestressed.   

It might be noted that for wood I-Joists (wood plate girders) there isn't a
horizontal grain orientation in the web panel; instead there is horizontal
shear at the connection of web to horizontally-grained flanges. Fittingly,
shear is figured at the face of support and without credit being available
for load concentrations nearby.

Charles O. Greenlaw, SE    Sacramento CA
At 07:17 AM 1/9/99 -0600, you wrote:
>I have often wondered about this inconsistency between the ACI and NDS
codes (I am sure
>there are others) but have never been able to find an answer for this other
>different committees developed those code requirements. Granted these are
>materials, but I think the mechanics must be similar.
>I believe it is important to address these inconsistencies to eliminate
>Maybe a letter needs to be written to each organization pointing out the
>and suggesting that they get together to try to resolve it.
>Jim Kestner, P.E.
>Green Bay, Wi.
>> > *******************
>> >
>> > From: Suresh <suresh(--nospam--at)>
>> > To: seaint(--nospam--at)
>> > Subject: Critical section for shear in wood beams
>> >
>> > ACI requires that critical shear stress be calculated at the face of
>> > support rather than a distance  'd'  from the face of support when a
>> > concentrated load occurs within a distance 'd' from the face of support.
>> > But UBC 94 chapter 23 without any "Ifs" or "Buts" allows designers to
>> > neglect ANY loads within a distance 'd' from the support. Is wood
>> > different than other materials? Anybody know any other relevant code
>> > provisions?
>> >