Civil Engineering Reference
In-Depth Information
Figure 10.5
Continuity for live loads
upwards creep defl ection of the beams, by differential settlement and by temperature
gradients in the decks, Figure 10.5. This is probably the most economical form of
construction for these standard precast beams.
Other countries have developed different types of standard ranges of precast beams.
For instance, in Australia the Supertee beam is used [2], AASHTO beams are widely
adopted in the USA, while in Italy very thin webs characterise some of their standard
products. Local conditions, including the aggressiveness of the climate, live load
intensity, codes of practice and the habits and skills of contractors make it diffi cult to
export such technologies without careful consideration.
10.3 Customised precast beams
10.3.1 General features of beams
The great majority of customised precast beams are 'T' beams although there have been
a few very signifi cant bridges built with customised 'U' beams. The most notable is the
Kwai Chung Viaduct in Hong Kong, designed and built by Campenon Bernard-Franki
JV and value engineered by Benaim, Figure 10.6. However, this bridge was very much
an exception, and this topic will not cover the design of customised 'U' beams.
Customised precast T beams are one of the most traditional forms of prestressed
concrete deck and they remain very versatile and economical. In the author's own
experience, they were used for one of the fi rst bridges he worked on, the Viaduc de
la Porte de Versailles built in Paris in 1964, and again for the 28 km of decks over the
Pearl River Delta in China for the Guangzhou-Shenzhen-Zuhai Super Highway in
1991, and they are currently being used in a design by Benaim for the 3.5 km long
Contract C250 of the 350 km/hr Taiwan High Speed Rail Project.
These essentially statically determinate beams are between 1.5 m and 3 m deep,
and are typically used for spans between 25 m and 45 m, although longer spans have
been built. In general, the most economical span/depth ratio of such decks is about 15,
although they are still viable at a ratio as high as 20. They are usually post-tensioned,
with a roughly parabolic prestress centroid that balances a large part of the shear force
as well as the bending moments.
