4 . DECK 4.1 Introduction
The structural steelwork for the deck was fabricated partly in China ( main cable-stayed deck and components for the North Approach Viaduct ) and partly in the UK ( South Approach Viaduct , and assembly of the North Approach Viaduct ). The main cable stayed deck is formed by a single box and constructed by the balanced cantilever technique , working on the two fronts away from each of the towers .
The South and North approaches are twin box constructions and they were assembled on site behind the abutments and launched across temporary bearings on the piers towards and over the water .
4.2 Approaches
The approach spans are twin open topped steel boxes , with composite decks and double composite at the supports . The South Approach Viaduct ( SAV ) totals 543m over eight spans , and the North Approach Viaduct ( NAV ) has just two spans . The maximum approach span is 104m in both the north and south approaches .
4.3 Launching of the South Approach Viaduct
The approach viaducts were both assembled and welded on site in twelves phases , alternating west and east . Each phase , equating to one span length of east or west box , was launched across the abutment towards the river , before the next phase could be delivered to the assembly area . The composite decks were cast after all launching was completed and the steelwork was transferred onto its permanent bearings .
The progressive launch was carried out using strand jacks fixed to the bearing pedestal of the abutment structure with the strands attached to welded “ lugs ” near the rear of the assembled box girder . Vertical and horizontal alignment was controlled on each of the six piers . The tip of the deck and the deflections at the cantilever were controlled by strands attached to the 35m high steel king post .
Launch phases ranged from 60 to 120m which with an average speed of 7m per hour gave an operation time of just two days .
Figure 13 : Cross section through approach viaduct decks with twin boxes
1 / 2017