CHAPTER X.

DOCK BRIDGES.

versus

CLASSIFICATION-FLOATING BRIDGES-TRAVERSING BRIDGES-DRAWBRIDGES-BASCULES LIFTING BRIDGES SWING BRIDGES SINGLE-LEAF DOUBLE LEAF BRIDGES-STRESSES IN MOVABLE BRIDGES-CASE OF THE DOUBLE CANTILEVER-CASE OF THE CANTILEVER AND BEAM-CASE OF THE ARCH-CASE OF THE CONTINUOUS BEAM-THE THEOREM OF THREE MOMENTS-EFFECT OF COUNTERPOISE— LOADS IMPOSED ON MOVABLE BRIDGES-WEIGHT OF STRUCTURE-WEIGHTS OF TYPICAL LOCOMOTIVES-EQUIVALENT LIVE LOADS WEIGHT OF VEHICLES AND MEN PRACTICAL EXAMPLE OF THE CALCULATIONS FOR A SWING BRIDGE DISTINCTIVE FEATURES OF MOVABLE BRIDGES-THE PIVOT-BALANCED ROLLERS AND WHEELS-THE COUNTERPOISE SETTING APPARATUS-INTERLOCKING APPARATUS-NOTES ON DESIGN-ILLUSTRATIONS OF MOVABLE BRIDGES AT GREENOCK, ANTWERP, ROTTERDAM, CHICAGO, MARSEILLES, LIVERPOOL, LEITH, AND KIDDERPUR.

NARROW waterways and locks, linking together the various parts of a dock system, are generally spanned at convenient points by bridges, in order that vehicular and foot traffic may be transmitted across them and access provided, as uninterruptedly as possible, to all quarters. On account, moreover, of the necessity of maintaining the navigation of these passages, it is essential that bridges crossing them should be of a movable nature and characterised by great rapidity of action, so as to avoid lengthy stoppages and interference with the use of either road or waterway.

Such bridges are, of course, used in a variety of situations and in branches of engineering not necessarily connected with docks. Their importance, however, to the dock engineer is indisputable.

Classification. For the purpose of this treatise, movable bridges may be divided into five classes :

Floating bridges.
Traversing bridges.
Drawbridges.
Lifting bridges.
Swing bridges.

Floating Bridges, as the name implies, are water-borne, either continuously and wholly, or partially and during such times as they are being moved. The former variety, which are generally formed of pontoons, either singly or in combination, are rarely used otherwise than for purposes of a purely temporary nature, such as the crossing of rivers and streams during military operations. A striking instance of their application to

more permanent ends is afforded by the Liverpool and Birkenhead landing stages on the River Mersey, which, themselves constructed on the same principle, are connected with the shore by floating bridges, consisting of a series of pontoons, flexibly linked together so that they are able to adapt themselves to the fluctuations of tidal level. The length of the Liverpool bridge is 550 feet and its width 35 feet. The Birkenhead bridge is 678 feet in length by 30 feet in width. Neither of these bridges is, however, a movable bridge in the sense intended in this section.

There is a pontoon bridge, which is movable in the true sense of the word, over the Kaiser William Canal at Holtenau. It consists of two main or turning pontoons, meeting at the centre of the canal, united to two bearing pontoons at their shore ends. The bridge, which carries a 15-foot roadway and two 2 feet 6-inch footpaths, is opened by turning the pontoons round their shoreward ends, and this is accomplished by having a chain, one end of which is attached to a mushroom anchor in the bed of the canal, and the other to a bollard on the bank, wound round the barrel of a winch, which is on a small pontoon alongside of, and fixed to the main pontoon.

The second kind of floating bridge is represented by caissons, which, however, only act incidentally as bridges, their primary function being that of closing a waterway. It has already been noted that one of the advantages appertaining to a caisson, in comparison with a pair of gates, is this capacity to discharge dual duties, whereby the additional expenditure for a bridge is avoided. Caissons as a class have already been dealt with in Chap. viii., so that there is no need to pursue this branch of the subject further.

Traversing Bridges are supported by the quay at or about the coping level and are projected forward or withdrawn in a straight line-in other words, their motion is rectilinear and approximately horizontal, or with just sufficient inclination to enable them to clear the edge of the roadway abutting on their recesses; for, except in the case of footbridges, which may be provided with approach steps at each end of the bridge, forming part of the moving structure, the wheel track of a traversing bridge must lie somewhat below the quay level in order that its floor may form a continuous horizontal plane with the roadways. Consequently, for the purpose of removal, the tail or inner end of the bridge must be raised to the height of the roadway before it can be drawn backwards.

Several arrangements have been devised for the working of traversing bridges, of which the following are a few typical instances :

(a) The nose or forward end of the bridge rests upon rollers driven in between the bridge girders and the wall-bearing plate. In order to open the passage these rollers are withdrawn, and, at the same time, the tail end is lifted. The bridge tilts about intermediate wheels, fixed at the quay edge, and upon these and the tail-end wheels the structure is supported during withdrawal.

(b) The same effect of tilting the bridge is obtained by making the tail

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.

(d) 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 residences 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 overhead 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 cannot 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

bridge, being both horizontally behind and vertically below it. At the instant of commencing to open the bridge, the moment of the counterbalance is slightly in excess of the moment of the bridge, thus assisting it to rise. The excess continues until the centre of gravity of the bridge comes vertically over the axis, at which stage the line of chain also intersects it, producing equilibrium. As the bridge continues its rotation the con

Fig. 395.- Bascule Bridge.

Sectional Elevation at outside plummer-block.

Section along centre line of Bridge.

trary effect is set up, the moment of the bridge tending to increase its travel, while the moment of the counterpoise acts as a check. In closing the bridge the action. is the same, but in reversed order.

Bascule bridges are usually in two leaves, meeting at the centre of span. The under side of each leaf is then perfectly curved in form, or is provided with raking struts, fitting into pockets or recesses in the side walls when the bridge is lowered. This type of bridge forms an arch, and, accordingly, it derives very considerable support from the mutual abutments at its centre and the skewbacks at the sides. These parts can be adjusted to a nicety which is not realisable in the case of other types.

The main objections to the employment of bascules are their liability to come in contact with the yards and spars of passing vessels, and also the very large surface which they expose to wind pressure. The leverage exerted by the wind materially