Ceiling Diaphragm plus Roof Diaphragm 1

[haynewp]

What is the standard way of analyzing a wood building that has an attic with plywood and a roof with a plywood diaphragm?

For example, a large single story residence with wood trusses with a 5 on 12 roof pitch and with a plywood or osb attic floor. It seems to me the attic floor is more stiff than the high pitched roof since the attic floor is horizontal, so the attic floor would naturally take more of the wind from the exterior walls than the roof diaphragm. But what is the normal assumption? Design the roof nailing to take 1/2 the wind from the wall and all the roof, or send 1/2 wall wind plus 1/2 the roof wind into the attic diaphragm and only design the roof diaphragm for what 1/2 the roof sees?

(ie. I don't want to provide a nailing pattern for the attic floor and a pattern for the roof and have the contractor laugh about..)

 

[msquared48]

Normally in an attic with trusses, the web members would interrupt any attempt at a diaphragm at the ceiling level. Are these premanufactured open web trusses, or sloping TJI's or similar?

If it is open web trusses, the roof diaphragm is the normal structural element. If sloping TJI's, then the attic space could be DESIGNED to be a structural diaphragm, as well as the roof diaphragm. If it is an occupied floor, it should be. If not, the call is yours as the structural engineer.

Mike McCann
McCann Engineering

 

[haynewp]

Good point, I initially missed the obvious interruption of the diaphragm with the chords. I guess it is moot post now, I will just provide a floor for storage space but not count a diaphragm at this level.

 

[haynewp]

I meant webs.

 

[msquared48]

To quote Mik from Crocodile Dundee - "Oh...is that what you meant..."



Mike McCann
McCann Engineering

 

[haynewp]

I used to always think of Crocodile Dundee when I heard anything about Australia, now always think about The Wiggles. I guess that is what having kids will do to you.