Straight and curved steel box girder bridges - Computational Analysis and Design of Bridge Structures - page 244

locations of the vertical bracing, respectively. If the aspect ratio a / b is relatively
small, the postbuckling mode appears as shown in Figure 8.11a. As the
aspect ratio increases, the critical mode changes, tending toward the mul-
timode situation, all depending on the a / b aspect ratio. In this case, the
stiffened bottom flange (as shown in Figure 8.11c) is recommended to
assure higher buckling mode for higher strength capacity.
8.2 PrinciPle and ModelinG of Steel Box
Girder BridGeS
There are many methods available for analyzing curved bridges. Of all the
available analysis methods, the finite element method (FEM) is considered to be
the most powerful, versatile, and flexible method (FHWA/NSBA/HDR 2012).
Among the refined methods allowed by AASHTO LRFD specifications
(2013) the three-dimensional (3D) FEM is probably the most involved
and time consuming method, and it is the most general and comprehensive
technique for static and dynamic analyses capturing all aspects affecting
the structural response. The other methods proved to be adequate but are
limited in scope and applicability. Due to the recent development in com-
puter technology, the 3D FEM has become an important part of engineer-
ing analysis and design. FEA packages are used practically in all branches
of engineering nowadays. A complex geometry, such as that of continuous
curved steel box girder bridges, can be readily modeled using the finite
element technique, in which steel plates and concrete deck of a box girder
may be modeled as plane shell elements. The method is also capable of
dealing with different material properties, relationships between struc-
tural components, boundary conditions, as well as statically or dynami-
cally applied loads. The linear and nonlinear structural response of such
bridges can be analyzed with good accuracy using this method. Live load
application is the same as that shown in Chapter 7 (Section 7.2.3) where
girder influence surfaces are generated to obtain the maximum effects due
to live load.
8.2.1 2d and 3d finite element method
In a two-dimensional (2D) grid analysis, the entire tub girder section with
concrete slab, steel top and bottom flanges, webs (with or without longitu-
dinal stiffeners), and top-flange lateral bracing is modeled as a beam. The
stiffness of the beam can be calculated from the whole cross section of the
girder or empirical estimates as shown in the illustrated example. When cal-
culating sectional properties, internal vertical diaphragms or cross frames
can be ignored. A relatively torsionally stiff beam element along the cen-
terline of each box (i.e., the shear center) is used to connect the slab at the