Shear Wall Overturning

[BadgerPE]

Good Morning/Afternoon All,

Looking for a little clarification on ASCE 7-05 Section 6.4.2.1.1 Minimum Pressures. In that section it says that all pressures in Zones A, B, C, and D should be +10psf while Zones E, F, G, and H should all be 0psf. This is where my question comes in. If my Zone B pressure calcs out to be 5 psf should I then use 10 psf as a minimum design for overturning of a shear wall and set uplift pressures equal to zero? (Case 2 in attachment) It seems highly unconservative to neglect uplift forces for overturning of shear walls. Or is the correct procedure to use the calculated value of 5 psf + Zone C + Net Uplift for overturning and the minimum 10 psf + Zone C when determining shear only forces. I have attached an arbitrary interior shear wall for review.

Thanks much!

 

[JAE]

I think the key word here, in section 6.4.2.1.1, is "load effect".

This means that any force, moment, shear, etc. found from your straight-up load case (your 5 psf example) must not be smaller than that found using the 10 psf projected area approach.

You are comparing individual shears, moments, etc (i.e. effects) rather than global forces.

 

[ash060]

Unless you have a steep roof slope your roof pressure zones will be negative (acting away from the building). The main thing ASCE 7 is talking about is to not subtract your horizontal roof pressure component from the horizontal wall pressure component.

If the two loads oppose each other treat the roof component as zero, if they are additive (steep roof slope) than add the A & B zones (or C & D) together.

Check Note 7 on Figure 6-2