Civil Engineering Reference
In-Depth Information
F
Cable
α
W
Girder erection segment
Figure 11.17
Determine cable quantities during scheme design.
Having cable forces due to the dead loads, plus an estimated percentage
of the live loads, the strength of cable strands, and a guided safety factor
of cables, quantities of each can be determined quickly. In most cases, it is
preferred that all cables, except anchor cables and end cables, be the same
size. The quantities determined earlier can be used in initial analyses.
11.2.3 desired state
As the dead loads on the girder can be redistributed to pylons by jacking
cables, tuning cables will reach a desired moment distribution on the girder
(Wang and Fu 2005). However, determining the jacking stress of each cable
so as to reach a desired state is a unique question in the design of a cable-
stayed bridge. If only the girder is considered, it is easy to conclude that the
ideal state of a cable-stayed bridge is the state in which the total bending
energy accumulated along the girder is minimal. In practice, it is equivalent
to adjusting the girder moment at anchor to zero (or even negative) or verti-
cal displacements to zero. If the pylons have to be considered together with
the girder, having no longitudinal displacement or no bending moment
would be perfect. Because most bridges are not symmetrical about pylons,
bearing a minor moment is unavoidable.
Moment and displacement distribution along the girder and towers can
reach the ideal state by adjusting cable stresses. The moment or the dis-
placement of an ideal state
Z
can be written as (Wang and Fu 2012)
=
{
}
T
Z
z
z
z
n
(11.2)
1
2
where:
n
is the total number of targets that need to be satisfied
T
stands for the transformation of a matrix or a vector
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