Cable-stayed bridges - Computational Analysis and Design of Bridge Structures - page 348

or the stiffness of one degree of freedom (DOF), depends on the existing
stress of another degree. For example, the so-called P-Delta effect, or ini-
tial stress problem, is one such phenomenon. As introduced in Chapter 3,
all nonlinear effects of initial stresses, large displacements, and cable sag
are due to geometric nonlinearities. When a large displacement problem
is considered in cable-stayed bridge analysis, a full geometric nonlinear
analysis should be employed.
Large displacement behavior in long-span cable-stayed bridges should be
investigated case by case. In general, when large displacements are consid-
ered, the lateral stiffness of a bridge will be enhanced due to the cables' geo-
metric stiffness under significant tensions, that is, the tendency to maintain
its lateral positions. When the girder is cambered as a shallow arch, as most
long-span bridges are, the girder will behave as with stronger stiffness than
the girder not considering large displacement. This characteristic comes
from the geometric stiffness along the shallow-arch girder, similar to the
behavior of a shell under pressure.
11.2.9 stability
Stability is one of the factors governing long-span bridge design and analy-
sis. It will be discussed in more detail in Chapter 14. Stability includes
static stability and aerodynamic stability. Static stability can be further
categorized as elastic stability and ultimate plastic stability. Elastic sta-
bility deals with scenarios where material is assumed linear but geomet-
ric deformations and stresses are coupled. The P-Delta effect in columns
and beams is one of these types of problems. Plastic stability focuses on
scenarios where material enters a plastic stage so that local components
yield. In general, the geometric nonlinearity in long-span bridges is more
significant than material nonlinearity and the elastic stability should be
investigated first. On the other hand, the material nonlinearity in middle-
and short-span bridges is more significant than geometric nonlinearity,
and the plastic stability becomes more important. Elastic stability can
further be grouped as bifurcated stability (Class I), which considers the
coupling at only the current geometric configurations, and full geometric
stability, which traces the changes of geometric configurations under each
increment of load. Both elastic stabilities in cable-stayed bridges should be
analyzed. When plastic stability is considered, geometric nonlinearity will
be considered at the same time, or the so-called dual-nonlinear analysis
will be performed.
Aerodynamic stability, which includes structural and cable oscillations
under wind and rain, is a critical issue for long-span cable-stayed bridges.
The design of a cable-stayed bridge should follow special guidance for
aerodynamic issues. Wind tunnel testing may be unavoidable for the design
of long-span cable-stayed bridges. The aerodynamic stability issue is not