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Re: horiz. diaph. blk'g.

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There!  You've forced my hand!  :-)
I opine that this is a detail open to the debate between the "it calcs!" and the "it doesn't calc!" camps.  I have done the math on a 2x on flat for carrying the shear loads I have found in diaphragms (and shear walls).  The shear diagram changes across the diaphragm and the numbers suggest to me that the required block thickness will be driven by the shear flow at the location of interest.  You would have to decide if you need a 2x or 3x or even 4x and nail configurations etc.  Once I am at that decision then I may call for 2x (3x...) on edge, the same as in a shear wall.  The flat fits nicely into joist chord depths.  Better still I will often call for full depth blocking (spanning top and bottom chords).  In real life I grudgingly accept 2x etc on flat only after a battle with the contractor and only if the main members are not too far apart.  E.g. I probably wouldn't spec it for 2ft+ spacings but might for 12" and 16" spacings, again dependent on the shear flow.
I believe that "on flat" blocking is not irrational if the shear is assumed to travel predominantly through the plane of the sheathing.  If it goes elsewhere as well then... who knows?  The blocking is for transferring shear at edges and the only secondary effect to consider would be, IMO, the local twisting, buckling, incompatibility issues, etc at the join, hence maybe making a limit on the main member spacings a good idea.

Thor A Tandy P.Eng, MCSCE
Victoria BC
e-mail: vicpeng(--nospam--at)
----- Original Message -----
Sent: Thursday, March 09, 2000 1:14 PM
Subject: horiz. diaph. blk'g.

So was my post so trivial no one is interested in responding or is everyone scratching their head ?