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Photo, Sir Alexander Gibb and Partners
REINFORCED CONCRETE ROADWAY, with its top layer of asphalt, was built on steel buckle plating between the top, of the girders. The roadway is 30 feet wide. Footpaths, 5 feet wide, were cantilevered out from the outer girders.
...pile projecting through the plug. The cylinder was then filled up with concrete to within about three feet of low-water level and the ends of the piles still projecting were built into the base of the tapered column rising from the cylindrical concrete base. The steel cofferdam was in each instance finally removed, and every pair of columns was provided with a reinforced concrete portal as in the north piers.
The piers for the 50-feet reinforced concrete spans of the bridge were built in a manner similar to that used for the l00-feet spans. Cylinders were unnecessary, however, as the saltings on which the piers were built are covered only by spring tides. The viaduct at the south end of the bridge was built on 18-in. piles, 65 feet long, driven 50 feet into the ground. The piles are spaced at 10-feet centres longitudinally and at 15-feet centres transversely; they support a reinforced concrete roadway 10 in. thick. Concrete footpaths, 8 in. thick, are cantilevered out from the sides.
The most important pier in the whole bridge is undoubtedly Pier 11, the huge mass of concrete on which swings the great central span. The original design for this pier provided for a huge hollow drum of concrete 42 feet in diameter, with walls 5 feet thick, founded upon the rock below the river bed. For the carrying out of this work a steel cofferdam 50 feet in diameter was lowered through 12 feet of sand and gravel to bedrock. In December 1934 the pumping of water from the interior of the cofferdam was begun, and at one time the pumps were handling 6,500 gallons a minute. Several minor inrushes of river water took place despite the thick layer of sand outside the dam. At high water on January 9, 1935, there occurred a violent onslaught by the river and some 20 feet of sheet steel were burst inwards, completely flooding the great cylinder.
It was decided that, rather than attempt a resealing of the cofferdam, the design of the pier would have to be modified, and six cylinders 14 ft. 6 in. in diameter were sunk under air pressure to rock level. The cylinders were equally spaced out on a diameter of 33 feet and were lowered by the use of hydraulic jacks from a temporary staging. The concrete of the column was placed between the cylinder wall and the circular shuttering surrounding the central air shaft. As the concreting proceeded, the jacks were adjusted and the cylinders sank slowly to their foundations. When built to a level just below low water the tops of the six concrete columns were united by a huge slab of heavily reinforced concrete, 4 feet deep, and the steel cylinder sections were removed. Above the concrete slab the enormous drum, 42 feet in diameter, was built as originally designed.
The top of the pier consists of two steel lattice girders 6 ft. 6 in. deep, arranged at right angles. At the junctions of the girders are eight U-bolts that secure the central pivot of the swing span. The girders are totally enclosed in concrete and the sloping roof of the pier is of reinforced concrete from 27 in. to 19 in. thick. The interior of the pier is ventilated by gratings, and a manhole in the roof provides access down an iron ladder. The total weight on the foundations of this pier is 4,200 tons.
The experience gained in the building of the central pier proved of great value in the erection of the neighbouring north pier, Pier 10, and similar methods were adopted. Both piers are more substantial than the remainder because they have to support, in addition to the roadway, the ends of the swing span when it is in the road position, and the vertically sliding gates that close the bridge ends. These piers also afford protection against passing...
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