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Re: AASHTO-timber-shear

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The limiting shear capacity condition in timber beams is shear stress
horizontally, parallel to the wood fiber, not across the fibers. In ordinary
principles of mechanics, the intensity of shear stress is the same in vert
and horiz directions at a given spot.

My impression of rationale behind the calculation of maximum/design shear 
at 3d or L/4 off support for moving loads is that the load is a concentrated
load which acts to vertically press the wood fibers together in that general
vicinity and thereby improve their resistance to shear slippage along any
horizontal faying surface. 

The traditional deck-of-cards demonstration for shear slippage in a bent
beam (the deck of cards) shows that the tighter the cards are pinched
together, the greater the horizontal shear resistance.

I am interested in other explanations of rationale that might be offered.

I don't have a reply on the decking part question. The NDS for building
structures may have some add'l info.

Charles O. Greenlaw  SE  Sacramento CA
------------------------------------------

At 07:20 AM 05/25/2000 PDT, you wrote:
>I would appreciate a knowledgeable comment on the AASHTO requirement on 
>shear in timber beams (AASHTO SSHB 13.6.5.2).
>
What would be the rationale behind the calculation of maximum/design shear 
at 3d or L/4 off support for moving loads?  If used for wood transverse 
decking, does this approach assume distribution of wheel load along the tire 
width (partial distr. load vs. concentrated load)?
>
>What would be a good source of additional information on the subject?
>
>Steve Gordin, SE
>Irvine CA