Stability Bracing Req'd Strength 1

[jake1ryan]

Hello all. I need some assistance with stability bracing. I have a condition in a very long clerestory where a cantilevered masonry wall is bracing the other clerestory wall. It is braced by the steel joists, but the problem is with the brace force. According to the AISC 13th ED in Appendix 6, the required brace strength for lateral bracing is 2% of the design moment divided by the height of the member. I understand that part, but the problem is how do i distribute that load to the cantilever wall through the joists. Is the req'd brace strength per spacing of the braces (joists)?? (i.e. 5' on center) Is the brace strength per foot of the wall?

I understand that the brace strength is based on the moment of the girder, and that the req'd brace force is highest at the middle. My question is, how do i provide that brace force on the supporting wall for designing the cantilevered masonry wall?

Thanks

 

[271828]

I don't think I understand your situation. I apologize if it's worded correctly and I'm too dense to get it.

App. 6 is for bracing of steel members, but it's not clear to me that's what you have. It sounds like you're trying to brace a wall using the joists, which would be a mis-application of App. 6.

 

[jake1ryan]

It's a very unique situation. I probably should have provided more information and I'll try to head you in the right direction with the problem.

What I have is a clerestory above a gym that is about 10' tall. The clerestory is supported on one side by a masonry wall that goes down to foundation. The other side is a 10' deep joist girder that spans the large gym. The clerestory uses steel joists for the roof. Becuase the joist girder cannot provide lateral resistance perpendicular to it, the steel joists act as braces. Because the joist girder is so large, the steel joists brace the girder from the loads due to sidesweep. These loads are transfered from the steel joists into the top of the masonry wall. The wall has a lower diaphragm on the opposite side for a smaller gym, so the masonry wall is cantilevered about 8 feet to the point of the joists at the clerestory. I am trying to design the masonry wall, so I need the brace load from the joists to add to the design of the wall.

I hope this helps!

 

[JAE]

jake1ryan,

A couple of thoughts here:

1. I would first try to get the upper roof diaphragm to transmit any lateral brace force in your upper joists to sidewalls of the clerestory rather than to resist the lateral force via bending in a cantilever masonry wall. Much more stiffness in plane than out of plane. Force follows stiffness so your cantilever wall may not really receive all that much load if there is a stiffer path out of the building. Follow?

2. If you can't use the diaphragm, or there aren't any side shearwall/braced walls at either end, then the cantilevered wall is your next option.

3. The issue of brace spacing and resulting brace force (and stiffness) is not explicitly discussed in the 13th Edition but you can surmise the following:
a) If you had one joist (brace) at midspan you would calculate its brace force based upon the value of Mr at that brace point which is at midspan. The brace force would be for that one joist-brace and be taken into your wall at the far end. The force would probably be spread out, via diaphragm action, across the full width of the span (maybe at some 1:1 diagonal spread). Since your Lb is quite large (Lb = L/2) the girder truss will naturally be quite strong and the lateral force required to brace it would be just that of the single brace.
b) If you instead placed two braces on the truss, one at each of the 1/3 points, the Mr value would be based upon the moments at the 1/3 points, and, accordingly, be a bit smaller since moment there is somewhat smaller than at the midpoints. Each brace would have its corresponding brace force based upon the formula in AISC. Since the girder truss is now braced at 1/3 points, it will naturally be less resistant to LTB and thus need more brace strength support....in this case almost twice as much.
c) If you kept adding braces, the girder truss progressively needs less "beef" and needs more bracing.
d) At some point, the addition of braces goes beyond what a natural girder truss of depth h_o would need and those additional braces would not require the 0.02 brace strength since they are technically not needed to brace the truss.