The beam bridge is the most common
bridge form. A beam carries vertical loads by bending. As the beam bridge
bends, it undergoes horizontal compression on the top. At the same time, the
bottom of the beam is subjected to horizontal tension.
The supports carry the loads from the beam by compression vertically to the foundations.
When a bridge is made up of beams spanning between only
two supports, it is called a simply supported beam
bridge. If two or more beams are joined rigidly together over supports, the
bridge becomes continuous.
All bridges need to be secure at the foundations and
abutments. In the case of a typical overpass beam bridge
with one support in the middle, construction begins with the casting of
concrete footings for the pier
and abutments. Where the soil is especially weak, wooden or steel piles
are driven to support the footings. After the concrete piers and abutments have
hardened sufficiently, the erection of a concrete or steel superstructure
begins. Steel beams
are generally made in a factory, shipped to the site, and set in place by
cranes. For short spans, steel beams are usually formed as a single unit. At
the site, they are placed parallel to each other, with temporary forms between
them so that a concrete deck can be cast on top. The beams usually have metal
pieces welded on their top flanges, around which the concrete is poured. These
pieces provide a connection between beam and slab, thus producing a composite structure.
