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...erected a piledriver capable of working vertically or at an angle. It was not practicable to use wooden piles for the southern section of the temporary bridge, which was built on a raft of wooden sleepers laid on top of the coarse grass that covered the saltings of the south bank.
The building of the northern section of the temporary timber bridge was completed in August 1934 after six months' work. The southern section was begun in July 1934 and was nearly completed by February 1935 when a steamer, inward bound for Alloa, crashed into the staging early one morning. Considerable damage was done to the timbering and the crane. This unfortunate accident caused much delay, but finally the southern section was completed to within 150 feet of the staging from the north bank of the river. The whole of the staging was illuminated by electric lamps for the guidance of shipping at night time, and the gap for shipping was left between Piers 11 and 12.
The permanent timber jetty in midstream, which protects the swing span when it is opened for shipping, was completed by the autumn of 1935. This structure comprises four rows of 14-in. piles, 50 feet long and driven 10 feet into the river bed. The ends of the jetty are rounded and strongly braced to withstand any collision. The timber used was Canadian Douglas fir and the logs were "incised" before leaving Canada. These incisions were 3/4-in. deep, spaced 1 in. apart with 2 in. between the rows, and their object was to assist the penetration of the creosote used for preserving purposes.
The building of the concrete piers for the main bridge, with its half-mile of girderwork, occasioned not a little anxiety to the engineers in charge. With modification of design, however, and with the adoption of special methods of construction, all difficulties were finally overcome.
"Skin" of Concrete
Nearly all the concrete piers on the northern section were founded on solid rock. The exceptions were the north abutment and the first two piers carrying the three 62 ft. 6 in. end spans. These structures were built on piles driven down to the solid rock through soil that had been reclaimed from the river during the building of the railway embankment along the north bank. Pier 3 was the first to be built on the solid rock below the river bed. For the excavation of the foundations two steel cylinders were lowered by a crane from the temporary bridge to the mud of the river bed on either side.
The cylinders, having a diameter of 15 ft. 6 in., were.built of 1/4-in. steel plate stiffened by angles, and they comprised a number of sections bolted together. Timber guides were provided to maintain the cylinders in an upright position. As the material was removed from within by a grab and crane the steel shell sank down to the rock. The final excavation was done at low water by men in the cylinder. During excavation for the foundations of this pier a seam of good quality coal, 18 in. thick, was pierced through. With its base firmly set into the rock each cylinder was filled with concrete to a line about midway between Jow and high water marks. Tapered steel shuttering was then built on top of the concrete for the construction of the pier columns. These columns consist of a "skin" of concrete, 9 in. thick, heavily reinforced by steel rods.
As the outer ring of high grade concrete rose, the interior was filled in with special concrete and the steel shuttering was removed for use on other columns. The surplus sections of the foundation caissons were also removed when they had served their purpose as cofferdams. The twin pier columns were finally united at the top by a portal beam, also of reinforced concrete, to support the girders of the bridge.
Piers 4 and 5 were built in a similar manner. At Pier 5, still farther out in the river, the water found its way into the cylinders and it was necessary to employ a diver to seal the gaps at the base with bags of concrete. Similar difficulty was experienced with Pier 6, and it was decided that the time had...
Photo, Cleveland Bridge and Engineering Co., Ltd.
A MASSIVE CONCRETE PORTAL is built above the pier abutting on the swing span. A portcullis, weighing nearly four tons, is raised and lowered electrically inside the portal to open and close the road to traffic. Seven spans of 100 feet carry the road up to the portal.
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