Civil Engineering Reference
In-Depth Information
Consequently the entire operation of creating continuity had to be deferred until all the
decks on one carriageway had been placed, signifi cantly lengthening the construction
programme. It would appear that continuity cannot be an economical option for such a
bridge, as each deck carries, in its statically determinate state, the weight of a complete
prefabricated deck plus the transporter, which is at least as great as its service load.
However, there were clearly other reasons why continuity was adopted in this case.
As the method requires a substantial investment in temporary works, it is probably
relevant to viaducts with at least 80 regular spans, and a deck area greater than
40,000 m
2
, when it will become more economical than the precast segmental technique.
If a contractor already had the falsework and the expertise from a previous contract,
the method would be viable for considerably smaller projects.
15.9.2 The Guangzhou-Shenzhen-Zuhai Superhighway Project: the Pearl
River Delta Viaduct
The author used the lessons learned in observing the Poggio Iberna site in responding
to a commission from Tileman (S.E.) Ltd, a subsidiary of Slipform Engineering, itself a
subsidiary of Hopewell Holdings of Hong Kong, to design a 17 km twin deck viaduct to
carry the Superhighway across the Pearl River delta in Southern China [6]. The design
was carried out with the assistance of Stuart Elliott of Tileman. The road consisted of
dual three-lane carriageways, and the viaduct included three major river crossings and
three interchanges. The non-typical interchange structures were designed by Tileman.
Programme was of the essence, and the period allocated to the erection of some 28 km
of deck, with a plan area of about 430,000 m
2
, was initially of the order of seven
months. Some 860 decks had to be built in 210 days, an average of over four spans
per day.
Full span precasting was adopted as the only technique that could meet the
programme. Placing gantries were designed that had a cycle time of eight hours. Thus
two spans per day or three spans with 24-hour working, for each of two machines,
could be achieved.
The deck consisted of a statically determinate single-cell box girder 32.5 m long,
15.75 m wide and 2.3 m deep, weighing 630 tons, Figure 15.47. As the core mould
had to be withdrawn through the end of the deck, it was necessary to reduce to a
minimum the end diaphragms. The diaphragm for the typical decks consisted of a
150 mm external thickening of the webs and bottom slab over a length of 1,300 mm,
and an internal thickening of webs and top slab over a 1,500 mm length, creating a
stiff frame. This geometry allowed the bearings to be placed close to the axes of the
webs, and provided the strength necessary to resist the sway forces. The side cantilevers
were stiffened at the deck ends by short brackets which also provided strong points for
lifting the unit.
The casting yard was designed to produce one complete deck per day per mould.
The precasting cell consisted of a soffi t mould beneath the webs, raised to allow a
transporter running on rails to penetrate beneath the segment, fi xed steel side shutters
and a hydraulically operated, collapsing core mould. The trapezoidal shape of the
box would allow the deck to be lifted directly out of its side shutters. There were also
suspended moulds for the inside face of the New Jersey parapets that were to be cast
together with the deck. It was planned to prefabricate the bottom slab of the box, to
shorten and simplify the casting of the concrete.
