Steel box girder bending and shear capacity

[Bridgeengineer2025]

Dear all,

For a steel box girder (built up plate section) what would be the easiest way to calculate the section properties and do a bending and shear check for preliminary design and sizing purposes. I appreciate this is straightforward to do in a software at detail design stage but how would you estimate this by hand for sizing purposes at the conceptual design stage.

- would it be acceptable to treat a portion of the top flange as a column in compression and do a column buckling check instead?
- Can I idealise this into an I beam and calculate moment and shear capacity accordingly?



Any help would be much appreciated. Many thanks in advance!

 

[wcfrobert]

Hi Bridgeengineer2025,

This is a unique section so you probably won't find any formulas. Instead, you would need to use those composite area equations we learned in school.

Or more simply 1.) calculate moment based on beam span 2.) divide moment by the internal lever arm which we will crudely assume to be the full depth of the section 3.) check buckling with this force. You still need to assume an unbraced length though.