3 Braces 1 Wall

[EngStuff]

Do you guys see an issue with this. A square building 100x80. On the 80'-0" sides we have brace system on both ends. On one side of the 100'-0" we have a brace system, but the other 100'-0" wall we have a masonry shear wall. This is what an architect firm reached out to me and asked me to design.

My first thought is it shouldn't mater. As long as I meet drift/deflection limits. But I am not sure how I would connect the ends? Do I even need to have a "return" or a column at the ends of the wall. Is framing joists into the wall sufficient enough?

I know seismic design is going to be a pain, but it's Category B so wind will most likely control.

FYI, I already suggested to remove the masonry wall and make it a brace system. They are trying to push away from that. At the end I told them I would look into it.

 

[MIStructE_IRE]

Is this steel frame? How many storeys? A sketch would get you more accurate responses.

Presumably your wall panel will be divided up with columns anyway, so if you’re concerned you could use a braced (flat plate) system with infill wall panel. If its low rise steel framing though I can’t say I’d be terribly concerned. Just follow the load paths through.

 

[r13]

If you have flexible roof system, then it shouldn't matter much, as the load is distributed evenly by tributary area. Not necessary true on drift concern though.

 

[EngStuff]

Here is a sketch. My wall will not be divided up by any columns. It's only 30'-0" High with 2nd story on the half between the center columns and masonry wall. The bottom side between the braces is full height to the roof.

It will probably end up being flexible roof system. I assume your concern of it being ridged/semirigid would be that we will see a rotation due to the high stiffness? If that was the case, would that create an issue?

 

[r13]

Yes. The twisting can complicate your design in many ways, though it is perfectly doable.

 

[jimstructures]

EngStuff,

Make the 100' wall braces stretch the same amount/distance as the masonry wall deflects in the 100' direction.

Jim

 

[human909]

Looks pretty fine to me. If those are interior columns then I'd make sure your roof is stiff enough to restrain them or otherwise provide roof bracing.

Make the 100' wall braces stretch the same amount/distance as the masonry wall deflects in the 100' direction

Pretty much this, though you will likely struggle to match their stiffness their stiffness and deflection. Given that is masonry you'll end up have to ensure relative deflections are kept low.

 

[XR250]

How much is too much differential deflection between the X-braces and masonry? When the 100 ft. braced frame line deflects, it will cause the masonry to bend/twist in the out-of-plane direction which will also be resisted by the 80 ft. side - braces (three sided building action)

 

[EngStuff]

Hunman909

If those are interior columns then I'd make sure your roof is stiff enough to restrain them or otherwise provide roof bracing

Is there a Rule of Thumb or literature on how stiff it needs to be to restrain columns? and/or design example?

 

[phamENG]

Note sure which code you're using, but take a look at 12.2.3.3 R, C_d, and Omega₀ Values for Horizontal Combinations in ASCE 7-10 (probably pretty close in 7-16). This is a pretty normal arrangement. I'd expect to see the masonry on the side walls, too, but there are reasons for not doing it.

 

[human909]

Is there a Rule of Thumb or literature on how stiff it needs to be to restrain columns? and/or design example?

Oddly enough I haven't seen much written on this, despite that it is definitely an issue if not considered. I haves seen 1/300 deflection to height ratio difference mentioned before. But it really depends on your roofing material or anything else subject to forces of differential movement.

In reference to portal frames I've seen the their frame 'racking' used.

 

[phamENG]

To consider a point on a column "braced" or restrained from a stability standpoint, I'd look to AISC appendix 6, Stability Bracing for Columns and Beams. Look for this stiffness within the diaphragm if it is not connected to a brace, shear wall, or a collector. You should also check out Section 2 of the manual (General Design Considerations) and the Stability Bracing information there (page 2-17 in the 14th edition).

Your P-Delta effects will come from diaphragm deflections. However far your diaphragm deflects at the location of the column is how much you need to consider for second order effects (generally speaking - various stability analysis methods have slightly different requirements).

 

[r13]

If I am not wrong, I guess human909 is concerned with the lateral displacement of those interior columns, caused by the deformation of the flexible roof, and pushing from the steel frame towards more rigid wall, thus may require roof level braces.

 

[BAretired]

If this were a single storey building, I would not be concerned about it, but the second storey adds a complication which needs to be addressed. Is there any bracing in the second storey other than the masonry wall? If not, I think there may be a problem.

BA

 

[human909]

If I am not wrong, I guess human909 is concerned with the lateral displacement of those interior columns, caused by the deformation of the flexible roof, and pushing from the steel frame towards more rigid wall, thus may require roof level braces.

Correct.

The two points I'm raising are:
-the lateral displacement of the interior columns.(easily solved with plan bracing in the roof.)
-differential displacment betweeen the masonry and non masonry wall. I don't believe this would be an issue as it is normally realatively simple to get a braced wall to have low deflection. While there will be some differential displacement compared to solid masonry, it should be enough to be of concern.

If this were a single storey building, I would not be concerned about it, but the second storey adds a complication which needs to be addressed. Is there any bracing in the second storey other than the masonry wall? If not, I think there may be a problem.

Depends on the roof diaphragm I suppose. I'm used to dealing with light weight steel roofing and assuming zero diaphragm stiffness.

 

[BAretired]

The second storey is 40'x100' (see green area below). To date, there has been no mention of second level bracing other than the masonry wall. I am concerned about the stability of the second storey. Also, the high roof might need in plane bracing.

EDIT: In re-reading the OP's post, I think I misinterpreted the geometry of the building. I believe the green area is a second storey, but the roof covers the entire 80'x100' plan area, which means the south half is a thirty foot high single storey. If that is the case, please ignore this post.

BA