This bridge, which was inaugurated in 1868, was constructed under the direction of Mr. Mantion, then engineer-in-chief of the Belt Railway. Fig. 1 shows the bridge raised.
The solution adopted in this case was the only feasible one that presented itself, in view of the slight difference between the level of the railway tracks and the maximum plane of the canal water. This circumstance did not even permit of a thought of an ordinary revolving bridge, since this, on a space of 10 inches being reserved between the level of the water and the bottom of the bridge, and on giving the latter a minimum thickness of 33 inches up to the level of the rails, would have required the introduction into the profile of the railroad of approaches of at least one-quarter inch gradient, that would have interfered with operations at the station close by.
FIG. 1.--LIFT BRIDGE OVER THE OURCQ CANAL.
Besides, in the case of a revolving bridge, since the bottom of the latter would be but ten inches above the water level, and the rollers would have to be of larger diameter than that, it would have been necessary to suppose the roller channel placed beneath the level of the water, and it would consequently have been necessary to isolate this channel from the canal by a tight wall. The least fissure in the latter would have inundated the channel.
As the Ourcq Canal had no regular period of closing, it was necessary to construct the bridge without hinderance to navigation. The idea of altering the canal's course could not be thought of, for the proximity of the fortifications and of the bridge over the military road was opposed to it. Moreover, the canal administration insisted upon a free width of 26 feet, which is that of the sluices of the St. Denis Canal, and which would have led to the projection of a revolving bridge of 28 feet actual opening in order to permit of building foundations with caissons in such a way as to leave a passageway of 26 feet during operations.
For these reasons it was decided to construct a metallic bridge that should be lifted by means of counterpoises and balanced after the manner of gasometers.
The free width secured to navigation is 28 feet. The bridge is usually kept raised to a height of 16 feet above the level of the water in order to allow boats to pass (Fig. 2). In this position it is balanced by four counterpoises suspended from the extremities of chains that pass over pulleys. These counterpoises are of cast iron, and weigh, altogether, 44,000 pounds--the weight of the bridge to be balanced, say 11,000 pounds per counterpoise. Moreover, each of the four chains is prolonged beneath the corresponding counterpoise by a chain of the same weight, called a compensating chain.
The pulleys, B and C, that support the suspension chains have projections in their channels which engage with the links and thus prevent the chains from slipping. They are mounted at the extremity of four latticed girders that likewise carry girder pulleys, D. The pulleys that are situated at the side of the bridge are provided laterally with a conical toothing which gears with a pinion connected with the maneuvering apparatus.
The two pinions of the same side of the bridge are keyed to a longitudinal shaft which is set in motion at one point of its length by a system of gearings. The winch upon which is exerted the stress that is to effect the lifting or the descent of the bridge is fixed upon the shaft of the pinion of the said gearing, which is also provided with a flywheel, c. The longitudinal shafts are connected by a transverse one. e, which renders the two motions interdependent. This transverse shaft is provided with collars, against which bear stiff rods that give it the aspect of an elongated spindle, and that permit it to resist twisting stresses.
The windlasses that lift the bridge are actuated by manual power. Two men (or even one) suffice to do the maneuvering.
This entire collection of pulleys and mechanism is established upon two brick foot bridges between which the bridge moves. These arched bridges offer no obstruction to navigation. Moreover, they always allow free passage to foot passengers, whatever be the position of the bridge. They are provided with four vertical apertures to the right of the suspension chains, in order to allow of the passage of the latter. The girders that support the pulleys rest at one extremity upon the upper part of the bridges, and at the other upon solid brick pillars with stone caps.
Finally, in order to render the descent of the bridge easier, there are added to it two water tanks that are filled from the station reservoir when the bridge is in its upper position, and that empty themselves automatically as soon as it reaches the level of the railroad tracks.
A very simple system of fastening has been devised for keeping the bridge in a stationary position when raised. When it reaches the end of its upward travel, four bolts engage with an aperture in the suspension rod and prevent it from descending. These bolts are set in motion by two connecting rods carried by a longitudinal shaft and maneuvered by a lever at the end of the windlass.
At the lower part the bridge rests upon iron plates set into sills. It is guided in its descent longitudinally by iron plates that have an inclination which is reproduced at the extremities of the bridge girders, and transversely by two inclined angle irons into which fit the external edges of the bottoms of the extreme girders.
FIG. 2.--ELEVATION AND PLAN.
The total weight of the bridge is, as we have said, 44,000 pounds, which is much less than would have been that of a revolving bridge of the same span. The maneuvering of the bridge is performed with the greatest ease and requires about two minutes.
This system has been in operation at the market station of La Vilette since the year 1868, and has required but insignificant repairs. We think the adoption of it might be recommended for all cases in which a slight difference between the level of a railroad and that of a water course would not permit of the establishment of a revolving bridge.--Le Genie Civil.