A STIFFENED SUSPENSION BRIDGE ON A STEEP INCLINE AT THE WESTPORT-CARDIFF COAL COMPANY'S MINE, NEW ZEALAND. (INCLUDING PLATE AT BACK OF VOLUME)
Minutes of the Proceedings of the Institution of Civil Engineers
THE problem presented for solution in designing this bridge was the spanning of " Chasm Creek " Gorge, 200 feet in width, by meam of a bridge on a gradient of 1 in 4, for the extension of the coal-workings beyond the creek. The bridge was not expected to be in use for more than 5 years, and was required to carry a double line of tramway with a working load not exceeding 10 tons, and consisting of a simultaneously ascending and descending string of mine-tubs attached singly, and at intervals, to
... nd at intervals, to a slow-travelling endless haulage-rope. The conditions only permitted of a bridge of the most economical design, consistent with the requisite strength and rigidity; and the erection also was to be carried out with the greatest possible despatch. If it had been found practicable, short spans with trussed or untrussed timber beams carried on timber trestles would probably have been adopted; but this was found to be out of the question, as the depth from formation-level to the bed of the creek is about 100 feet, the banks being precipitous and the creek subject to heavy floods, rising rapidly to a height of 20 feet above the normal level, when it becomes a roaring torrent carrying down large trees and boulders ; under these conditions nothing but a very substantial pier in the centre would have sufficed, or trestle piers of a considerable height erected on the banks, with a centre span of not less than 106 feet; the estimate for this was, however, found to exceed the sum available, and the time required to get the long lengths of timber on the ground, for the trestles and the necessary staging, was also a serious drawback. It was therefore decided to construct a bridge which could be quickly and economically erected, without expensive staging or scaffolding, and without the large end piers which a suspension-bridge of the ordinary type would entail. Downloaded by [ UNIVERSITY OF OXFORD] on [12/09/16].