There are five basic types of steel bridges:
(1) Girder bridges – flexure or bending between vertical supports is the main
structural action in this type. They may be further sub-divided into simple
spans, continuous spans and suspended-and-cantilevered spans as illustrated
in Fig. 1.1.
(2) Rigid frame bridges – in this type the longitudinal girders are made
structurally continuous with the vertical or inclined supporting members
by means of moment-carrying joints; flexure with some axial force is the
main structural action in this type.
(3) Arches – in which the loads are transferred to the foundations by arches
as the main structural element; axial compression in the arch rib is the main
structural action, combined with some bending. The horizontal thrust at
the ends is resisted either by the foundations or by a tie running longitudinally
for the full span length; the latter type is called a tied or a bowstring
(4) Cable-stayed bridges – in which the main longitudinal girders are
supported by a few or many ties in the vertical or near-vertical plane,
which are hung from one or more tall towers and are usually anchored
at the bottom to the girders.
(5) Suspension bridges – in which the bridge deck is suspended from cables
stretched over the gap to be bridged, anchored to the ground at two ends
and passing over tall towers erected at or near the two edges of the gap.
The first three types and the deck structure of the last two types of bridges
may be either solid-web girders or truss (or lattice) girders.