AISC Unbraced Lengths Lb

[engpes]

I have a RISA FEA lifting analysis that I am working on. As you can see in the attached plots, the top rail of the basket is returning a “no calc” on the unity check because the KL/r value is greater than 200.

If I reduce the unsupported length Lbyy to 72in, the unity checks are calculated and the code checks are adequate. My question is this:

1.) Is 72in a reasonable assumption for the Lbyy unsupported length if the AISC Appendix 6 equation A-6-2 checks out for the stiffeness requirements? (see attached diagram).

2.) Do lateral unsupported lengths need to be perpendicular to the beam in question?

Thank you in advance for your help!

Best regards.

 

[JAE]

It looks to me that you would not use 72 inches but rather twice that - 144 inches - because the diagonals on the top of the box truss extend for 144 inches.
At 72 inches there is nothing perpendicular to the top chord member to brace it.

The vertical that comes up from below might be able to brace it if there are full moment connections at each end. But typically you don't see that in trusses like this.

 

[engpes]

Sorry JAE, but I think my image may have been unclear. There are not lateral members at all for the top side rails. See attached plot from a different angle.

These are all welded members (full pen) so I believe that we can safely say the the columns have fixed end moment conections.

My question is:

1.) Can I calculate the stiffenes of the columns as a cantelever and use this stiffenes for AISC equation A-6-2 in Appendix 6? (see original attachment hand calcs)

I know traditionally lateral bracing should have beams in the lateral direction, but in this case I think my column stiffenesses are adequate to say that the unsupported length is 72" in the Lbyy axis.

Please let me know your thoughts.

Thanks a lot!

 

[BAretired]

The Canadian code has provisions for lateral bracing which I believe are similar to the ones in the AISC. Those provisions have a strength requirement and a deflection requirement. If you satisfy both requirements, the unbraced length can be established as the distance between braced points.

By the way, why don't you lift the thing symmetrically with lift points equidistant from each end?

BA

 

[engpes]

Thanks BAretired. That is what I thought, just wanted to make sure I wasn't missing anything where the braced member had to be in the lateral direction. I have calculated the stiffness and strength of the brace (as a cantilever) and everything seems to check out fine.

The reason for the unsymmetric lift is a special tool that is heavily off balanced to that side of the basket.

Thank you for your help!

 

[spats]

If you sling the lifting cables as you have shown, you will be imparting a significant lateral load at the top of the truss at the lifting points. Seems to me that you need cross memebers at the top between lift points, if only temporarily during the lift.

 

[engpes]

This is accounted for in the FEA, and the moments in the rail and columns are not excessive. The challenge is the axial loading (lateral buckling) of the top long rails.

This is why I am concerned about the Lbyy unsupported length in the lateral direction.

This is similar to a welded / fixed end truss. Thanks.

 

[spats]

You'll need to specify the minimum cable lengths, unless it really is not that critical.

 

[dhengr]

Engpes:
You have two distinct buckling problems. One is the individual top chord members braced every 72", can they act as beam/columns over that distance. And, I believe you can develop your side/truss verts. & diags. as cantilevers off the floor structure to provide the needed lateral resistance for this. The resisting force has to be in the lateral direction, the resisting member does not have to be in that direction. Many codes have an approx. lateral load req’rd. for this resistance of about 2 or 3-5% of the column loading. The second buckling problem relates to the 48' long top chords of the two side trusses, and this is a bit more difficult to pin down, because it is so dependant upon the top chords not having any initial lateral deflection, which is some P-delta affect in the buckling problem. To simplify this issue, I would carry Spats’ temporary cross member idea a step further. Btwn. the pick points, I would put a temporary cross member at the joint of every vert. (with no diags. framing into the top, that’s 4 total) and they are about 12' apart. Each pair of cross beams, I would actually join into a light x-braced frame, so that 2 cross members where part of the same frame and installed/bolted down as a unit (thus, 2 units).

I would also study the welding details of this basket. The idea that all the welds might/should be full penetration welds, when more than likely not needed, will just raise the cost of my gas 10 cents a gallon. I would clean up the welding details, maybe even add some doubler plates at some joints, and provide plenty of drain holes. I would also put my four pick points right at panel points so I could get that loads down into the side trusses without inducing any secondary bending into the top chords. This might cause the basket to rotate clockwise (in your sketches) a couple degrees. Or else, that can be fixed by adjusted the sling lengths a bit. Seems to me that you want to detail and fab. this as two side trusses all welded up, they are the real backbone. Then the floor framing, and my bracing frames and your end frames which might have x-bracing in them too, are other sub-assemblies which are tacked to the standing trusses. Then the whole thing can be rolled for final welding.