Steel truss bridges - Computational Analysis and Design of Bridge Structures - page 299

Figure 10.3 Construction of George P. Coleman Bridge in segment. (Courtesy of VDOT,
Virginia.)
segmental trusses are unique for long spans and offer very efficient use of
materials. Due to relatively few live examples, they are not discussed in
this chapter.
Lateral cross bracing in the plane of both the top and bottom chords
of the trusses is essential. Its main purpose is to provide shear stiffness
on these planes so that sufficient torsional stiffness of the truss bridge as
a whole will be ensured. Also, it will enhance lateral stability and help to
distribute lateral loads applied on truss members. Sway bracing is pro-
vided between the trusses in the plane of either verticals or diagonals,
and its primary purpose is minimizing the relative vertical deflections
between the trusses. Portal bracing is a sway bracing placed in the plane
of the end posts.
Based on the deck location, there are three basic truss types: (1) deck,
(2) through, and (3) half-through trusses (Figure 10.4). For deck trusses,
the entire truss is below the bridge deck. Deck trusses are generally desir-
able in cases where vertical clearance below the bridge is not restricted.
Through trusses are detailed so that the bridge deck is located as close
to the bottom chord as possible and are generally used when there is
a restricted vertical clearance under the bridge. Half-through trusses
carry the deck high enough that sway bracing cannot be used above the
deck. It is very difficult to design a half-through truss if the chosen truss
type does not have verticals. Many of the recent trusses designed in the
United States have been designed without verticals to achieve a cleaner