Steel truss bridges - Computational Analysis and Design of Bridge Structures - page 306

bottom chord, which ties these tips together, taking the thrusts as ten-
sion, rather like the string of a bow. Therefore, a tied-arch bridge is often
called a bowstring-arch bridge. The structure as a whole was described
as nonredundant; failure of either of the two tie girders would result in
the failure of the entire structure.
10.3 PrIncIPle and ModelIng of
Steel truSS BrIdgeS
For truss bridges, a 2D truss model with planar truss only or a 3D finite
element model of the whole superstructure can be defined. For the 2D truss
model, truss on only one side is modeled and the vertical load coming from
the deck is considered linearly distributed between two parallel trusses and
loaded at the connection points between truss and floor beams. For the 3D
truss model, two trusses plus floor beams and stringers are modeled as their
actual position in space.
When modeling a truss member, as introduced in Section 10.2,
1D-truss/2D-frame or 1D-truss/3D-frame elements can be used in 2D and
3D truss models, respectively. The deck is represented by a combination of
transverse beam elements and plate elements. The beam elements provide
the load transfer characteristics of the concrete deck, whereas quadrilat-
eral plate or steel elements are used only to receive the wheel loads and
distribute the wheel loads to the beams. To provide the ability to repre-
sent the actual boundary conditions, hinges, rollers, or linear displacement
springs, depending on the bearing situation, can be placed at the truss sup-
port locations.
It is regarded that pin-connected analysis model is applicable and accurate
as long as the truss bridge is properly cambered (Kulicke 2000). Further,
most long truss bridges are already on a vertical curve. Thus, in many
practical truss bridges, a parabolic curve exists over at least part of the
length of the bridge. When a truss is analyzed as a three-dimensional (3D)
assemblage with moment-resisting joints, the inclusion of camber, usually
to a no - load position, becomes even more important. If the truss is inde-
terminate in a plane, just like any other type of indeterminate structure, it
will be necessary to use realistically close cross section areas for the truss
members and may be important to include the camber of the members to
get realistic results in some cases. A sample calculation of the cross section
is shown in Figure 10.10.
An influence line is a graphical presentation of the force in a truss
member as the load moves along the structure. If the truss is statically
indeterminate, then the influence lines will be a series of chords to a
curve, not a straight line like the statically determinate case. It is often
found efficient to calculate the influence lines for truss members using the