Steel truss bridges - Computational Analysis and Design of Bridge Structures - page 302

(3D) frame elements. Compared with axial force, moments at connecting
nodes and along truss members are negligible for a perfect truss bridge,
in which all connecting members are so assembled that all centroid lines
intersect at a working point (WP) with deck loads transferred through
nodal points only. When any member is assembled off its theoretical
position by purpose or due to construction error, its secondary bend-
ing effect can be obtained from this modeling. Another reason that the
frame model should be encouraged whenever possible is the inaccuracy
in counting for the secondary bending effect in an idealized truss model.
When an offset exists for a member and if the idealized model is adopted,
the bending moment for calculating its extra bending stress in the design
phase can only be obtained by multiplying its axial force with the offset.
In reality, this bending moment may be redistributed because all other
connecting members do have bending stiffness. With a frame model, all
will be considered internally and automatically.
Steel truss bridges are generally considered to be fracture-critical
structures. The simplified approach during design has been to des-
ignate all truss members in tension and members subjected to stress
reversals as fracture-critical members (FCMs). Fracture-critical stud-
ies can be performed based on analyses that model the entire fram-
ing system, including the bracing systems and member end fixities, to
determine whether certain lightly loaded tension or reversal members
are truly fracture critical. In many cases the number of FCMs can be
reduced through this process, which reduces fabrication costs. More
details of FCM and structural redundancy are covered in Chapter 15—
Redundancy Analysis.
10.2 BehavIor of Steel truSS BrIdgeS
10.2.1 Simple and continuous truss bridges
Simple-span truss bridge, like simply supported beam bridge, is made
up of trusses spanning between only two supports. A continuous truss
bridge is a truss bridge that extends without hinges or joints across three
or more supports. A continuous truss bridge, which behaves the same
as a continuous girder bridge as a whole, may use less material than a
series of simple trusses. It is possible to convert a series of simple truss
spans into a continuous truss. For example, the northern approach to
the Golden Gate Bridge was originally constructed as a series of five sim-
ple truss spans. In 2001, a seismic retrofit project connected the Marin
(north) approach viaduct five spans into a single continuous truss bridge
(Figure 10.6).