Exterior Load Bearing Walls - OOP Wind + Uplift 1

[EngineerEIT]

Per ASCE 7-10:

When you are designing wood load bearing studs supporting low slope wood roof trusses, do you include an uplift value in your axial load determination due to wind? Do you use MWFRS (say Cp from Figure 27.4-1) or C&C? Say tributary/effective area is less than 700sf.

For the out-of-plane wind, C&C loading makes sense but the uplift from C&C will be higher than MWFRS and will result in a lower axial load than MWFRS.

 

[milkshakelake]

I check wood stud to truss, and stud to stud, and stud to foundation connections for C&C uplift because they can fail locally. MWFRS is intended for parts of the structure that handle large portions of the wind load, like shear walls, not single studs. For axial, I check wood studs for load combo without uplift, so it won't result in lower axial load. Load combinations aren't just 0.6 x dead load + 1 x wind load; they include other things like dead + live.

By the way, the tributary area for a single wood stud would be much, much less than 700sf. It would be closer to 100 to 150 sf if you used the definition of effective tributary area.

 

[jayrod12]

Agree 100% with milkshakelake's response.

 

[EngineerEIT]

I understand multiple load combos exist and I understand the difference from tributary area and effective area.

I think everyone knows that the out-of-plane pressure on the wall should be C&C but what about the uplift from the roof trusses?

I am asking when you design the load bearing wall studs, the axial load on the stud due to the reaction from low sloped roof trusses should you use MWFRS or C&C pressures? You could ignore this uplift force, but I would think in reality it exists and I am asking how to quantify it.

 

[JAE]

I think everyone knows that the out-of-plane pressure on the wall should be C&C but what about the uplift from the roof trusses?

You'd also use C&C for the roof uplift. Certainly not MWFRS wind roof uplift in combination with lateral C&C for a single stud design.

If it was also part of a shear wall, then we would do a single stud C&C design and then check it with MWFRS loads as part of the shear wall.

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[jayrod12]

Your truss reactions in uplift are based on the C&C wind loads for the individual truss, therefore assuming 2ft spacings on the trusses and likely no span larger than 40-50ft, you're at most looking at 100sqft of trib area.