Straight and curved steel box girder bridges - Computational Analysis and Design of Bridge Structures - page 245

web positions. This can be done with short dummy transverse slab beams
modeled with either no stiffness before the hardening of concrete during con-
struction or assigning transverse slab bending stiffness. This form of 2D grid
model for a twin-box bridge with cantilevers is illustrated in Figure 8.12.
For a box girder bridge, a 3D finite element model can be used to
more accurately simulate each part of the section and bridge component.
As shown in Figure 8.13, web and flange plates of a box girder bridge are mod-
eled by plane shell elements, whereas bracing or diaphragm components
are modeled by beam or truss elements. As far as finite element modeling
is concerned, the same five modeling techniques described in Chapter 7
(Figures 7.10 through 7.14) can be adopted for box girders. Among the
five, 3D brick-shell model and 3D shell-shell model are more suited for
box sections where the bottom flange is modeled by using shell elements
and longitudinal stiffeners by eccentric beam elements to correctly quan-
tify the lateral and torsional stiffness of the cross section. Girder flanges
can be modeled by beam or, more commonly, shell elements; webs are
modeled by using shell elements (at least two to capture the parabolic-
curved shear); and cross frames and bracing are modeled by using truss/
beam elements with their respective proper areas and bracing configura-
tion. The deck typically can be modeled by using eight-node solid ele-
ments (Figure 8.14) or four-node plane shell elements (Figure 8.15).

Main beam element
(box section)

Transverse element
(slab)

Dummy members
(between main beam
and transverse elements)

Support

(a)

Acctual
section (b)

Figure 8.12 (a, b) 2D grillage model for a twin-box girder bridge.