Lateral Support

[121202]

What constitutes lateral support to prevent lateral torsional buckling in steel I-Section beams?

For example, is it adequate to provide transverse stringers connected to the compression flanges of parallel beams, i.e. to provide a grid structure? If that is acceptable I presume that you would then only need to check the spans between stringers for buckling?

Alternatively if steel plate was placed on the top of parallel beams and welded to the compression flanges would that be adequate? Would it be permissible to then ignore lateral torsional buckling in the analysis of the beams?

 

[VirtualEngineer]

Torsional buckling occurs when the whole section rotates in between the lateral supports.

Yura does a great lecture on the subject. AISC sponsers the lecture from time to time.

Usually when I provide transverse stringers and bolt into the web of the beam needing lateral support, since it cannot rotate, I consider that fully lateraly braced.

A steel plate welded to the top of parallel beams also does a good job of providing lateral support, however the field will want to just lay the plate across several parallel beams without having to go in and make overhead welds between the plate and beam. Just make sure that weld is made.

Regards,


JPJ

 

[UcfSE]

Check out this thread from a couple of months ago.

http://www.eng-tips.com/viewthread.cfm?qid=82919

If a grid is constructed but the braces are not connected to some sort of lateral system such as a roof or floor diaphragm, the beams may not buckle individually but may instead buckle as a group. The thing to remember about bracing besides providing the proper strength and stiffness is that the force must still be taken to the earth.

 

[FalsePrecision]

This is somewhat related to the issue, but is there an easy way to graph the allowable bending moment in a steel beam with Lb>Lu?. The AISC Manual has the graphs included, but is there a source somewhere that shows how to plot those graphs exactly, mathematically?

 

[VirtualEngineer]

The graphs are based upon the mathematical equations F1-6, F1-7 and F1-8 in Chapter F of the AISC Specification.

Look on page 5-47 of the 9th edition AISC Manual if you are using the ASD method.

Regards,

JPJ

 

[FalsePrecision]

thanks Virtual
Someone had snatched my 9th Edition, and I didn't see anything in my 8th Edition. Rather than spend my free time playing video games, I am developing an Excell spreadsheet to design steel beams. (Except when my wife asks me to watch "ER" with her).

 

[VirtualEngineer]

Look under Specification section 1.5.1.4.5

I don't have my "red" manual with me but my "green" manual refers back to this section for the "Members with Compact or Noncompact Sections with Unbraced Length Greater than Lc".


Regards,

JPJ

 

[FalsePrecision]

thanks again Virtual...same formulae on p. 5-22 of the "red', except limit of <=0.60Fy was not included (obvious ennyway).