Design of the Shoots Bridge Deck and supporting Cables.

The Shoots Bridge Deck.

The bridge deck is constructed from individual full width units, each 7.3 m long and containing four separate components:-
• a reinforced concrete slab at the top;
• two 2.15 m deep steel plate girders, set well apart to support the slab;
• transverse members, at 3.6 m centres, between the plate girders;
• open steel trusses, below the items mentioned above, to add stiffness to the girders.
The reinforced concrete slab varies in thickness between 200 mm and 350 mm. It is 470 mm thick in the vicinity of the anchorages.

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Cross section of Shoots Bridge Deck showing internal construction of Deck Units.

The cable spacing of 7.3 m at deck level, dictates the transverse girder spacing, each girder being located at the mid point between adjacent pairs of deck anchorages. The anchorages are contained in special steel assemblies on the outside of the deck. The main girder web thickness varies between 20 mm and 25 mm, increasing to between 35 and 40 mm in the regions of the pylons and the back-spans. The design of the stiffening truss beneath the bridge deck is dictated by the need for a rectangular space to accommodate the monorail-suspended access train. The truss is constructed out of steel T-beams, I-beams and cruciform sections. Solid plate-girders are needed at the back-span piers, and at the connections to the segments of viaduct on either side of the bridge.

The government stipulated that the cable-stayed bridge must be able to function with any one cable removed for replacement – and that, when combined with 10% reduction in live loading, safety must not be jeopardised by the accidental removal of any two cables. This, and the desire to avoid using large cables, explains the relatively close spacing, at 7.3 m, of the cable anchorages along the deck.

The whole structure is stiffened by the presence of a pair of back-span piers at each end of the bridge. They are all built in a similar manner to the viaduct piers. This is to counter the fact that the long cables supporting the end of back span distant from the pylon, are far from vertical and so less efficient in dealing with vertical loads, making that part of the deck susceptible to vertical movement caused by loading on the centre span. The deck needs to be tied down to the back span piers, to avoid the possibility that a sufficient part of its weight might otherwise be lifted off the isometric bearings on the piers to allow it to move laterally, relative to the piers. The stressing cables are located within the hollow piers and they are made up of wire strands that are housed in wax-filled ducts to protect against corrosion.

Cables.
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View of cablr stays and secondary cables (very feint). Copyright; Neil Thomas of Photographic Engineering Services.

The deck of the bridge is slightly wider than the viaduct deck because the cable stays pass through the deck, to be anchored to the steelwork below. The cables consist of between 19 and 75 parallel seven-wire strands, held together in a sheath. The actual number of strands depends upon the position of the cable within the fan; the greater the length of a cable, the more inclined it would be and so the less efficient in dealing with a vertical load. In other words, the longer the cable, the greater the number of strands it would contain.

The Royal Fine Arts Commission recommended the use of a light colour for the cables and a pale green was chosen. The black sheathing of the cables was covered in adhesive plastic sheeting of that colour, prior to installation. The pale green colour enhances the bridge appearance. It is carried through on the wind shielding and street lighting, blending well with the wide expanse of estuary, sky, and Welsh mountains in the background.

Erection of the bridge on the basis of the geometry of the completed, but unstressed, structure would inevitably have resulted in distortions. It was necessary therefore to take full account of the stresses that would be experienced when the bridge structure is complete and fully dead-loaded, and the effect that these stresses would have on the ultimate dimensions of the connected elements. Two independent methods were used for these calculations and the results compared. The calculations were then repeated at each erection stage to allow any necessary adjustments to be made. In the event, very good agreement with the design geometry was achieved.

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