This view shows the manner of mounting and operating it on a steel tower 70 feet high. This bridge has been duplicated by the Soudan Government for a crossing over the Nile.
SPANNING a river is always a problem of considerable moment to the railway engineer; there are so many factors to be borne in mind. Not only is he forced to keep the essential and rigid requirements of the steel highway to the forefront, but he is compelled to fulfil the demands of other interests. The issue is aggravated in its complexity when the crossing is over a busy waterway, in a congested city or town, and when the banks are somewhat low-
In meeting such different conditions, the engineer has displayed considerable ingenuity. Three methods of meeting the situation are available -
Here again, however, difficulties arise. What is the type of bridge most suited to the prevailing conditions and able to meet to the best advantage the claims of both railway and navigation? This is an issue which cannot be decided without the most minute investigation. The bridge may be built at a sufficient height above the waterway to afford adequate headroom to the traffic passing beneath, but in this event the natural contour of the land must be propitious to the provision of the approaches, unless expenditure is a secondary consideration. If these favourable conditions do not prevail then the engineer must provide some means of interrupting the railway link when and as required in coincidence with marine requirements.
This end may be fulfilled in many ways. The engineer may introduce a vertical lift method as described in the chapter “A Telescopic Double-
The bascule bridge probably is one of the oldest bridge-
About eight years after this monumental work had been commenced, an American engineer, William Scherzer, conceived a new type of bascule bridge, the salient features of which were greater simplicity, both of design and operation, extreme cheapness in installation, maintenance and working, and enhanced speed in operation. The invention aroused instant attention, as its engineering features were too pronounced to be ignored. In some quarters the claims of the inventor were criticised, but the opportunity arose whereby he was able to demonstrate the practicability of his idea, and to establish his contentions in no uncertain manner. The Metropolitan West Side Elevated Railroad desired to get into the business centre of Chicago, but was faced by the Chicago River. The company wanted to lay down four roads, for which the right of way had been secured. This extended between two existing swing bridges, and although the railway company at first concluded that the swing bridge would meet their requirements, they found it impossible to adopt this solution, for the simple reason that there was no space in which to place the structure, as the existing bridges were too close together.
This view shows the bridge open. The front part is 255 feet above the water.
As a solution of the difficulty, an eminent American engineer suggested that a bridge, similar in character to the Tower Bridge and working upon the same principle, should be adopted. The authorities acceded to the proposal, and the plans were taken in hand. As the latter matured, however, certain drawbacks were observed. At last the railway management called in Mr. Scherzer to determine whether his new idea was applicable to the situation, and, if so, would it be suitable to the purposes demanded. Time was pressing, since the remainder of the track was almost completed. Mr. Scherzer had been turning over in his mind the possibility of evolving what he termed “a rolling lift system”, and he saw that it could be incorporated in this case very efficiently, so he prepared plans in accordance with his ideas. The first designs were submitted to the management, and they were so impressed with its advantages that they placed the contract in his hands.
The bridge is, in reality, in two sections. Two duplicate structures, each carrying two sets of metals, are built side by side, and firmly coupled together so as to be operated as one bridge. Inasmuch as it might be desired to use each section separately, the designer introduced facilities whereby, within ten minutes, the coupling can be severed, and each bridge operated independently of the other, so that, the railway has a crossing at all times. The movable span is 114 feet, centre to centre of bearings, and the channel between the masonry piers is 108 feet. This structure is designed to act upon either the arch or cantilever principle. When acting as a cantilever the live load is supported by the tail girders, which are locked under the projecting approach spans, the latter being firmly anchored into the masonry. The counter-
Each bridge carries two sets of metals. This photograph shows the rolling segment and elevated side counter-
This particular bridge constituted a severe test for the new idea, inasmuch as the Chicago waterway is very busy, while the railway traffic is heavy. On the average, 1,200 trains cross the bridge during the twenty-
Another illustration of the possibilities of the system Avas furnished also in the same city, and this example served to show how economically such a structure can be built and operated. The Chicago Terminal Transfer Railroad Company selected the system in order to provide them with access to the Grand Central station. The design called for a clear opening of 255 feet between the piers -
Needless to say, this very marked difference served to emphasise the possibilities and economy of the Scherzcr Rolling System in no uncertain manner. Therefore it is not surprising that engineers recognised that in this invention they were offered a complete solution of the difficulties which harass them under such conditions from time to time. The various railways running into Chicago which were forced to cross the navigable waterways embraced the idea; while the civic authorities, on their part, recommended the bascule bridge because it rendered a wider navigable channel available to the busy maritime traffic.
This is the widest and heaviest bascule bridge in the world.
It may be pointed out that the Scherzer is by no means the only expression of the bascule bridge of to-
The idea can be adapted to meet any requirement. It can be worked upon the double-
in the opposite direction, until the leaf has descended below the 40 degrees mark, when, the power being brought in once more, the bridge is returned to its horizontal position. One advantage of this method is that should anything go wrong with the mechanism the leaf cannot fall from its elevated position. The energy consumed in operating the bridge is so small as to be a negligible quantity; indeed, it is so low that in many instances the authorities working the bridge do not trouble to levy charges for the opening operation.
So far as construction is concerned, interference with the navigable channel is reduced to the minimum. The leaf is erected in the vertical position, and upon completion is swung, to be taken up by the operating machinery. While the majority of bridges of this type which have been built up to the present are designed for utilitarian purposes purely and simply, it can be adapted to any artistic requirement that may be desired, so that aesthetic considerations need not necessarily be ignored.
Some remarkable bridges have been carried out upon this principle, especially in the land of its origin. The New York, Newhaven, and Hartford Railroad was presented with a puzzling proposition when it essayed to improve the facilities for entering and leaving the huge South Terminal station at Boston, Mass. The banks on either side of Fort Point Channel are low-
This is the longest double-
Another large installation, unique at the time, was the building of an eight-
Thereupon the authorities advertised for fresh designs, and the Scherzer proposal was selected out of seven submitted plans. Furthermore, the company operating the patents offered to complete the structure for £68,428. These plans meeting with the unanimous approval of the authorities and railways, the contract was placed. The bridge comprises four through spans placed side by side and in duplicate, each carrying two sets of metals. This is the most ambitious example of Scherzer bridge building that has been attempted. At this point the waterway is 894 feet wide, but 120 feet clear was considered to be adequate for navigation purposes, and so the foundations for the rolling lift spans were set out into the river, the approaches on either side being carried over deck truss spans. The bridge, set at an angle of 63 degrees to the centre line of the waterway, has an over-
The invention has met with approval among British engineers. The late Sir Benjamin Baker, the eminent engineer and designer of the Tower Bridge, was the first to introduce it into England by installing such a bridge across the River Swale for the South Eastern Railway.
Either leaf may be opened independently if desired.