Regarding Lateral Torsional Buckling Bracing 1

[kaffy]

Good Morning Fellow Engineers,

I am working on a structure with two W40x324 beams spaced 10 feet apart, each carrying a concentrated load of 150 kips at mid-span on the top flange (attached file showing AISC, YURA reference and loading condition) . My goal is to prevent lateral-torsional buckling (LTB) in accordance with AISC requirements. Since LTB is a concern with the length, I am designing bracing to reduce the effective lateral span. From my research, AISC Appendix 6 provides the necessary checks for both required flexural strength and flexural bracing. I plan to use a W21x44 section diaphragm for bracing, attaching it to transverse stiffeners positioned near the top flange of the beams. While the required flexural strength checks out, for bracing stiffness, I’ve found guidance from Yura, which suggests that if a diaphragm is used, the available stiffness can be either 6EIb/s or 2EIb/s. Since there’s no slab, I was planning to use the 2EIb/s as available stiffness. As this is my first time designing bracing for LTB, I would appreciate it if a senior engineer could confirm whether I’ve covered all the necessary checks or if anything might be missing.

Thank You

 

[lexpatrie]

You should look for a structural engineer that offers peer review for this kind of thing, ideally. Illinois has a structural peer review program and you can look at that list of persons/firms that do that work as it might lead you to some good candidates. That way you aren't leaning on random advice from the internet on something fairly consequential.

 

[KootK]

Can you refer us to the particular Yura document that you're looking at? I have most of them.

Your diaphragm connection strategy sounds about right but do keep in mind that you are targeting a moment connection at each end of the diaphragm.

Is there only the one diaphragm along the length of the girders? I would expect there to be several. That said, your girder size is substantial so maybe a single diaphragm checks out. You might be able to reduce your beam size significantly by adding bracing at more locations.

I agree with lexpatrie that this is pretty serious stuff for a tourist to be undertaking. Like you, I'm an Alberta engineer. Feel free to reach out if you'd like some IRL help with this.

If you'll be doing this without the help of real world structural engineer, I would favor:

- Cross braced diaphragms at the girder ends and 1/4 points and;

- Light horizontal trussing between the top flanges the whole way.

That's a lot more stuff but, at the same time, it's pretty fool proof. No doubt there will be spatial constraints that prevent this.

 

[kaffy]

Thank you very much for the response kootk

- I am using fundamentals of beam bracing as reference (page 10/16)
- I am planning to have multiple diaphragm (2 on the ends & then approx. 9 ft apart).
- By cross brace diaphragm, do u mean W beam as cross brace or any other k or x frame?
- I can not put trussing on the top flange since I will have a structural sliding on top flange.

Do you have any other reference that can be used for the design of cross frame?

- According to BS 5950, the design requires considering an axial load equal to 2.5 times the compression load. Another option I was considering was replacing the diaphragm with a truss system. Based on the 2.5/ compression load acting on both ends of the frame, I planned to design each truss member to be specifically sized to resist buckling. Any thoughts on this?

 

[KootK]

- By cross brace diaphragm, do u mean W beam as cross brace or any other k or x frame?

K or X

Do you have any other reference that can be used for the design of cross frame?

You are already familiar with the ones that I have close to hand (Yura / AISC etc). You might look to US Bridge design guides.

Any thoughts on this?

I like it. It's a robust scheme.