Civil Engineering Reference
In-Depth Information
The optimum tower height is a compromise between the cost of the tower and that
of the stays. As the tower height is reduced the inclination of the stays becomes fl atter
and their force and thus their steel area must increase, while the deck becomes more
fl exible. If the tower is taller it will cost more and be more visually dominant, but the
stays will be more economical and the deck stiffer.
Cable-stayed bridges derive much of their stiffness from the fact that the head of
the tower is anchored by back stays to a fi xed abutment. Where it is necessary to
design a multi-span cable-stayed deck, means must be found to cope with the lack
of this fi xed point [1]. When one span is subjected to live load, the additional force
in the stays must either be absorbed by bending in the tower, or transferred to the
adjacent spans. Proposals have been made for rigid towers consisting of longitudinal
'A' frames, capable of resisting the overturning forces, Figure 18.6 (a). However, rather
than this stiff solution, which will require a major increase in the cost of the piers and
foundations, it is better to adopt a fl exible solution, whereby a proportion of the live
load tension in the stays is transmitted through a fl exible tower to the adjacent spans.
The live load bending moment is thus shared between tower and deck, requiring both
to be strengthened and stiffened, Figure 18.6 (b). For a deck to have adequate stiffness
it most probably has to be a box section. As box section decks have torsional strength,
it would be logical to adopt a single plane of stay cables (
18.4.3
). In some bridges,
notably the Ting Kau Bridge in Hong Kong, the towers have been stiffened by linking
their summits to the base of adjacent towers with stays, Figure 18.6 (c), and it has
Figure 18.6
Multi-span cable-stayed bridges
