Civil Engineering Reference
In-Depth Information
Figure 1.9
Ah Kai Sha Bridge towers (Image: David Benaim)
a main span of 360 m, and is exceptionally wide, at 42 m. The bridge is described in
more detail in Chapter 18.
The towers rise 100 m above the ground, some 70 m above the deck. Their functions
are to provide the height necessary to attach the cable stays that support the deck, to
carry the loads down to the foundations and to give stability to the deck under the
effects of typhoon winds and earthquake.
The tapering shape of the towers has been determined by the progressive increase
in weight applied to them by the stay cables and due to their own considerable self
weight, by the longitudinal forces imposed by earthquakes, wind and the expansion
and contraction of the deck, and by the lateral forces due to earthquakes and wind.
As the towers cantilever from the foundations, all these forces increase their effect
towards their base, particularly below the level of the deck. Consequently, the rate of
taper increases below deck level. This tapered shape of the towers expresses the forces
acting on them as well as providing the necessary strength.
Most cable-stayed bridge towers have at least two cross-beams, one of which
supports the deck, which make the towers into portals. These cross-beams are always
heavily reinforced and slow down the construction of the towers, particularly when
it is intended to build the towers by slip-forming, as in this case. Furthermore, the
great width of the deck would have required the beams to be very substantial. As a
consequence it was decided to omit the cross-beams, and to provide the necessary
stability by designing the towers as vertical cantilevers, built into the pile caps.
