How does a beam bridge carry the weight of traffic?

A beam bridge carries the weight of traffic primarily through the mechanical process of bending. As a horizontal beam supports vertical loads, it experiences two simultaneous internal forces: the top of the beam undergoes horizontal compression, while the bottom of the beam is subjected to horizontal tension.12

The bridge's supports—such as piers or abutments—then transfer these loads from the beam downward to the foundations through vertical compression.12

To manage these forces effectively, modern bridges often use prestressed concrete, which leverages the material's natural ability to resist compression while utilizing embedded steel to withstand tension. For longer spans, engineers may use plate girders or "haunch" the beams—making them deeper at the supports and shallower at mid-span—to stiffen the structure and reduce bending.23