Is a sheathing considered a blocked diaphragm?

[ronstruc]

In an existing building seismic retrofit, I have seen a design where the engineer used a wood sheathing (OSB) overlay as a wood blocked diaphragm. The walls are masonry and the designer placed anchors and straps to connect the wall to the diaphragm for out-of-plane connections without using blocking below the strap. The designer considered the two layers of sheathing (original plywood and an OSB overlay) as blocking. The designer did this for all straps for continuous cross-ties and subdiaphragms. Our firms approach is to remove sheathing where straps are to be placed and place blocking below the straps. What are your opinions and do you have any references for or against treating an overlay as a blocked diaphragm or as blocking for straps.

 

[msquared48]

Without a detail, your description tells me that your firm’s approach is correct and conservative. I would inquire as to why the second layer was needed - deteriorated original plywood later, or need for more diaphragm capacity. In the first case, I would not use a deteriorated member for the support of another. In the latter case, I do not as the nails used could not be longer than the total thickness of the plywood and OSB.

Mike McCann, PE, SE (WA, HI)

 

[ronstruc]

I believe the main purpose for the second layer of sheathing was so the contractor didn't need to remove parts of the existing wood sheathing to install blocking for a blocked diaphragm or below straps. It could be a quicker and cheaper solution to just install an additional sheathing layer.

 

[dold]

Sometimes we've used additional layer of plywood if there is a heavy concentrated load on the deck, specifically for storage racks on a mezzanine in my case. But yea, I'd put blocking at the straps if you want to do it right. There are no values for straps anchoring to two layers of ply/osb. Most use 10d nails which would poke out the back side of the plywood.