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Load Path for roof or floor Diaphragm Shear and CALCS

Load Path for roof or floor Diaphragm Shear and CALCS

Load Path for roof or floor Diaphragm Shear and CALCS

I am confused with the design of Diaphragm shear calculation in floor or flat roof for plywood. Let say I have five shear walls carrying 20kips of shear force on each shear walls of length 20 ft. My roof and floor width is 40ft wide by 120 ft long. So, I when i calculate the diaphragm shear force along floor or roof (width side of 40ft) it will be 20K/20ft = 1kips/ft of diaphragm shear. So i provide the sheathing size and nailing pattern for diaphragm shear on floor per 1kips/ft??? Is this correct? Do i take the full diaphragm width of 40ft and diaphragm force will be 20k/40ft = 0.5k/ft and design sheathing and nailing pattern per this load? Please suggest which is correct. I am told to take the full length of diaphragm width as 40 ft????????

RE: Load Path for roof or floor Diaphragm Shear and CALCS

Either solution could be correct depending on how you've chosen to arrange your diaphragm collectors and chords. If it were my building, I'd likely arrange things to utilize the full 40' depth. If you post a sketch showing your walls, collectors, and chords, we'll be happy to provide comment.

This article gets into the theory a bit: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Load Path for roof or floor Diaphragm Shear and CALCS

You are mixing apples# and oranges.

Simplistically with no sub diaphragms to consider, when calculating the diaphragm shear nailing here you would use the 40 foot length giving only 500#/ft for the diaphragm nailing. Drag link(s) will collect this load and carry it to the shear wall for the area away from the shear wall. The design load(s) in the drag link(s) plus the nail force of 500# X 20' SHOULD equal the 20 kip load needed.

Similarly for the other direction.

Clear as mud?

Mike McCann, PE, SE (WA)

RE: Load Path for roof or floor Diaphragm Shear and CALCS

Thanks Mike and Koot K for clarification......It helped a lot...

RE: Load Path for roof or floor Diaphragm Shear and CALCS


Consider getting the book "The Analysis of Irregular Shaped Structures" By Terry Malone. It's a fantastic reference for all things wood diaphragms and shear walls. It's the best technical book I own and will help you understand load paths extremely well. Wouldn't need it for a rectangular building, but for anything else, it's fantastic.

I just wish Malone would write more books!

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