Double Top Plates as Diaphragm Chord

[palves]

I'm currently working on a wood mezzanine and not visualizing how the diaphhragm chord forces will be resolved. I understand the theory behind the forces, but just don't see how the load path runs from the plywood nailing, down through the blocking/rim joist, and into the top plates. My joists are 16" deep so am I really supposed to believe that a 16" deep 2x4 block will transfer this tension force down into the plates? It seems too tall. Should I use 4x blocking and strap across the sheathing underneath the upper floor sill plate or should I strap across the blocking on the outside underneath the shear wall plywood? Any advice?

 

[BAretired]

No, a 2 x 4 block will not transfer lateral forces. It is sometimes called a "squash block" and is intended to transfer gravity loads through the joist system.

To transfer lateral forces, you need blocking panels fitted between joists. See some of the many TJI catalogs for more information.

BA

 

[msquared48]

As BA pointed out, your idea of what blocking is, is false.

It fills the void between the joists, and between the top plate and underside of the plywood. You really need to use TJI blocking here as suggested.

Mike McCann
MMC Engineering

 

[palves]

Even though my wording might not indicate it, I do understand about the blocking between the joists. That wasn't the issue, but my explanation wasn't clear. I mistakenly used "16" 2x4 block" when I meant 16" deep blocking between the joists. After talking with a friend I realized that I was visualizing the load from the diaphragm being transferred to the top plates through the sheathing only when I should have visualized the "diaphragm" as the integrated system of plywood, joists, and blocking. When seen that way I understand how this diaphragm system transfers a shear force to the top plates, which in turn puts the chord tension force into the top plates.