Offset Masonry Shear Walls 2

[JBBMN]

I am designing a single story warehouse box type building with non-load bearing masonry shear walls at the exterior for the building. The roof is framing with steel joists and joist girders with a flexible steel deck diaphragm. The roof deck is attached to the masonry wall by a continuous angle that is welded to embedment plates. The overall dimensions are roughly 300’x 600’ with an expansion joint in the middle.

On one side of the building, the 300ft long wall has a jog in it. The back third of the side wall is offset by approximately 25ft. (Approximately 8% of the dimension from the side wall to the expansion joint.)

Under seismic loading, the walls can be offset up to 5% of the building dimension perpendicular to the direction of lateral load and still be considered in the same line of resistance.

Is there a similar provision for wind loading?

 

[JAE]

Well, kinda sorta maybe.

For your metal deck building, you really have a sort of flexible diaphragm (although technically speaking you can measure whether it's code-considered flexible or not based on the definitions in the IBC code for Diaphragms-Flexible).

Thus, your two offset shearwalls could be considered as separate walls - the inner one taking the brunt of the load.

But if you make sure that there are adequate collector elements running the full depth of the diaphragm, then I'd consider them working together to some degree.

Force follows stiffness so in some sense much of the lateral wind will be dragged into the inner wall.

I see I'm not saying much here. On one extreme you would have the inner wall taking 1/2 of the 25 feet and 1/2 of the 275'. The outer wall would take 1/2 of the 25 feet. This "extreme" option would imply that your deck is 100% flexible.

On the other "extreme" you would assume that both walls take 1/2 of the 300' - each accepting a share of the total shear based upon their relative rigidities (lengths).

The true answer, of course, is somewhere between these two extremes....I would think that it is closer to the one that puts more load on the inner wall.

 

[AAHstruct]

Can you please provide an IBC or ASCE 7 (or anything else) reference for assuming that offset walls lines within 5% of the building width can be assumed to act on the same line? I'm having trouble figuring out if this is codified or if it is a generally accepted engineering practice.

To answer your question, I would say that wind loads and seismic loads follow the same load path. Seismic loads are generated in walls and diaphragms, and wind loads are applied to the exterior walls. The loads are resisted by the walls (in bending) and are transferred to the diaphragms, which then distribute them to the lateral resisting elements. When transferring loads to the lateral resisting elements, the diaphragm doesn't know if the loads were generated by wind or seismic forces.

Now, the question is how does the diaphragm distribute the load into the lateral resisting elements? Some engineers probably design by the strict letter of the law, and assume that the walls will resist a load based on the tributary width of the flexible diaphragm. Some engineers will assume that walls relatively close to each other will act on the same line. And there's probably a few that say that the diaphragm doesn't act in a pure flexible or pure rigid manor, but somewhere in between, and will analyze the structure using both methods and assume that the design load is somewhere in between (I would think that if you're going to go through all that work, though, you might as well calc the stiffness of the walls and diaphragm, and model the diaphragm using a 2-D beam analysis in RISA or some other modeling program -- you'd probably get a more accurate result anyway).

 

[JBBMN]

The Code Reference I know of is from the IBC 2000, Section 1617.4.4.2 Flexable Diaphragms.

This section was dropped from the IBC2003 and refers to ASCE 7-02 section 9.5.5. Unfortuanetly, ASCE 7 does not directly speak to this issue.

Thanks for your reply.

 

[csd72]

Why not put a diagonal strut from the inner corner of the wall to the outer wall to provide a continuous load path. us a 3 to 1 angle if possible to limit sidewards load on the wall or better still line the connection up with a beam.

Are you sure that by offset, they mean a step not a slope?