A Brief History of Movable Bridges

Movable bridges are only necessary on navigable waters, where wooden Dutch-style draw bridges were used prior to the introduction of swing bridges (possibly c 1650). When draw bridges were open it was difficult for the bridge deck to clear the bridge's navigable passage completely, and this was probably the main reason for the development of the swing bridge. The increasing size of bridges in the early nineteenth century, to cope with both increases in road traffic and the size of ships, meant that greater force was needed for operating an opening bridge. Swing bridges were easier than draw bridges to balance, and this would also have encouraged their use. Vertical lift bridges were also built, but they were more expensive and had limited clearance so were unsuitable for docks. In the first half of the nineteenth century, cast iron was used for most large swing bridges, something of a compromise because of its brittle nature. Outside girders were often damaged by glancing blows from ships passing through the bridge passage. The width of passage was also restricted to around 40 feet, too narrow for the paddle boats which came into use in the mid-nineteenth century. From this time iron, and later steel, lattice and plate swing bridges were introduced. They were still vulnerable to damage from passing ships and many were subsequently replaced by rolling bascule bridges. As the deck lifted it moved away from the edge of the passage so was unlikely to be damaged by a passing ship. Other types of movable bridge were developed, but they tended to be used in locations where there were specific problems, such as a poor foundation or restricted space.

The range of movable bridges is extensive, so some limitation is necessary to make the compilation of a list manageable. For this reason, most bridges where the passage is less than 18 feet will be excluded. These tend to be canal bridges, and are probably already quite well documented. A few may be included to give an indication of their type and survival. This will also restrict the list to metal bridges, though the earliest swing bridges were built from timber, and an example could be included for completeness. The following gives an overview of the development of the movable bridge.

Swing bridges

Cast iron for large swing bridges was first used early in the nineteenth century and continued to be used until the 1850s. The maximum navigable passage capable of being crossed by a cast iron swing bridge was about 45 feet. The development of paddle steamers in the 1850s led to navigable passages of over 60 feet being required, and to cross these fabricated wrought iron girder bridges were built. Cast iron bridges also had much higher wind loadings than open lattice wrought iron girder bridges, and this was a further obstacle to their being used on wide navigable passages.

Cast iron swing bridges can be divided into two types depending upon how the weight of the bridge is supported. On the earliest examples the load is taken by wheels or rollers which run on a large diameter circular track under the bridge. On a few cast iron swing bridges built around 1850 the load is supported by both the central bearing around which the bridge turns and the roller track. On some later wrought iron and steel swing bridges, wheels at the inner end of the bridge act to stabilise the bridge when in motion.

Ralph Walker produced the first drawing for a cast iron swing bridge in 1800 as part of the design stage of the earliest London Docks. Wooden swing bridges were already widely used on canals at that time, though they were usually of the single leaf type. Walker's design was for a double leaf bridge. When the bridge was in use by road traffic the two outer ends of the leaves were locked together and there were bearing pads fitted to the abutments close to the edge of the bridge passage. When the bridge came under load, as a vehicle passed over it, the locked outer ends of the leaves would descend slightly and this would cause the ironwork of the bridge to press against the bearing pads, forming an arch and thus increasing the bridge's load bearing capacity. The one major difficulty today is to understand whether how the bridge distorts moves to create the arch. Distortion of the bridge girders is unlikely, so it is more probable that clearance of the rollers and central bearing allowed the movement.

Fixed bearing pads could cause the bridge to jam whilst turning or, because of the close tolerances required, they would were perceived to be vulnerable to breakage during turning. It was possibly to overcome the problem that John Rennie, in one of his drawings which probably dated from 1803, suggested having the use of a movable wedge operated by gears. This system survives on the cast iron swing bridge in Leith Docks, Edinburgh, and it was probably used in London. In later designs, such as Humber Dock bridge, fixed bearing pads are continually under load, the bridge sliding across several pads as it turns, and further horizontal pads taking the load when the bridge is closed and in use by road traffic. Cast iron was first used for swing bridges in London. In his notebooks in the National Library in Edinburgh, Rennie suggested the use of cast iron swing bridges for London Docks on 4 April 1803, and the decision to use cast iron was noted on 16 April 1803. Although cast iron swing bridges were erected in London Docks around this time, the earliest bridges built to Walker's design were actually wooden. On such wooden bridges, the framing of the bridge leaves would have been able to distort to create the arch. It was more difficult when the bridge was of cast iron, particularly as bridges increased in size, such as when double carriageways were introduced in the 1840s. It In order to maintain a sliding fit for the bearing pads, is probable that the bridges were balanced about a point over towards the roller track/bearing blocks closest to the navigable passage and not about the central bearing. Until around 1850 the central bearing did not carry any vertical load, and bearing clearance may have been such as to allow the inner end of the bridge to rise slightly when loaded at the outer end of the leaves, and this would allow the bearing pads to become active.

Cast iron is only suitable for swing bridges with a navigable passage of up to 45 feet. In the second half of the nineteenth century, after wrought iron was introduced to allow for wider passages, bearing design divides into two types. On one the weight of the bridge is carried by the rollers, the central bearing just keeping the bridge in place. On the second, a load-carrying bearing was fitted to the top of the central bearing and the load was divided between the central bearing and the rollers. Wheels on the inner end of the bridge stabilised it when it was being turned, as at Sculcoates Bridge, Hull. The central load-carrying bearing also allowed the bridge to rock, and on the last of the cast iron swing bridges, built in the 1850s, this may have been used as part of the mechanism for forming an arch. On these bridges, the angled bearing pad found on earlier cast iron swing bridges was omitted, and horizontal bearing pads with vertical stops, located close to the edge of the navigable passage, were fitted. Similar horizontal bearing pads, but lacking a vertical stop, were used on bridges with angled bearing pads dating from the 1840s, such as the recently restored Humber Dock Bridge, Hull. Wrought iron bridges do not seem to have been designed to create an arch, though they did have locking mechanisms at the outer ends of the leaves. Hydraulic wedges for stabilising the bridge and for load carrying were also introduced from the 1860s.

Bascule bridges

The term 'Bascule' is derived from the French word for a see-saw - 'Bacule'. A bascule bridge is one in which the deck or roadway can be raised around a horizontal hinge or axis.

The earliest type of opening bridge was the drawbridge such as those forming the entrance to a castle, the bridge deck being raised by chains fitted to its outer end. In Holland this method was developed to form lifting bridges where one end of the deck was raised by chains fixed to overhead beams which were pivoted in the middle and counterbalanced at their opposite ends. This type of bridge was common in the eighteenth century for roads crossing river navigations and canals. Several, with the wooden structure typical of this period, can still be found on the Llangollen Canal. The earliest bridge over the entrance to the Queen's Dock, Hull, was of this type, while Wheldrake Bridge on the River Hull remained as an excellent example until it was burnt down in March 1976. In this century many of this type of bridge have been built from steel, with recently modernised examples on the New Junction Canal between Goole and Doncaster dating from circa 1900. New wooden drawbridges are sometimes built as part of conservation area developments, such as that at Castlefields in Manchester.

A true bascule bridge is one which has its counterbalance fitted to the tail of the bridge deck on the opposite side of the bridge's horizontal hinge. This has the major advantage that when the bridge is down there is little to hinder passage of road vehicles. Ideally, as the bridge opens, the counterbalance weights descend into a recess which is free from water. If the counterbalance enters water, its displacement will materially affect the balance of the bridge. Bascule bridges could be of the double leaf type, where the bridge decks meet in the middle when closed. The carrying capacity of the bridge can be increased by ensuring that when the two leaves meet they form an arch, the outer ends of the leaves butting against the masonry of the bridge foundations.

The simplest form of bascule bridge, such as those found on the Oxford Canal, are opened purely by the weight of the operative pressing against an arm extending beyond the bridge hinge. This was fine for a seven foot wide narrow canal, but for wider canals an opening mechanism was necessary. The double leaf bascule bridges on the sixteen foot wide Forth & Clyde Canal are typical of this type, with gears and a windlass opening the bridge via a quarter segment gear. These early bridges were usually made of wood with cast iron and wrought iron fittings.

The increasing size of maritime boats during the nineteenth century created the need for greater channel widths on bridges, particularly after the introduction of the paddle steamer. The corresponding increase in weight of the bridge made the introduction of improved opening mechanisms necessary. The first bascule bridge to be fitted with a hydraulic drive was erected over Bowcombe Creek, Kingsbridge, Devon in 1831. Designed by J. M. Rendell, it was a single leaf bridge, 32 feet long and 15.75 feet wide. The bridge was opened by a manually operated hydraulic pump which moved a rack connected to a pinion on a drum shaft, a chain attached to the drum and bridge raising the end of the leaf.

As technology developed, cast iron became the usual material for the framing of movable bridges. They were common in nineteenth century docks, such as the manually-operated bridge in Whitefriargate (between Queen's and Prince's Docks) in Hull which was only altered from this form in the 1920s. The use of cast iron was also necessary for the increased loading found on railway bridges. The original railway bridge at Selby, erected in 1839, was a cast iron double leaf bascule bridge. Manually operated, it had a span of 45 feet. It was replaced by a wrought iron swing bridge towards the end of the nineteenth century as heavier trains required a more substantial structure. The move from cast iron to wrought iron was also because of the better characteristics of the latter.

Skeldergate Bridge, further up the Ouse in York, also has a bascule opening section to allow sailing vessels to reach wharves upstream of the bridge. Completed in 1881, it appears at first glance to be a three-arch cast iron bridge. However, one arch was actually a bascule bridge. It was originally hydraulically powered, two cylinders, of 12in. dia. and 5ft-6in. stroke, acting on chains to open and close the bridge. A hydraulic pressure of 700 lbs/sq.in. was obtained from a small pumping station powered by a gas engine. The bridge was converted to electric power in 1938/9, but has recently become fixed and the machinery scrapped.

Towards the end of the nineteenth century bridge spans were increasing, with swing bridges more often being built, rather than bascule bridges. As span increases, so does the size of each leaf of a bascule bridge, with corresponding problems in balancing and powering the opening sections. Windage on the open leaves also increases dramatically, always a problem in exposed situations. However, bascule bridges require less space, and this has resulted in their continued use, particularly in smaller ports. Two electrically powered bascule bridges, dating from circa 1930, are still in use at Poole and Weymouth on the south coast.

Rolling Bascule Bridges

The invention of the rolling bascule bridge by the American, William Scherzer, in 1893, has resulted in the replacement of many movable bridges in docks, such as Liverpool, Birkenhead and London. In Hull, Drypool and North bridges are of this type, their light framework allowing them to span much greater passages than with conventional swing or bascule bridges, whilst keeping power requirements to a minimum. Keadby Bridge, built in 1916, is one of the best known English Scherzer type rolling bascule bridges. It carries both road and main line railway.

Although the majority of modern bascule bridges in England were built using the Scherzer 'rolling lift' system, there were other types. The Rall system uses a rolling trunnion and a linking strut to the bottom of the bridge deck such that less space is required for the deck to be raised. Joseph Strauss built the first bridge to his system in 1905. It uses a counter balance connected to the bridge deck by a parallelogram linkage. As the deck rises, the counter balance moves closer to the fulcrum to maintain a balance. The Brown system, dating from 1896, also uses a counter balance connected to the middle of the moving span by wires or a linkage. Finally there is the Abt system, where the counter balance moves in the opposite direction to the bridge deck

Technical Aspects of Movable Bridges

The technical design of opening bridges has developed continually. The earliest were the simple draw and bascule bridges described above. Swing bridges were introduced during the period of canal construction in the late eighteenth century, and they were used in dock works from the early nineteenth century when cast iron was the usual construction material. Whilst good in compression, cast iron is poor in tension and this restricted the span of an opening bridge.

Although with opening bridges, the bridge deck may seem to be, in effect, a cantilever where the upper surface is in tension, this was not necessarily the case. When double leaf cast iron bascule and swing bridges were introduced for dock works in the nineteenth century, the two leaves locked together when closed and formed an arch. This reduced the tensile forces and increased compressive ones, making wider and greater load bearing bridges possible with cast iron.

For docks in particular, bascule bridges had two major disadvantages. One was the effect of wind on the raised leaf which could increase the power needed to open and close the bridge considerably. The other was that the raised leaves tended to catch on the rigging of the large sailing ships of the time. (Swing bridges had folding railings on the side next to the navigable passage when open) Bascule bridges were, however, quicker in operation than comparable swing bridges and required less space as the decks were raised rather than swung out of the way. The introduction of wrought iron and steel from the middle of the nineteenth century resulted in changes in the design of opening bridges. They are much stronger in tension than cast iron, and this allowed swing bridges to be built with spans sufficiently large to accommodate the increasing size of ships, which were also increasing in size due to the introduction of steel and steam power. Lattice girders were also used to keep down the weight of the bridge and they also helped to reduce wind forces.

The development of steel rolling technology enabled angle sections to be produced in the 1890s, and it was this which led to Scherzer's bridge design. Cheap steel angle sections allowed for the construction of light framework cantilever bridges of which the Scherzer is the best-known opening type. The majority of nineteenth century bascule bridges in docks were replaced either by swing or Scherzer bridges in order to provide greater spans. Bascule bridges tended to be used where the confines of the site dictated their design, ie where there was insufficient space for the bridge to swing sideways.

Cast iron bridges today

Few cast iron bridges have survived, as most would have been removed during dock or road improvements. Cast iron swing bridges similar in size to those in Hull have survived in Liverpool (Albert Dock), Leith and at Plymouth. The one at Albert Dock is still operable, but there are increasing problems in balance and consequent ease of operation. The bridge was restored about twenty years ago, and is used occasionally to allow access for large boats to Albert Dock. Problems may have developed with the roller track and segmental gearing. Only one set of operating gears are used on this single carriageway bridge, and it has been suggested that a further set of gears could be fitted as on the double carriageway bridges in Hull. The Leith bridge is now fixed as there is no water-borne traffic. None survive in London Docks.

There is a cast iron swing bridge over the Caledonian Canal at Moy, near Fort William, and operational problems are also encountered here. The bridge is used by a farmer who may use the bridge with larger loads than the 2.5 tons allowed, an earlier load limit being 5 tons. The bridge had a major rebuild a few years ago when the cast iron frames were strengthened by bonded steel plates and is currently undergoing restoration to make its operation easier by refurbishment of the turning gear. Originally only one of the original swing bridges on this canal was to have been constructed of cast iron, the others were to have been wooden to a similar design to the cast iron bridge proposed by Ralph Walker in London which incorporated bearing pads. Instead, around 1812 it was decided to use cast iron for all the swing bridges as local timber was difficult to find and expensive _ cast iron was cheaper. According to the surviving drawings, several types were built. The large road bridge at Muirtown had fixed bearing pads on the stonework below the roller track level and a simple central bearing; the smaller road at the head of the lower lock at Gairlochy had a simple central bearing with three bearing pads fitted to the upper and lower roller tracks; Moy (the only surviving example on the canal) has an angled roller track and an angled central bearing with no bearing pads. This type was probably the first use of a load-taking bearing on cast iron swing bridges.

There are other cast iron canal swing bridges, but these are of smaller dimensions and cannot be compared with those found in docks or on large canals such as the Caledonian. Bridges on the Gloucester & Berkeley Canal, the only other contemporary large British canal, are currently being investigated as some original ones may have been cast iron.

Balancing cast iron swing bridges

Humber Dock (Wellington Street) bridge was built in 1846 under the direction of J.B. Hartley, the son of Jesse Hartley, Liverpool's Dock Engineer of the time and one of Britain's foremost civil engineers. The bridge is virtually identical to the one built a few years earlier in 1842 over the passage between Salthouse and Canning Docks in Liverpool. Any slight variations in dimensions can probably be ascribed to the casting methods of the time. For instance, large patterns were necessary, and these may have been constructed in sections for assembly during moulding, resulting in slightly different overall dimensions. The two cast iron swing bridges erected in Albert Dock, Liverpool, at the same time were narrower than the Salthouse Dock bridge, incorporating a single carriageway, as opposed to the double carriageway for the bridges at Salthouse and Humber Docks.

Swing bridges with load-carrying central bearings built in the 1840s 1850s mark an important development in swing bridge design. Subsequent wrought iron and steel swing bridges were of two distinct designs: those balanced around the centre of rotation, with much of the weight of the bridge being taken by the central bearing, and those where the weight was taken mainly by rollers or wheels running on a circular track and balanced around a line through the wheels closest to the edge of the water passage. Variations in this design include bridges which had additional wheels at the inner end of the swinging sections for stabilising the bridge whilst in motion. Sculcoates Bridge, Hull, is an example of this type.

Cast iron swing bridges of the 1840s are usually of the second type, the central bearing being used just to locate the swinging section of the bridge and not to carry any of the weight. The surviving 1848 cast iron swing bridge at Alexandra Dock entrance, Hull, is also of this type, though in place of the weight being carried by numerous rollers working around a track, as at Humber Dock, two sets of three wheels are used, each set being directly under the bridge carriageway. This may be a fore-runner of the second type of wrought iron/steel swing bridges mentioned above. The first type of wrought iron/steel swing bridges probably developed from the later design of cast iron swing bridge. Surviving drawings of swing bridges erected in Liverpool Docks in the late-1840s and the 1850s show that they had some of their weight taken by a bearing fitted on top of the central bearing also used for locating the bridge.

Thus it can be seen that the centreline for the balance of cast iron and wrought iron swing bridges could be in one of two places dependent upon the type of bridge is not necessarily around the main bearing. With reference to Humber Dock bridge, the centre of balance should probably be towards the edge of the water passage, and this may account for the insufficiency in kentledge (here, random masonry blocks) calculated for balance around the centre bearing. Similar results for the kentledge required have been calculated for the Moy Bridge on the Caledonian Canal. However, no contemporary published details regarding the balancing of cast iron swing bridges has been found, only those from the 1870s and after when wrought iron was the structural material.

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last revised: 27 March 2014