Sloped Roof Diaphragm - Post and Beam Roof

[cuels]

I am designing an addition onto an existing home that is going to be 27'w x 68'L. The roof will be made up of a ridge beam (to carry TJI rafters) and posts every 16'. The roof is a 6:12 pitch. The bearing walls will be 16' high. I am not sure how the code works with a sloped roof diaphragm and post and beam supporting structure. The wind loads control the design. The wind loads will be 195 plf for 6ft, 150 plf for 32 ft, and 110 plf for the remaining 30 ft transferred from the walls (and roof) to the roof diaphragm. All of my calculations show that the diaphragm can handle the load across the 68' span to the resisting shear wall lines. Am I missing something in the code I should be considering for engineered design (i.e. aspect ratio, sloped diaphragm load reductions, etc.)? Your comments would be appreciated.

I know in the prescriptive code it requires there to be a shear wall line every 35', but I am engineering the diaphragm. I am trying to decide whether to include a tall shear wall midway or if I can get away without it.

 

[a2mfk]

Whoa, an 1800sf ADDITION! I'd like that addition, I'd call it my new house...

Check your diaphragm deflection, and is that compatible with your structure? Can your walls perpendicular to the loading direction accommodate that P-delta at the top from the diaphragm deflection?

And I'll be the first to say it, a sketch showing some loads and calcs on it is always helpful... Especially a plan view in this case showing the openings. I didn't really understand your different shear values but maybe because I haven't had any coffee yet..

 

[cuels]

Here is a picture of the addition with the loads that I am applying. There is the 2a from IBC, 8' wall tributary, 4' wall tributary, and a cantilevered overhang on a porch. I agree, huge addition that gets more rediculous when you look at the existing home.

Anyway, I had thought of the p-delta, but how do I calculate the shear in the diaphragm with it being sloped and with no bottom tie? Is there a reduction factor for the allowable shear?

 

[DaveAtkins]

First, make sure you are using the correct code. IBC usually does not apply to a single family residence. In my state, we still use the old Uniform Dwelling Code for single family residence design.

Second, your diaphragm does look a little long relative to its depth. You may be able to justify the diaphragm itself, but can you get the shear walls to work for the imposed load?

DaveAtkins