A Treatise On The Principles And Practice Of Dock Engineering

DRAWBRIDGES. 407 end lighter than the overhanging portion. The nose end is then provided with movable supports, and when these are lowered, the bridge naturally inclines into a position suitable for removal. (c) The intermediate support is formed by a pair of wheels surmounting hydraulic rams which lift the bridge bodily. The nose end of the bridge is the lighter end, and is checked in its tendency to rise by a bracket which engages in the abutment. This allows the tail end to clear the roadway prior to being drawn over fixed wheels at its edge. (cl) The main girders of the bridge have prolongations in the form of bent levers, inclined upwards and counterweighted, so that, with a slight additional pressure, the inclined tail is brought down to the level of the roadway, and the bridge, with its nose end now tilted, moves backwards over wheel tracks provided for it. Traversing bridges are much inferior to swing bridges, in that the working friction on the axles is considerably greater than that on a pivot, but they afford decided advantages where it is desirable not to curtail the length of the quayage, since they only occupy a frontage equal to their width. They share this feature in common with the class of bridges next to be considered. Drawbridges are the most ancient of all movable bridges, dating back to mediæval times, when a militant nobility were in the habit of girdling their résidences with moats or ditches, spanned by bridges which could be raised for defence or lowered for sortie, as occasion might require. Such a bridge consisted of a single flap. It was raised by chains attached to the nose end ; these passed over pulleys at the summit of uprights fixed near the hinged end. The later development of this type of bridge is known as a Bascule Bridge. Like its prototype, it revolves about a horizontal axis, but it is also provided with a counterpoise in the form of a weighted prolongation of the bridge, whereby the power required for working the bridge is reduced to a minimum. An alternative method of counterbalancing is by means of over- head beams, set a little back from the axis of rotation. The first method needs a deep pit to receive the tail end of the bridge when in the vertical position, and this is not always easy to provide without some portion of the counterpoise becoming submerged. Hence the second method, which is much in vogue in Holland, where the quays are very little above water level. A third method of counterbalancing the structure is by means of weights attached to chains connected with the bridge and passing over pulleys carried by independent posts. This method has the objection that, the moment of the bridge about its axis being variable at different stages of the lift, while the moment of the counterpoise remains constant, the bridge canhot be maintained in even approximate equilibrium throughout. A compound arrangement of self-contained and extraneous balancing is afforded by the design in fig. 395, due to Mr. W. R. Browne. The axis of rotation is fixed some little distance away from the centre of gravity of the