From the Transactions of the Institute of Mining and Mechnaical Engineers, vol. XXII, 1872-3
THE system of dock accommodation has of late years called into existence several kinds of opening bridges, differing according to the varying conditions of span, quay space, traffic, etc. Many navigable rivers also are crossed by opening bridges, some built in answer to the requirements of railway extension, and others on the sites of fixed bridges, removed as obstructions to the course of river improvement. The introduction of wrought iron as the material of construction afforded the means of increasing the size to almost any extent, and opening bridges are now designed for spans at one time deemed almost impracticable for fixed structures. About twenty years ago Sir William G. Armstrong introduced the central press system for Swing Bridges, and shortly afterwards applied the lifting press to Draw Bridges. This, together with the employment of hydraulic power as the moving agent, forms the next great epoch in the history of opening bridges. From the steadiness of the action of hydraulic pressure, and its thorough adaptation to machinery working at irregular intervals, this appears to be the most suitable means by which power can be applied. The writer is aware of only one instance of the direct application of steam power -bridge over the entrance to a dock at Glasgow-although in several cases steam engines of small capacity have been erected for the purpose of pumping a supply of water to work a bridge alone, apart from any other machinery.
The opening bridges at present in use-and it is to these that the writer purposes to confine his remarks-may be divided into three great classes-Swing, Draw, and Lift Bridges.
There are few situations in which a Swing Bridge may not be used, either on a central press or on rollers. It is peculiarly suitable for crossing wide rivers, as it affords an opportunity of having, if necessary, two opening spans with comparatively little extra cost in first construction, and none in working expenses. Where, however, both ends are unsupported in the act of turning, it is evident that the central press system will not apply, and that the roller bridge is the only form of swing which can be adopted. There are situations where a Swing Bridge is impracticable, sometimes on account of the space occupied on the shore, and occasionally for river bridges the large central pier may interfere either with the navigation or with the flow of the current. In such cases the choice rests between the Lift Bridge, whether Bascule or otherwise, and the Draw Bridge. For long spans the Lift Bridge is unsuitable, and the Draw is the only available system, for, although it has not yet been used for spans of more than about 60 feet, if the weight were distributed by means of a bogie, bridges on this principle might be applied to spans equal to any hitherto crossed by opening bridges. Where there is a choice, however, between a swing and a draw bridge, supposing the first cost to be equal, the former will in most cases have the preference. The Draw Bridge requires much greater power to work it, as the entire weight of the bridge has to be carried to a considerable distance. Comparing also the central press swing with the Draw Bridge on lifting presses, the former can be worked much more easily by hand power in the event of a failure in the water supply.
The Bascule is, of necessity, confined to comparatively short spans; and the writer is not aware of any instance of its application to an opening of more than 45 feet. For a large bridge of this kind, either the pit in the masonry to receive the counter-balance levers would require to be of an inconvenient size and depth, or, with shorter levers, the amount of ballast would be out of proportion to the weight of the bridge. The Bascule at present in use, is easily kept in repair, occupies little lock or quay space, and, if properly weighted, can be worked in calm weather with very little power.
The unbalanced Lift Bridge (Plate XVIII., Fig. 2), obviously confined to short spans, and requiring considerable power to work it, occupies little space, does not cut into the quay, and may be used in situations where there is room for no other description of bridge.
SWING BRIDGES.
Of opening bridges, the swing, in its various forms, takes the most prominent position. It is worked on two principles; one where the bridge rests and is turned on a circle of live rollers, and the other where the weight is carried by a hydraulic press under the centre. The tail-end is commonly made from a third to half the length of the other. If much less than one-third, the total weight is necessarily much increased; and if much ;renter than one-half, the advantages gained in the reduction of the weight will not, in most cases, compensate for the additional space occupied on the shore, and the increased cost of both girder-work and masonry. The rollers are generally made of cast-iron, although in some recent instances wrought-iron has been introduced. In most bridges the lower tread-path for the rollers presents no difficulty, being simply bedded into the masonry; but for the upper path it is necessary to provide, by cross-girders, a rigid and almost continuous bearing, between the points where it bears directly beneath the main longitudinal girders. The castings forming the roller-path should also be of a depth sufficient to afford considerable stiffness in themselves.
The rollers seldom exceed 3 feet in diameter. As the conical bevel increases with the diameter very large rollers are objectionable, from the greater outward thrust, and the excessive friction thrown on the retaining washers. There is no fixed rule for the proportion between the diameter of the rollers and of the roller-path; the angle of which varies greatly in the different bridges at present in operation. The extreme cases of which the writer is able to speak, are the Shannon bridge, with an angle of 1° 22', and a bridge at Birkenhead (of 100 feet span), with an angle of 7° 39'. The latter bridge has now been at work for some years, and, notwithstanding the great angle of the rollers, they show no signs of failure. The rollers of the South Bridge at Hull work at an angle of about 3° 50'; and those of the Ouse Bridge, near Goole, at 5° 23'; and the action is smooth and easy in both cases. The rollers of the Duke Street Bridge, at Birkenhead, are at an angle of about 10° 15', but as the outward thrust is taken by a flange on the roller, the example is scarcely a fair one. In practice it is found that swing bridges work best when the rollers run loose on the axles, especially where the axle-rods act also as ties from the central boss. Where motion is given to the boss, by the rods acting as levers, the friction of the boss has a tendency to cause flexure in the rods, and, if these turn with the rollers, rapid wear of the bearings.
The bridge over the northern entrance of the Alfred Dock, at Birkenhead (Plate XV., Fig. 1) carries a double line of railway and two wide footways, by three wrought-iron box girders 180 feet long and 10 feet deep. This bridge, the total weight of which, with counterbalance, is about 450 tons, turns on twenty-two cast-iron rollers 4 feet in diameter and 5 inches wide. Although, three minutes are generally allowed, it can be turned in two, by a hydraulic engine fixed in the ballast-box at the tail-end, and working through spur-gearing into a rack bolted to the masonry. The engine receives its supply of water through a universal joint at the centre of the bridge. In case of an accident to the hydraulic machinery, provision has been made for working by hand-power through the same spur-gearing.
The Hull South Bridge has a clear opening of 100 feet, and weighs 800 tons. There are two single-web fish-back girders, carrying a double roadway and two large footways. It turns on thirty solid wrought-iron rollers, 2 feet in diameter and 12 inches wide. The bridge was erected, in the first instance, to be worked by hand-power, but as this was found too slow for the requirements of the traffic, hydraulic power was subsequently applied. It can now be turned in less than one minute by a hydraulic engine sunk in the ground beneath the tail-end, and working through gearing on to a rack fixed immediately outside the circle of rollers. The Duke Street Bridge, at Birkenhead, 100 feet span, is another example of the after application of hydraulic power to a bridge originally intended to be worked by hand. This bridge can now be opened in a minute and a-half: by hand-power, with thirteen men, it required fifteen minutes. This is, however, an extreme case; the friction at the flanges of the rollers, previously alluded to, absorbing a large proportion of the power.
The roadway bridge over the Medway, at Rochester, has a span of 50 feet, and revolves on thirty cast-iron rollers. It is turned by handpower, by means of a chain and drum.
Other examples of single span roller bridges are found at Liverpool, Sunderland, etc.
Perhaps the best example of a Swing Bridge crossing two contiguous openings, is that of the North-Eastern Railway Company over the River Ouse, near Goole (Plate XV., Fig. 2), designed by Mr. T. E. Harrison, and erected under the superintendence of the writer, as agent for the contractors, Sir W. G. Armstrong and Co. The turning bridge, which forms a portion only of the entire viaduct, is 250 feet long, and spans two openings, each 100 feet wide. The total weight is 670 tons, and it is carried on twenty-six cast-iron rollers, 3 feet in diameter, and 14 inches wide. The rollers and upper and lower roller-paths are all of cast-iron, faced with Bessemer steel, 24 inches in mean thickness. The bridge is turned by hydraulic machinery, which, with the steam power for pumping the water, is placed beneath the centre of the bridge, within the circle of the rollers. Before the hydraulic machinery was completed, the bridge was frequently turned by temporary hand gear; on a calm day, five men on the end, working a single purchase crab with one part of a 16- inch chain, could turn the bridge through 90° in about twenty minutes. If each man exerted a force of 35 lbs. on the handles, there would be, after deducting for the friction of the gear, a strain of 1,680 lbs. on the chain,
Proportion of gear = 6 to 1, 25 lbs. X 5 men X 16 inches handle X 6 X 0.7 = 1,680 lbs.
5 inches barrel X 1
which from the angle at which it worked would give an average strain of about 1,460 lbs. at right angles to the end of the bridge, and rather more than 17 lbs. per ton traction at the radius of the rollers.
1,4601bs. X 125 feet radius of bridge = 17 lbs.
16 feet radius of rollers X 670 tons
Allowing a wide margin for so rough an estimate, this shows an unusually low co-efficient of friction, attributable, probably, to the broad hard surfaces of the steel paths and rollers. At present, with the hydraulic power, the time occupied in opening the bridge, including the withdrawal of the resting blocks under the end of the girders, does not exceed one minute.
The York and Doncaster Branch of the North Eastern Railway crosses the Ouse at Bishopthorpe, by a bridge smaller in every respect, but very similar in construction.
The bridge over the River Foyle, at Londonderry, has two spans of 45 feet each, which open by a swing. It was designed by Mr. Hawkshaw ; and is, perhaps, the only example of an opening bridge being employed for two roads at different levels, the railway being below and the roadway above.
The railway bridge over the River Shannon, at Athlone, has two openings of 43 feet each, and turns on thirty-two cast-iron rollers 8 inches in diameter. The bridge of the Manchester, Sheffield, and Lincolnshire Railway over the River Trent, at Keadby, has wrought iron rollers on a wrought-iron tread-path. There are two openings of 60 feet each. These last three bridges are worked by hand power; but the writer is not able to give the time required for opening.
The supporting gear under the ends of the bridge must also be of sufficient power and range to raise them to the proper level, ensure a sufficient bearing, and counteract any additional depression arising from the action of the sun on the tops of the girders. For effecting this object, cams worked by levers or spur gear are adopted in some instances; in others, the resting-gear consists of wedge-shaped blocks, drawn into position by screws or spur gearing. The ends of the six main girders of the Rochester Bridge, before alluded to, are lifted by vertical screws, 4 inches in diameter, worked by worm-wheels from one horizontal shaft. An efficient arrangement of resting-gear is shown in Plate XVI., Figs. 1-2. It consists of an upright strut, with a joint in the middle, by which the head, being confined horizontally, may be raised or lowered as required. The power is obtained from a hydraulic ram acting horizontally at the joints of the struts, or by hand-power, through a rack and spur gearing. This gear works with' less friction than those previously mentioned, and has the advantage of increasing in power as the strut approaches the vertical position, when the resistance is greatest. When lifted to the full extent, the centre of the strut is allowed to pass slightly beyond the vertical line, to prevent it working back. In the case of the Goole Bridge, the action of chocking the ends is effected by two distinct motions. The weight is first lifted by a modification of the strut arrangement just described, and so held until parallel cast-iron resting-blocks have been thrust into position under the ends of the girders. This gear is attached to the moving part of the bridge, and worked by hydraulic power from the centre. The end-gear of the Keadby Bridge differs from any previously mentioned, inasmuch as the varying level of the ends of the bridge, caused by differences of temperature, is not counteracted by any lifting power in the gear. The wedge blocks are simply thrust home until the ends of the girders have a firm bearing; any want of level being met by adjusting a moveable rail on the end of the fixed part of the viaduct.
The central press Swing Bridge (Plate XVII., Fig. 1), when in position for traffic, is supported on each side of the opening and under the tail-end on cast-iron blocks resting on the masonry. Across the centre of the bridge, and immediately beneath, and bolted to the longitudinal girders, is the main lifting girder, and under the centre of this is the hydraulic press. The head of the ram is concave and fits a casting of corresponding form bolted to the bottom of the girder. This arrangement is adopted to allow of the slight oscillating motion developed in the working of the bridge. When not employed, the ram falls slightly clear of its bearing on the lifting girder, in order to insure the longitudinal girders resting solidly on their supports. Two small rollers, A, are fixed at the opposite sides of the tail-end to bear upwards against a cast-iron tread-path, bolted to the sides of the circular recess in which the tail-end turns. The ease with which the whole bridge oscillates over the ram affords the means of an accurate adjustment of the ballast, which is arranged to allow the long end to preponderate to the extent of two or three tons. When water is admitted to the press, the tail-end being lighter is first lifted, and continues to rise until the rollers, which have been set with a clearance of about 1 inch, are brought into contact with the tread-path, and so arrest the upward movement. The press continuing to work the nose-end next rises; and the bridge, wised entirely off' its bearings, is now supported altogether on the press and steadied laterally by the rollers at the two back corners. It is now in a condition to be turned, and this is done either by spur gear working into a rack and actuated by hydraulic or hand power, or by chains passing round a drum and drawn in opposite directions by hydraulic rams. Examples of this class of bridge are numerous. One, of 80 feet span, at the entrance to the Canada Dock, Liverpool, has a single roadway and two footways, and weighs with ballast about 140 tons. At the Birkenhead Docks there are two similar bridges 70 feet span, carrying a double line of rails and two wide footways, each weighing about 490 tons. The heaviest bridge yet constructed on this principle is that over the Regent's Canal, which weighs 450 tons. There are smaller bridges at the Penarth Docks, at the Sunderland Docks, at the East and West India Docks, at the London Docks, and at Leith and Swansea. Bridges of this class are generally connected with a system of hydraulic machinery (cranes, machines for opening dock gates, &c.), and obtain the supply of water from a steam engine and accumulators in the usual way. When this supply is not available they may be worked by hand power, either by pumping previously into a small accumulator a supply of water to lift and swing the bridge, or by pumping direct into the central press to lift the bridge, and turning it by a rack at the tail-end. Hand power, of this latter description, is fitted to most bridges, in the event of an accident to the hydraulic machinery. The bridge at the Sunderland Docks was at first lifted and turned entirely by hand power, but the lifting press is now supplied from the hydraulic mains.
The circular recess in which the tail-end turns is, of necessity, 3 or 4 feet deep, to obtain over the tread-path a sufficient weight of masonry to counteract the upward pressure of the rollers. To meet the objection urged against this large pit, a modified form of this bridge has recently been introduced. The tail-end is more heavily weighted, so as to allow the nose to rise first. The rollers bear downwards, and travel on a path at about coping level. The bridge over the 80 feet entrance to the Albert Dock, at Hull, is of this class, and there are others at the West India and millwall Docks.
For dock entrances, Swing Bridges are sometimes constructed in halves, dividing in the middle of the span, and each leaf turning on rollers or strong grooved wheels, to its own side of the entrance. One form of this double-leaf Swing Bridge (Plate XVIL, Fig. 2) consists of cast iron arched ribs, the lower parts of which are, in turning into position, brought in front of, and in contact with, projections on the cast-iron bed-plate on which the rollers travel. This takes the horizontal thrust of the arch, whilst, at the same time, to _relieve the rollers of part of the weight, the ribs rest on cast-iron plates on the edge o? the quay; these plates being carefully dressed to level that the ribs may slide easily on to their respective bearings. To admit the necessary freedom in all states of temperature, a certain amount of clearance (on a cold day probably about 1 inch) is allowed at the junction of the two leaves. This want of perfect contact at the crown of the arch, is supplied by tapered cast-iron keys dropped into position from above. The moving power is obtained by a rack and pinion on the tail-end of each leaf. To bridges of this class there are several objections; the large cutting into the face of the quay, the cost of double foundations, the multiplicity of working parts, and the greater expense of working and maintenance. The very fine fitting required at the abutments of the different ribs is often disturbed by the wear of the wheels and other parts, and frequent adjustments are rendered necessary. There are examples of double-leaf Swing Bridges of this class at the East and West India Docks, at Liverpool, at Hull, etc.
The central press arrangement has also been applied to double-leaf bridges. At the Waterloo Dock, at Liverpool, there are three bridges of this description, each leaf of which consists of four single web wrought-iron girders, resting, when in position, at the edge of the quay and at the tail-end of the bridge. No attempt is made to obtain strength from the arch, each half being sufficiently weighted and strong enough in itself to carry the overhanging load. These bridges are turned by hydraulic power, by a chain and drum, as previously described, means being also provided for working by hand, if necessary.
Of Draw Bridges, there are various systems in operation, although the principle is the same in all, viz., a girder of length sufficient to bring the centre of gravity of the bridge when in position, entirely over one pier, on which it may be rolled backwards in the line of the roadway. In one arrangement (Plate XVIL, Fig. 3) the bridge is lifted by hydraulic presses and drawn back on a horizontal tramway. A press is placed under each longitudinal girder, as near as practicable to the edge of the quay, the head of the ram being so constructed as to form, when raised, a continuation of the tramway on which the bridge travels. The tail-end is made heavier than the other by two or three tons, and to carry this excess and steady the whole, two small grooved wheels (A) are fixed to the lower part of the end of the bridge, in such a position, that when it has been lifted and drawn back about one foot, they are brought on to the rails of the permanent way approaching the bridge, or other rails laid for the purpose. Until, by this backward movement of the bridge, these wheels have been brought on to the rails, the nose is held down by guide-pieces or "horns" (B), projecting about three feet from under the ends of the girders, and working upwards against rollers fixed in the quay wall. The bridge is moved horizontally by chains worked from hydraulic cylinders beneath the tail-end. When it is closed and lowered into position, the rams fall slightly clear of the wheels, leaving the girders resting on the masonry.
The bridge over the South Outlet (60 feet) at Sunderland, erected by Sir. W. G. Armstrong and Co., about the year 1855, is a fair illustration of this principle. Here a single line of rail and two footways are carried by two single web girders, 112 feet long and 6 feet 6 inches deep, placed 17 feet apart, centre and centre. The long end of the bridge projecting across the opening is 68 feet, and the inner or ballast end 44 feet long. The total weight, including ballast, is about 130 tons. The presses for lifting the bridge, one under each main girder, are 19 inches in diameter and about 20 inches stroke. When water is admitted to the presses, the nose-end, being lighter, first rises until the "horns" are arrested by the rollers above. The tail-end, which is the heavier by about five tons, next rises until the small wheels at the end are about 4 inches above the level of the rails behind. The presses are now at the top of their stroke; the tops of the rams on which the large wheels rest, are level with the rest of the tread-path; and the bridge is in position for being drawn back. For the first 18 inches of the backward movement, until the small wheels have been brought over the rails, the tail is supported by the "horns" working under the rollers at the opposite end, the sloping form of the "horns" allowing the wheels to come gradually down on to the rails. The movement of the bridge backwards and forwards is effected by chains J-inch in diameter, acted on by an arrangement of hydraulic rams and sheaves, similar to that now so well known in connection with hydraulic cranes, &c. The rams are each 9 inches in diameter, with chains, multiplying 6 to 1, providing a maximum tractive force of about 35 lbs. per ton. The large carrying wheels are of cast iron, solid, about 3 feet in diameter and 7 inches in the tread. The bearings are 6 inches in diameter. This bridge, which is worked from hydraulic mains supplying other machinery as well, can be lifted and drawn back in about three-quarters of a minute. Although it has now been at work for nearly eighteen years, the writer is informed that very few repairs have been required. Neither the wheels, tread-path, nor chains have been renewed, and even the brass bearings of the large wheels are the same that were originally put in.
There are also bridges of this class at Birkenhead and Swansea. At the Morpeth Dock, on the Mersey, three wide roadway bridges, each of 25 feet span, work simultaneously, and can be lifted, hauled off, and again put into place in three minutes and three-quarters.
At the Docks at Barrow and Millwall a modified form of this bridge is in use. The long tread-path, in some cases involving expensive foundations and masonry, has been abandoned; and the main rollers, attached to and travelling with the bridge; replaced by fixed rollers on the cross-heads of the lifting rams; two others in the same line taking the weight of the bridge as it runs back. In all other respects the action is the same as in the bridge just described.
The bridge over the river Tovey, on the South Wales Railway, is an example of another form of Draw Bridge. When in position, it is balanced over fixed rollers, on a central pier ; the nose-end is slightly the heavier, and is supported on cams, the tail resting on the stone-work of the opposite pier. The cams being lowered, the tail rises to a height sufficient to clear the permanent railway, and is held down at that level by kep-rollers, which continue to act until, by the backward movement of the bridge, the tail-end becomes the heavier of the two. Whilst the bridge is being moved it is level, but it is inclined when in position. The two Draw Bridges, both of about 36 feet span, over the rivers Leven and Kent, on the Lancaster and Ulverstone Railway, are drawn back, not above the fixed railway, as in the foregoing examples, but below it. The inner, or tail-end, of the bridge is the heavier, and is supported on cams, by which it is lowered until it can enter beneath the cross-girders of the fixed portion of the viaduct. In the Leven bridge it is run down an inclined girder-tramway, on wheels fixed to the centre and tail-end. In the Kent bridge, the inclined girders are dispensed with, and the bridge is rolled over fixed wheels on the centre and tail-end pieces. Both bridges are worked by hand-power from the centre pier, through a pinion gearing into a long rack on the under side of the bridge.
The Bascule (Plate XVIIT., Fig. 1) is the most common form of Lift Bridge. Each half works on a horizontal axle, placed as near as practicable to the edge of the quay, and is counterbalanced by levers and weights projecting beneath the roadway forming the approach. It is built up of cast-iron arched ribs, which, when lowered into position, rest on cast iron springgings, fixed in the walls to receive them, while at the centre, the two parts are in contact and in a position to afford mutual support. A bridge of this kind has, to a great extent, the property of an arch, and with the object of obtaining this advantage in a greater degree, many have been constructed with the roadway rising from either end to the centre. But this want of a level roadway is a serious defect in many of the earlier bridges, and has been remedied in those of later construction. The adjustment of the counterbalance is a matter of some little difficulty, for not only must the half-span and its balance-levers be in equilibrium when in the horizontal position, but when lifted into the vertical, the two opposing weights must still maintain their proportionate leverage, and this can be the case only when a straight line passing through the centre of gravity of the half-span and of its counterbalance passes also through the centre of the horizontal bearing. In many instances, the absence of this perfect adjustment is partially compensated by an arrangement of weights suspended by chains from the ends of the levers; the chains being of sufficient length to allow the weights to rest on the bottom during a certain portion of the movement, and to be brought successively into action as the preponderance of the opposite end may require. The gap in the roadway for accommodating the slight backward movement of the platform over the centre is, in some cases, covered by a flap working on an independent centre. There are Bascule bridges over three of the entrances to the docks at Hull, and one at Goole; and at Selby, the bridge carrying the North Eastern Railway over the River Ouse, has one span on this principle. These bridges are all worked by hand, the power being applied through a segmental rack of 6 or 7 feet radius, keyed on to the horizontal shaft.
For short spans a bridge is sometimes used, hinged only at one side of the opening (Plate XVIII., Fig. 2). It is simply a flat platform strengthened by wrought-iron girders or trussed beams, with a short weighted tail-end, working into a pit in the masonry. The difficulty of adjusting the counter-balance, experienced in the Bascule, is felt here also. There is a bridge of this kind, of 26 feet span, carrying the public road, over the Aire and Calder Canal, at Goole.
The entrance to the Corn Warehouse Dock, at Birkenhead, is crossed by a lift-bridge of unusual construction (Plate XIX). It is of 30 feet span, provides for one line of railway, and is constructed in halves; each part, consisting of two single-web wrought-iron girders, 12 feet apart, being drawn up by chains into a vertical position on its own side of the entrance. When lowered, the two outer ends are supported by diagonal struts, hinged to the lower part of the girders, and guided by kepchains into sockets let into the face of the wall. The hauling power on the chains is obtained from hydraulic rams below the level of the ground, and not shown on the drawing.
The writer has not considered here the principles which determine the strength of the various parts of the different opening bridges, nor the details and proportions of any arrangement of gearing or other machinery employed in working them. A thorough description in detail would extend far beyond the limits of a paper like the present; the object of which is merely to give a general outline of each class of bridge, and its application to particular situations.
The CHAIRMAN said, it was not customary to make many remarks on subjects of this kind until the papers were in their hands, but even then, in the present case, it was hardly a question for discussion; it was a one of facts which would certainly prove very valuable matter in the Transactions of the Institute, more especially to the mechanical members, to whom it must be very interesting.
Mr. BOYD (Ex-President) begged leave to propose a vote of thanks to the writer of that very excellent paper; in which he was sure they would all join. It was a paper which required no observation, and would be especially appreciated by the mechanical members.
Mr. DAGLISH had much pleasure in seconding the motion, which was carried unanimously.
Mr. WAWN thanked them for the patience with which they had listened to his paper, more particularly as it was a subject which must be very familiar to most present.