CATWALK AND HAULING SYSTEM
Walkways (or catwalks) are provided just below each main cable to access during the spinning and subsequent erection operation. Typically these are about 3 m wide, consisting of hardwood tread boards mounted on galvanised mesh all carried on wire strands, with two additional strands acting as handrailing. Stiffening frames are spaced regularly (approximately 50 m) along the span and cross bridges provided to link the two walks. Also two tramway support strands are stretched above the catwalk between the main towers and from towers to abutments to carry the stabilising cross beams (at approximately 90 m spacings) and spinning wheel haul rope sheaves. The whole system is stabilised by means of strong ropes anchored to the towers and abutments. The walkway strands are the first to be placed in position using barges. The free end of each is raised and connected to the top of the tower while the rest is unwound off the reel along the bed of the river, and subsequently fixed to the abutment or other tower. Adjustment is generally necessary to produce the desired sag for the spinning operation.
The rest of the catwalk equipment such as mesh, stiffening frames, main bridging beams, etc., are usually slid down the strands working from the tower tops. 'A small working carriage running on the handrail strands is generally set up to transport the workmen.
The tramway support strands are next erected in a similar manner to those of the footbridge, with the cross beams also being slid into position under control from the tower tops and tied to the catwalk frames. With these operations complete the tramway haul rope can be winched along the catwalk.
Walkways (or catwalks) are provided just below each main cable to access during the spinning and subsequent erection operation. Typically these are about 3 m wide, consisting of hardwood tread boards mounted on galvanised mesh all carried on wire strands, with two additional strands acting as handrailing. Stiffening frames are spaced regularly (approximately 50 m) along the span and cross bridges provided to link the two walks. Also two tramway support strands are stretched above the catwalk between the main towers and from towers to abutments to carry the stabilising cross beams (at approximately 90 m spacings) and spinning wheel haul rope sheaves. The whole system is stabilised by means of strong ropes anchored to the towers and abutments. The walkway strands are the first to be placed in position using barges. The free end of each is raised and connected to the top of the tower while the rest is unwound off the reel along the bed of the river, and subsequently fixed to the abutment or other tower. Adjustment is generally necessary to produce the desired sag for the spinning operation.
The rest of the catwalk equipment such as mesh, stiffening frames, main bridging beams, etc., are usually slid down the strands working from the tower tops. 'A small working carriage running on the handrail strands is generally set up to transport the workmen.
The tramway support strands are next erected in a similar manner to those of the footbridge, with the cross beams also being slid into position under control from the tower tops and tied to the catwalk frames. With these operations complete the tramway haul rope can be winched along the catwalk.
SADDLES
The cable is usually spun in place, strand-by-strand. When the required number of wires in a strand is achieved they are grouped together with straps or bands placed at about 2 m intervals. The saddle at the top of each tower and the splay saddles are cast in steel and grooved, stepped and divided with steel spacers to aid wire location during spinning. A cover plate is is provided for protection against corrosion. The whole unit is bolted down to resist movement.
The cable is usually spun in place, strand-by-strand. When the required number of wires in a strand is achieved they are grouped together with straps or bands placed at about 2 m intervals. The saddle at the top of each tower and the splay saddles are cast in steel and grooved, stepped and divided with steel spacers to aid wire location during spinning. A cover plate is is provided for protection against corrosion. The whole unit is bolted down to resist movement.
ERECTION METHODS
The two systems most commonly selected for the deck placing sequences are:
1. Start erection from centre of of main span.
2. Start erection from the towers.
The deck section joints are left unconnected until the last unit is in position as significant displacement of the main cable and towers takes place while the dead loading is progressively increased. Calculations should therefore be made of the cable tension and shape, and distortion of the towers during the erection sequence to ensure tolerable limits of deformation are maintained. A typical sequence of operations is as follows:
(i) Construct piers and anchorages.
(ii) Erect towers and saddles.
(iii) Hoist catwalk cables into place and build catwalk.
(iv) Place cross bridges and fix storm ties to stabilise catwalk.
(v) Haul out tramway ropes for spinning wheels.
(vi) Erect suspension cable by air spinning method adjusting wires and strands for sag within tolerance specification.
(vii) Compact cable and temporarily clamp.
(viii) Fix permanent bands and hangers to approximate positions operating from work carriages running on the tramway cable.
(ix) Erect deck units.
(x) Connect deck joints when all units are in place and complete tightening of hanger bands.
(xi) Wrap main cable.
(xii) Complete road surfacing, etc.
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