Complex sentence: The original span was constructed entirely of riveted steel plates, and has an open-grate shoulder access walk while the second span was constructed mostly of welded steel plates (with riveted joints in crucial areas) and has concrete access walks.
Split into simpler sentences: The original span was constructed of riveted steel plates, and it has an open-grate shoulder access walk. In contrast, the second span was constructed mostly of welded steel plates (with heavy riveted joints in crucial areas) and it has concrete access walks.