Civil Engineering Reference
In-Depth Information
18
Cable-supported decks
18.1 General
This chapter discusses so-called 'extradosed' concrete decks, cable-stayed bridges,
under-trussed decks and stressed ribbons. All these bridge deck types fall outside the
logic of sizing deck members given in Chapters 8 and 9.
18.2 Extradosed bridge decks
This title covers a family of bridge decks whose one common feature is that the
prestressing cables at the pier section are raised above the extrados or top surface of
the deck, hence their rather clumsy name. When these cables are encased in a concrete
wall, the deck is typically called a 'fi n back'.
Extradosed bridges are transitional between girders and cable-stayed bridges.
Whereas cable-stayed decks have little stiffness or strength compared with the stays
which carry most of the loads, extradosed decks are stiffer compared with their cable
systems and the decks carry a signifi cant proportion of the loads. The decks are designed
as for any other prestressed girder bridge, with the difference that the eccentricity of
the prestress over the supports is not limited to the deck envelope.
Most such bridges are erected in balanced cantilever, and may be cast-in-situ
or precast segmental. When the deck erection is complete, there exists an array of
extradosed cables. If the cables are not encased in concrete, the fl uctuation of their
stress under the effect of live loads may affect their fatigue life and should be checked.
The Rambler Channel Bridge, designed by Benaim for tender, was intended to carry
four tracks of the Hong Kong Mass Transit Railway on two levels. It spanned 205 m,
and was 7 m deep, a span/depth ratio of about 1/30, Figure 18.1 and Figure 18.2.
Although the live load was very heavy, the deck stiffness was such that the fl uctuation
in cable stress due to live loads was little greater than that experienced in conventional
internal grouted prestressing tendons, and in this case did not constitute a fatigue
risk. For more slender decks it is likely that the stress variation in the tendons will
be greater.
It is also possible that the cables may suffer from vibrations due to wind, or to
excitation by the deck, which in turn could give rise to fatigue problems. On the Rambler
Channel, the cables rose 11 m above the deck and the longest were unsupported for
70 m. However, as the cables constitute a closely spaced array, with a wide range of
lengths, the longer cables may be stabilised by attaching them to the shorter ones or
