Subdiaphragm in light-framed wood building??

[WAstruc10]

I certainly understand the ASCE 7-05 Section 12.11 requirements for wall anchorage and subdiaphragms when it's a concrete or masonry wall tied into my floor. But what about light-framed wood wall structures...it seems like there is no exception to 12.11.2.2.3, so when I have exterior studwalls parallel to my floor joists, is it really necessary to put in long blocking/strapping lines to develop a subdiaphragm? It is quite difficult to imagine a light framed wall pulling away from the 1-1/8" subfloor nailed down to the double top plate at 6" on center...but the code won't let me use the diaphragm in tension. Please tell me I'm missing something?

 

[DaveAtkins]

I think you will have subdiaphragms anyway, because the top of the double top plate will be at the same elevation as the bottom of the joists. There will be a rim board, which is the same depth as the joists, on top of the double top plate. You will need blocking perpendicular to this rim board.

DaveAtkins

 

[WAstruc10]

Thanks Dave, we're actually using open-web wood chord floor trusses, so there is no rim or blocking per se. Wall double top plate is up at underside of subfloor ply. With a subdiaphragm ratio of 2.5 max this seems to mean, with 32' floor truss span, that I have to block/strap back every so often (8 ft oc? Depends on horizontal span capacity of double top plate?) for 32/2.5=12.8' subdia depth. Just seems like a waste, no? And what about gable ends for roofs, where we use gable end bracing up to a blocking piece between trusses...does this mean to support our wood wall out of plane that we need to make a subdiaphragm by blocking/strapping the roof trusses, instead of just calculating how much horizontal load that one piece of blocking transfers into the full diaphragm? Roof trusses can span pretty far, so the subdiaphragm requirement would mean really long straps in some cases. Hmmm....

 

[DaveAtkins]

I think if you can justify the forces you are working with, you are fine. As you stated earlier, I think the prescriptive rules in the code are more concerned with diaphagms bracing masonry or concrete walls.

DaveAtkins