LRFD C6.10.3.4-1 1

[TampaBridgeDesign]

LRFD has an equation (C6.10.3.4-1) that says the width of the compression flange shall be greater than or equal to the girder shipping length divided by 85. It goes on to say that this equation is provided merely as a guideline and is not an absolute requirement. Since this isn't a requirement how far can you push the 85. Is L/90 a more reasonable number?

 

[civilperson]

Shipping the girder with a narrow flange will increase the cost because of the likely failure if supported on the side or lateral torsional buckling of the flange. Thus shipping temporary braces and elaborate lifting rigging will be required. (The L is distance between splices.)

 

[TampaBridgeDesign]

Do you think that the L/85 criteria is specifically for steel plate girders? What about steel box girders? With those you have internal cross-frames and a top flange lateral bracing system in place. I would think that maybe you could increase "L" by 10% without losing any sleep at night.

 

[civilperson]

The width of the compression flange on a box girder is more than four feet, (The unbraced length will be the distance between the lateral braces/diaphragms and the compression flange width without composite action will be a few inches to a foot or so.) Using the width of just the steel compression top to compare to unbraced length, not overall length.

 

[TTK]

The L/85 criteria would usually only apply for I-girders, and your steel box (with internal cross frames) should more than satisfy this criteria.

Be very careful with this criteria for I-girders though. Many DOT's require contractors to submit specialized erection plans ($$$) for sections exceeding the L/85 rule and this will add cost to the steel.

As a designer you may think you have optimized the plate sizes, but if this solution requires a specialized erection sequence then you have likley not optimized the cost.

 

[crossframe]

I've seen states allow up to L/100... with permission. TTK has a good point in that sometimes pushing this limit will require a more complicated erection and shoring procedure. If you're anywhere near the contractor, you'll hear about it, especially since material is only a portion of in-place cost of structural steel.