Cable spinning was affected by strong winds as well causing time loss , oscillation and vertical motions and even some damage to the cable strands and bridge equipment .
To check the aerodynamic stability of the suspended steelwork in the main span during erection a series of tests were carried out in the wind tunnel on a model with a scale of 1:48 .
4.2 Side Towers
The side towers comprise two reinforced concrete legs and a cross beam , all of box section , with arch ribs stiffening the corners of the structure . The cable saddles , each of which imparts a maximum vertical reaction of 3500 tons , are located on thick capping slabs at the tops of each leg .
The south side tower was originally just off shore , but land has susbsequently be reclaimed so that it now sits on the shore line which is formed by shale and sandstone , at the pier the sandstone is overlaid by 50 ft ( 15m ) of hard boulder clay , 8 ft ( 2.4m ) of gravel and 13 ft ( 7m ) of sand and silt . The foundations were constructed in steel sheet pile cofferdams . The two cofferdams were built consecutively using the same sheet piles and waling frames .
The north side tower is on the shore and excavation in the whinstone rock was carried out by normal blasting methods . A bund wall of rock fill , sand bagging and sheet piling was used to keep tidal water out of the excavations . Foundation blocks are similar to those on the south side .
The walls of the tower legs are generally 2 ft ( 61cm ) thick and were concreted in 6 ft 8 in ( 2m ) lifts . Horizontal diaphragms are at every fourth lift . After completion of the concreting they were post-tensioned .
4.3 . Anchorages
Each anchorage had to resist a pull of 28 000 tons ( 14 000 tons imposed by each main cable ), acting at an angle of 30 " to the horizontal . The anchorages are tapered concrete blocks , prestressed longitudinally , which fill tunnels driven into the rock . During the final stages of the tunnel concreting , work was commenced on the construction of the reinforced concrete anchorage chambers and splay saddle foundations . The external surfaces of these chambers were tanked with asphalt to ensure watertightness .
At the upper face of each anchorage block there are 19 steel crosshead slabs with the bolts retaining the main cable strand shoes . Each crosshead slab is anchored by six cables consisting of four 19-wire galvanized strands of parallel lay , anchored in a common socket . The cables pass through ducts to the lower end of the concrete blocks where they are stressed . By prestressing the anchorage cables , movement of the steelwork at the anchorage face due to changing loading conditions is eliminated . After stressing , the cables were grouted and the chamber and anchorages backfilled .
Figure 4 : General Sections of South Anchorage Tunnel
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