lateral torsional buckling of a stiffened plate

[Buzzbromp]

I have a question regarding lateral torsional buckling. I have a plate that is stiffened with T-sections along its length, such that the web of the tee is welded to the plate. The plate is approximately 270 inches long and wide, with approximately 12 tees spaced 18 inches apart (approximate). The plate is about 5/8" thick, while the tee has a 5/8" thick web and a 1" thick flange. The loading is such that the plate is in tension and the flanges of the tees are in compression.

I'm analyzing this as a built up section with the plate having an effective width such that it is a compact shape. Since the web and flanges are all compact, should I consider lateral torsional buckling? Since I'm compact, local flange buckling is not an issue. I've been told that lateral torsional buckling is not an issue, because it is not an I-beam but more of a diaghram, and the diaghram will prevent the beam from having a rigid twisting deformation. I was hoping for any opinions on this.

Thanks in advance

 

[hokie66]

I think you do have to consider buckling, but don't know that any code provisions are going to help. The tension flange is continuously braced, but the top flange depends on plate bending of the web for bracing. I think it depends on the depth of the web.

 

[civilperson]

Local flange buckling and lateral torsional buckling are two separate failure modes. The LTB is still possible with compact sections.

 

[JStephen]

I would consider the flanges of the T's to be unsupported laterally, and design as if each strip were a separate beam.

You can get into some odd situations where things are sort of supported laterally, but you don't know if the support is adequate, and that may involve being a little overconservative on your design. If the construction were common enough, either pratical experience or some research effort would clarify what was required.

 

[UcfSE]

You might try checking the webs as torsional bracing for the compression flange of the tee's. See AISC appendix 6. You have to meet both the stiffness and strength requirements to count as bracing.

 

[271828]

3 options:

Buckling analysis using a program like SAP. If you're up to speed on this, it wouldn't take long at all. I think you'd find that the buckling moment would be very high, so phiMn=phiMp. Build the model out of shell elements.

Analyze the tee flange plus 1/3 the web as a column. I'd bump up the strength by Cb.

UcfSE's idea. Least confident in this option because Mcr fot a WT is different from Mcr for a W-shape.

 

[statiker]

hi,

there is a easy approximation approach:

take the flange an 1/5 of the web as a cross section and calculate the buckling value for that. thats all !

so you are on the save side