Complex sentence: A plate girder bridge is a bridge often seen supporting railroad roadbeds over short spans, this bridge may be a shallow U-shape in cross section with the roadbed at the bottom of the U or the bridge may be entirely under the roadbed.
Split into simpler sentences: A plate girder bridge is a bridge often seen supporting railroad roadbeds over short spans. This bridge may be a shallow U-shape in cross section with the roadbed at the bottom of the U or the bridge may be entirely under the roadbed.