Cable-stayed bridges - Computational Analysis and Design of Bridge Structures - page 356

to the girder become easy to simulate. For some particular analyses, such
as bridge instability in wind and spatial flutter analyses, 3D modeling must
be used and the torsional stiffness of the girder from the cables has to be
counted for.
Using an appropriate mesh density or element length also needs to be
considered in modeling a bridge. In general, due to the advancement of
computer technologies, the total number of DOFs is no longer a restric-
tion. A model of 100,000 DOFs, or 20,000 3D nodes, is very common
nowadays. Similarly, when meshing a component, computer capacity is no
longer an issue. However, an appropriate density should be overviewed and
controlled. Considering the common bridge dimensional scales in reality,
1 m (about 3′), in longitudinal and vertical directions, could be taken as the
minimum distance of elements. It is not necessary to mesh the girder or the
pylons smaller than 1 m. In the lateral direction, 1/2 m (about 1 1/2′) could
be used, respectively. Wherever there is a specific point of interest, it should
be meshed regardless. In most cases, cables can be simply simulated by
one element described by its two anchor points. When large displacements,
crosstie cables, or local natural modes of cables are of interest, cables will
be submeshed into smaller segments.

11.4.1 Main girders
Box girders (steel or concrete) and composite steel I-girders are two types
of girders commonly used in cable-stayed bridges. A box girder, as shown
in Figure 11.23, can be modeled as a beam at the centroid of its cross sec-
tion in the longitudinal direction. In the transverse direction, the rigid
connection from anchor point A to the beam centroid B is adequate and
should be used by default. From the perspective view, the girder looks like
a fish bone, as shown in Figure 11.24. The widely used Hambly formula
(1991) to simulate the vertical bending stiffness of transverse equivalent
beams is not necessary as the lateral distribution is no longer a concern in
the global analysis of a cable-stayed bridge. If stiffened transverse beams,
instead of rigid bodies, are used to simulate the connection of cables and
the centroid of the girder, its bending and shearing stiffness along the

A B A

Figure 11.23 Model of a typical steel box girder.