Chevron Out of Plane Bracing Necessity

[structengr23]

I have a vertical vessel supporting structure with an intermediate section that does not allow cross bracing at the chevron braces. I modeled it in Risa3D and used a compact W10x33 primary chord that spans 16' in the long direction. To account for no lateral bracing, I used the full member length of 16' for my Lcomp Top, Lcomp Bot, Lbyy, Lbzz, and Ltorque parameters so that it would be analyzed for reduced allowables per AISC. The braces have loads of around 14 kips, compression on one and tension in the other, not necessarily in the same load combination. Anyway, the analysis shows the members are well within allowable limits. I just want to make sure I am not overlooking anything. Attached are snapshots from the Navisworks and Risa models for a visual.

 

[structengr23]

I mislabeled the braced chevrons, so I reuploaded my views for clarification of where the unbraced chevrons are.

 

[r13]

The out-sized beam could be fine if incidental torsion from the braces below is included/considered. However, how about distortion of the unbraced plane? I think you need to find ways to stiffen it. Do you have room for secondary truss frame besides the edge beam, or add corner diagonals?

 

[JoshPlumSE]

Is this detailed for seismic OBF, SCBF? Or, just R=3?

 

[JoshPlumSE]

What I'm implying with my references towards seismic is the seismic detailing requirements to design the beam to resist an unbalanced load resulting from one brace buckling and the tension brace pulling down on the beam.

Also, you should design the beam to resist the applied gravity loads as if the bracing did not exist.... The desire being that if the braces buckle, the beam can still support all gravity load applied to it.

 

[KingKongdoor]

When I have bracing out of plan with the column/beam line I will model this by using rigid links to offset the brace from center of the column. Have not had a case that has made a significant difference in just assuming the load is centered but I always check. It will put some torque in the column.

 

[structengr23]

Thanks guys. Good points for sure. There is no seismic activity down here in southeast Texas, so I did not include those parameters. It may be good practice to follow those regardless. I did go ahead and install some horizontal diagonal braces in a diamond pattern around the perimeter of the vessel attaching at midspan.

 

[KootK]

I did go ahead and install some horizontal diagonal braces in a diamond pattern around the perimeter of the vessel attaching at midspan.

I like that, in particular for the reason shown below.

Some of your braced framing bays here are knee braces and moment frames, right?