ASCE 7-16, Figure 27.3-8, Case 3

[StrEng007]

Question #1:
For a rectangular building where all (4) walls act as full-length shear walls, it's clear that the south wall of Case 3 will carry 50% of the in-plane shear resulting from the 0.75PWX & 0.75PLX wind loads that travel from left to right. However, ASCE 7 isn't very clear if you're supposed to combine this in-plane shear with the out-of-plane wind resulting from 0.75PLY.

Since the south wall is part of the MWFRS in the x-direction, and also acts to deliver the y-direction wind load into the two side shear walls, why don't we see more design examples for combining these in-plane and out-of-plane checks on our wood or masonry shear walls? They are all part of the same MWFRS, right? I've always referred back to Case 1 and designed the wall for in-plane and out-of-plane separately. Do you agree?

Question #2: Consider all masonry shear walls with wood trusses.
When checking your truss connections for combined in-plane and out-of-plane point loads (in addition to wind uplift), assuming the in-plane is a result of diaphragm shear, what are you using to determine the out-of-plane point load; is it the horizontal component of the truss reaction only, or the out-of-plane load from the wall below? If it was the latter, how do you design this wall (refer back to Question #1)?

 

[ProgrammingPE]

Case 3 requires the shear walls to be designed for in-plane and out-of-plane wind loads at the same time. Depending exactly on what you are checking, you should be able to just make sure that the sum of the load/capacity ratios for the two directions is less than 1.

This may not matter though since if the load/capacity ratio due to wind in the X-direction is 3 times higher than the load/capacity ratio due to wind in the Y-direction, then Case 1 will control over Case 3.

[Load/Capacity Ratio in X-direction] = 3*[Load/Capacity Ratio in Y-direction]
LCRx = 3*LCRy

Case 3 <= Case 1
0.75*LCRx + 0.75*LCRy <= LCRx
0.75*LCRx + 0.75*(1/3*LCRx) <= LCRx

If you had masonry shear walls, the corners would have extra bars for in-plane shear as well as out-plane bracing due to the perpendicular walls so the out-of-plane load would be negligible there.

Technically, even without Case 3, a combined in-plane and out-of-plane check was already required since Figure 27.3-1 shows a suction load on the side walls. Your out-of-plane design, though, will likely be controlled by the higher C&C wind pressure, but you should still keep an eye on all of the cases required by ASCE 7 just to make sure that one of the combined load combinations doesn't control. The same thing applies for your truss connections.


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[StrEng007]

...make sure that the sum of the load/capacity ratios for the two directions is less than 1

Can you substantiate this approach? I'd like to see this written, code or commentary, in the design standards referenced by our building codes.

If you had masonry shear walls, the corners would have extra bars for in-plane shear as well as out-plane bracing due to the perpendicular walls so the out-of-plane load would be negligible there

This approach still allows you to consider each case separately. My original question regarded the combination of these stresses. Let's consider a tall masonry wall subject to the strength design method for combined bending and axial (P-delta). How do you combine this with the in-plane shear through the masonry wall? We currently have no standard that addresses this.

Technically, even without Case 3, a combined in-plane and out-of-plane check was already required since Figure 27.3-1 shows a suction load on the side walls

This figure illustrates wind load as it acts on the structure but is not explicit in stating the required combination of wind directions. If you carefully read ASCE 7-16, Section 27.3.1, it says:

"Pressures shall be applied simultaneously on the windward and leeward walls and on roof surfaces as defined in Figs. 27.3-1, 27.3-2, 27.3-3."

This statement did not include the word sidewall. Also, sidewall pressures carry a negative Cp as shown in Figure 27.3-1, the load acts away from the wall. This is not analogous to Figure 27.3-8, which IS intended to address the combined effect of wind as referenced in Section 27.3.5.

I'm not sure where you are deriving this 3x load/capacity ratio statement.

My original question still applies to Case 3 of Figure 27.3-8.

 

[StrEng007]

ProgrammingPE - Thank you for your previous response.

I found one document on the ASCE Library, "Design of Wall Structures for In-Plane and Out-of-Plane Forces." It was highly theoretical and didn't provide an interaction equation to check the combination of these forces.

Do you know of any texts that cover this topic that you can recommend?

I'm very interested to see what the consensus is and how other engineers handle this as well.