Dalmuir Swing Bridge 1915
The new swing bridge at Dalmuir, carrying the main road and the tramways between Glasgow and Dumbarton over the Forth and Clyde Canal, which was opened for public traffic on February 10 is specially interesting, on account of the very complete system of electric control which has been installed. The old bridge was a narrow timber double-leaved bascule bridge, a mere renewal of the bridge constructed when the canal was opened through to Bowling in 1790, and it had become hopelessy) inadequate for the traffic of the district.
It was the advent of electric tramways, however, which rendered the improvement indispensable. The Glasgow Corporation tramways, having been extended through Clydebank, came to a dead-end at this bridge, which was an impossible structure from a tramway point of view. But the terminus of the Glasgow lines was two or three hundred yards west of the bridge, and some seven or eight years ago the further question arose of through communication with the Dumbartonshire tramways, which were being opened, and whose eastern terminus lay near the western terminus of the Glasgow lines. To save through passengers a five or ten minutes' walk from the one system to the other, a new bridge with the tramways on it was necessary, and after the usual contention between local interests the matter was arranged, and the bridge was erected. The new bridge consists of a single swing span constructed of steel, the foundations and pit walls being of concrete. The clear opening of the canal is 20 ft., but owing to the width of the new bridge and the uncertain condition of the existing abutments, the total length of the swing span is 78 ft. 5 in., the radius of the nose end being 53 ft. 2 1/2 in and that of the rear end 25 ft. 2 1/2 in.
The two main girders are of the plate-girder type, 6 ft. deep over the pivot, and placed 21ft. 6 in. apart between centres. There is a clear roadway of 20 ft. in the centre of the bridge and on the outside of the main girders there are footpaths 5 ft. wide, carried on projecting brackets. The floor of the roadway consists of longitudinal runners of red pine 5 in. thick, laid on the top of the steel cross girders, which are 4 ft. apart. The wearing surface of the roadway is 2 in. larch planking laid across the longitudinal runners, and the foot paths are laid with longitudinal larch planking 2 in. thick. A double line of tramway is laid on the roadway, and the rails are of the Glasgow Corporation Tramways standard section, weighing no lb. per lineal yard. The rails are supported on the steel cross girders and on longitudinal steel joists.
The bridge is of the centre bearing type, and practically the whole swinging load is carried on a centre pivot. The pivot has a spherical bearing to allow of a slight bascule movement for the purpose of lifting the nose ends of the main girders out of the bearing sockets. At each end of the pivot girder there is a roller carriage with two rollers, but these are only for steadying purposes. The bridge is so balanced with cast-iron kentledge that there is an overturning moment at the rear end, and when the bridge is swinging the rear end is supported on a single roller running on the same track as the steadying rollers. When the bridge is in position for road traffic the rear ends of the main girders are lifted up by end jacks, and the bridge is then supported at both ends and on the pivot.
The turning gear is of simple construction. An electric winding winch fixed in the operator's cabin is connected by a continuous wire rope to a circular drum underneath the bridge. The end lifting jacks are of the ordinary screw type, and are operated through shafting by an electric motor in the operator's cabin. The safety gates on the approach roadways are also operated electrically. The gates are of the usual railway-crossing type, and each pair is connected together by sympathetic levers. An electric motor driving suitable worm gear operates each pair of gates.
The overhead wires on the bridge are carried on latticed steel poles and cross girders erected on the top of the main girders. Similar poles and cross girders are erected on the approaches, to which the overhead wires are suitably attached. The trolley wires on the bridge section are connected to those on the approaches by a flexible cable near the rear end of the bridge, thus avoiding sliding contacts in the overhead work. A complete system of electrical control has been installed, and all the operations are interlocked. The operator's cabin is close to the bridge, and the operator has a clear view of the canal and approach roadways. Various schemes of electrical control for the working of the bridge were considered by the Glasgow Corporation Tramways Committee and the officers of the Caledonian Railway Company, and finally it was decided to adopt the " Igranic " system submitted by Messrs. Osborne and Hunter, Glasgow. Under this system the whole of the operations are controlled by a small master switch in the operator's cabin.
When the bridge is being opened to allow canal traffic to pass, the operating movements are as follows:
1st Movement.—Semaphore signals on the approach roadways are set to danger and alarm bells are rung.
2nd Movement.—The electrical supply is cut off from sections of the tramway overhead wires on the approaches, and the catch points on the tramway tracks are opened.
3rd Movement.—The safety gates on the west side of the canal are partially closed.
4th Movement.—The safety gates on the east side of the canal are partially closed.
5th Movement.—The safety gates on the west side of the canal are fully closed.
6th Movement.—The safety gates on the east side of the canal are fully closed.
7th Movement.—The end lifting jacks under the short end of the main girders are lowered.
8th Movement.—The bridge locking bolt is withdrawn.
9th Movement.—The bridge is swung.
To open the bridge for road traffic the foregoing operations are reversed.
The engineers for the bridge were Messrs. Crouch, Hogg, and Easton, C.E., Glasgow, and they had the advantage of the advice and assistance of Mr. James Dalrymple, general manager of the Glasgow Corporation Tramways, and his staff. This bridge enables tramway cars to be run between makes possible a through service from Glasgow to Loch of the bridge were Messrs. Robert McAlpine and Sons, of the steelwork and machinery to the Brandon Bridge Glasgow and the Dumbartonshire district, and Lomond. The contractors for the construction Clydebank, and they entrusted the construction of the steelwork and m\chinery to the Brandon Bridge Building Company, Limited, of Motherwell.
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