Chevron braced frames - out of plane behavior

[SocklessJ]

I’m designing an industrial structure (no diaphragm) with some chevron, vee, and multistory-x braced frames, and would like to better understand how they behave out-of plane.

For the beams, I’ve set my weak-axis unbraced length for compression to the whole length of the beam. Infill beams are neglected, though in reality they’ll brace against weak-axis buckling and force some form of torsional buckling.

A slideshow I found called “steel design after college” recommends adding a kicker to laterally brace the bottom flange of the beam. AISC 341-05 requires both flanges to be laterally braced, and then sends you to AISC 360 Appendix 6 to check the strength/stiffness of the out-of-plane brace. This seems very tedious. Is there a similar requirement when using R=3? I see a lot of chevron beams in industrial structures without kickers or even infill beams.

For multistory-x braces, I assume that the braces in tension would stabilize the beam. Kind of like a regular x-brace.

Any advice/refereces would be appreciated.

 

[JoshPlumSE]

For OCBF's I consider infill beams to be generally sufficient to brace for weak axis buckling of the beam. Provided they are greater than half the depth of the beam. Though it is not a bad idea to run the numbers for Appendix 6 as well.

Note: AISC 341-10 says the following: "As a minimum, one set of lateral braces is required at the point of intersection of the braces , unless the member has sufficient out-of-plane strength and stiffness to ensure stability between adjacent brace points". To me this isn't saying top and bottom. Just weak axis buckling of the beam.

I just checked and the 2016 code has the same wording.

Note: For R=3 you would not be required to brace the beam at the Chevron connection. Though you do have to check the compression in the beam and make sure the beam can take it without buckling.

 

[KootK]

I’ve set my weak-axis unbraced length for compression to the whole length of the beam. Infill beams are neglected, though in reality they’ll brace against weak-axis buckling and force some form of torsional buckling.

In most cases, I would consider this excessively conservative. Depending on the connections etc, the infill beams probably restrain both weak axis and torsional buckling.

This seems very tedious. Is there a similar requirement when using R=3?

There is. Technically, it't not a brace unless you can prove it's a brace. As JP intimated though, we're a little more liberal in practice as few of us enjoy the tedium.

I see a lot of chevron beams in industrial structures without kickers or even infill beams

This can, and often is, done. Makes for a nice, clean structure which is nice in an environment that is prone to both corrosion an rampant refurbishment. There's a way to check it as JP mentioned. In these cases, I'd expect to see a lot of HSS or very stock WF as the beam.

For multistory-x braces, I assume that the braces in tension would stabilize the beam. Kind of like a regular x-brace.

I wouldn't normally do this for weak axis beam buckling but I'll be curious to hear what others have to say. With the right kind of brace and connection, I'd be willing to call the braces adequate LTB restraint at the node.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.