Overpasses & Undergirds

Why are the undergirdings of overpasses sometimes concrete and sometimes those steel beams? Does it have to do with weight? Distance between ground support poles?

It has to do with strength and span (length between supports).

A reinforced concrete beam can only span so far before its own weight will cause it to fail. If the distances to be spanned (distance between supports) are short, you can use concrete beams, which are less expensive than steel beams. OTOH if you must span a long distance, you will need to use steel.

Also, the longer a concrete beam is, the taller it has to be (again, for strength). A steel beam can be longer with less height. Sometimes a steel beam is used when clearance under the bridge is a problem.

And don’t forget that large spans might contain huge amounts of steel. I’m thinking of some local freeways that sport very long spans but look like mere concrete roadways.