Steel truss bridges - Computational Analysis and Design of Bridge Structures - page 318

evident gaps between sliding top plate and fixed bottom plate and is the
main cause of plate crack due to bending.
10.6 3d IlluStrated exaMPle—francIS
Scott Key BrIdge, Maryland
There are 22 bridges in various types that account for 4.1 miles (6.6 km)
of the 10.5-mile (16.8-km) facility, which includes the Francis Scott
Key Bridge over Patapsco River and its I-695 approaches in the state of
Maryland. This bridge structure combines the behaviors of an arch, truss,
as well as cantilever. The main section of the Key Bridge (Figure 10.9) is
a three-span, 219-366-219 m (720′-1200′-720′) for a total of 805 m
(2640 ft) through truss bridge. The as-is bridge is first modeled and ana-
lyzed in 2D by using Win-TRAP program. In this simple 2D model, only
one main truss is considered and each truss member is modeled as the
truss element—one-dimensional axial element. One main truss is modeled
by 416 truss elements. Figure 10.24 shows the elements of the 2D truss
model.
As introduced in the beginning of this chapter, a truss bridge can be
modeled as frame elements in 3D if analysis tools permit. To illustrate
this more sophisticated 3D approach and make a comparison to the sim-
ple 2D truss model, the Francis Scott Key Bridge is also modeled with
frame and truss elements in 3D and illustrated in this section. As shown
in Figure 10.25, all components of a truss bridge including main trusses,
bracings, sway frames, floor beams/trusses, stringers, and its diaphragms
and hangers are modeled.
In this model, the connections between floor beams/trusses and main
trusses are framed together at their centroid positions. The stringers are
placed at their centroid positions, and rigid bodies are used to connect them
to floor beams/trusses. As bottom chord bracings are aligned on the bottom
chord plane, rigid bodies are also adopted to connect them to the floor beams/
trusses in the middle. Stringers are modeled as four- or eight-span continu-
ous beams with joints inserted at as-is locations. Figure 10.26 shows the end
portal portion, and Figure 10.27 shows its corresponding photo. Figure 10.28
shows the detailed modeling of floor systems. There are 11,618 elements in
total with 62 truss elements for hangers and 11,556 3D frame elements for
all other components. The total 3D finite element analysis nodes and degrees
of freedom are 9,124 and 54,744, respectively. One hundred thirty different
cross sections are used in the model. The modeling and analyses are con-
ducted by using VBDS (Visual Bridge Design System, Wang and Fu 2005).
The axial force and moment distributions due to structural weight
are shown in Figures 10.29 and 10.30, respectively. Figure 10.31 shows
moment distributions in part of stringers.