Designing a concrete girder with a top and bottom flange 1

[GalileoG]

What value of b (width of beam) would I take if my concrete girder has a small flange top and bottom in the calculation of flexural resistance? I am curious what normal practice is. Also, how would shear reinforcement look for a girder of this shape? Much thanks.

 

[271828]

Depends on whether the top or bottom is in compression and the amount of reinforcement. The depth of compression block, a, is set by how deep it has to be for 0.85f'c to balance out Asfy on the tension side. If this isn't deeper than the flange thickness, then use the compression flange width. If it's deeper than the flange, then you'll hvae a T-shaped compression area. If the bottom is in compression, it's the same situation except upside-down.

Find a T-beam example in a concrete textbook and you'll see exactly how to do this. The tension-side b won't come into play for flexural calcs.

Stirrups depend on the proportions. Almost certainly will have a typical looking stirrup in the web, but might also have a rectangle enclosing the bottom flange.

 

[Lion06]

just to add to what 271828 said, the most common reason for having a bottom flange would be to fit more steel in there. Unless you are getting negative moments, that extra concrete at the bottom flange is irrelevant.
Consider prestressed concrete bridge girders with an I shape. They are only shaped that way to allow more prestressing strands at the bottom of the section.