The simplicity of the beam bridge made it the first type of bridge ever built. It’s still the cheapest to build. All you need is a crossbeam covering the span, supported by an abutment at each end. One type of beam bridge is a girder bridge, which employs steel girders as reinforcement.
Gravity is a bigger challenge when constructing a bridge because, unlike a building, most of what’s underneath is empty space. A beam bridge might be supported only by two abutments, one at either end, to counter gravity and bear the entirety of its load.
By adding supports in the middle, known as piers or stanchions, and connecting sections between them, you can create a very long, stable bridge.
In beam bridges, the force of compression pushes the load inward onto piers in the middle of the bridge. Simultaneously, the pulling or stretching force of tension pulls the load outward toward the abutments at both ends of the bridge.