Steel truss bridges - Computational Analysis and Design of Bridge Structures - page 297

Steel truss bridges
10.1 IntroductIon
Trusses, in principle, behave as large beams to carry loads but are com-
prised of discrete members that are subjected primarily to axial loads.
Joints, or nodal points, are the locations where truss members intersect
and are referred to as panel points. A truss bridge is a bridge constructed
using triangular units connected at joints, suspending loads through ten-
sion and compression. Traffic loads are applied to the bridge deck, which
is supported by longitudinal stringers, generally placed parallel to traffic,
that carry deck loads to the floor beams. Floor beams are usually set nor-
mal to the direction of traffic and are designed to transfer loads from the
bridge deck to the trusses, the main load-carrying members to supports.
Figure 10.1 depicts a truss bridge and terminology used.
In early years, truss bridges were built with wood. Then, metal gradually
replaced wood as the primary truss bridge-building material, leading to
extensive building of wrought iron bridges after 1870. The Bollman Truss,
patented in 1852, used cast iron for the compression members and wrought
iron for the tension, for which the 100′ bridge in Savage, Maryland, is the
only surviving example (Figure 10.2). The truss bridge is one of the oldest
types of modern bridges, and it became popular because of its economical
design and relatively affordable construction. There are a large variety of
truss bridge types, with most having been built between the 1870s and
the 1930s. Truss bridges have been widely used to carry automobile and
railroad traffic.
Many steel truss bridges built in early years are now either renovated
or replaced. One example is the George P. Coleman Bridge that carries
Route 17 over the York River in Yorktown, Virginia. In 1993, the state
of Virginia widened the existing 1143-m (3750′) two-lane bridge to four
lanes using the existing substructure. The original bridge was 9.5 m (31′)
wide with no shoulders, and the new structure would be 23.6 m (77′)
with full shoulders (Bergeron 2004). Figure 10.3 shows barges floating
a truss segment into place. Some of the truss bridges in Europe are built
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