Computerised Vertical-Lifting-Span Railway Bridge

Vertical-Lift-Span Railway Bridge    Send article as PDF   
Movable Vertical-Lift-Span Bridges have a movable portion deck which remains horizontal as it is lifted vertically. The amount of headroom available is determined by the variations between water levels and the heights of the lift towers. Water traffic beneath these bridges are usually restricted to low-masted craft, barges and tugs.

Virticle-Lift-Span Railway Bridge with single Lane Service Road.
Vertical-Lift-Span Railway Bridge with single Lane Service Road.

Movable Vertical Bridges are one of the oldest types of bridges known to humankind. The bascule or draw span was developed by Europeans during the Middle Ages. There was a resurgence of movable bridges during the late 19th century. Reliable electric motors and techniques for counterbalancing the massive weights of the bascule, lift or swing spans marked the beginning of modern movable-bridge construction. They are usually found in flat terrain, where the cost of approaches to gain high-level crossings is prohibitive, and their characteristics include rapidity of operation, the ability to vary the openings depending on the size of vessels, and the facility to build in congested areas adjacent to other bridges.

These types of Vertical-Lift-Span Bridges are very common throughout Australia on its navigable inland rivers. A large number of this type of bridge can be found along the Murray, Murrumbidgee and Darling Rivers.

These bridges have always sparked close inspection on my many travels along the Murray, Murrumbidgee and Darling Rivers especially. On several occasions I have been successfully halted in my travels by a Vertical-Lift-Span Bridge operating with its span up as watercraft sailed, paddled, or motored underneath.

Australian Vertical-Lift-Span Bridge History:

The period 1872 to 1884 was boom time for the economy in the Australian colonies around the Murray-Darling; the wool industry, with its partner the river trade, was part of that boom. But the rail network was spreading and would soon diminish the river-trade. Few people foresaw that change and in 1874 the Port of Echuca recorded its busiest year ever with 240 boats trading with the port.

In the same year the Victorian Government opened the final section of a railway line from Melbourne to Wodonga. Paddle-steamers previously carried freight to and from Albury and Wodonga via Echuca; now that commerce went by rail directly from Wodonga to Melbourne and the river trade lost business.

The New South Wales Government had watched the growing wool industry and river trade with concern. New South Wales sheep stations were producing more and more wool each year and it was going to build up ports and businesses in Victoria and South Australia.

Worse, many parts of the Riverina looked more to Melbourne than they did to Sydney because they conducted business with Melbourne via the river trade and Echuca. A programme of building railway lines to connect the sheep growing areas with Sydney was begun to provide competition to the river trade. Developments on the main southern line were incremental; from Goulburn the line went to Yass, then Cootamundra, then Junee, Wagga Wagga and Gerogery (reached in 1880) but not quite to Albury in case that encouraged trade between southern New South Wales with Victoria. A branch line from Junee went along the Murrumbidgee to Narrandera (in 1881) and then Hay, which was considered in Sydney to regard itself as virtually a Victorian town. The effects were immediately noticeable at Hay:

Prior to the railway in 1882 the swing bridge at Hay opened for as many as 6 paddle steamers per day. Conversely, in the whole of 1883 only six steamers passed through (Buxton 1967:217). River traffic was now rapidly on the decline and rail transport had taken over, or soon would.

A railway extension from Narrandera went to Jerilderie. To the west the train line from Bathurst was extended to Condobolin on the Lachlan River and from Orange, north to Dubbo on the Macquarie River and finally to Bourke in 1885. Now New South Wales could capture the pastoral trade previously carried along the Darling to Melbourne (via Echuca) and Adelaide (via Morgan) and divert it to Sydney. Paddle-steamers lost more business.

In South Australia a spur from the Adelaide-Melbourne railway line was laid to the wharf at Murray Bridge and the Port of Mobilong declared in 1886. A two level timber wharf 190 metres long was available for paddle-steamers. Murray Bridge as a working port meant the end for Goolwa and Mannum as river ports; trade through Goolwa had virtually ceased by 1890. About that time train lines were built to Loxton, Waikerie, Paringa, Berri and Renmark, Barmera and Glossop – all places which had relied on the river trade and now used the railway. Several of these South Australian places, and Mildura in Victoria, produced dried fruits which paddle-steamers carried to the nearest railhead. But the growth of irrigation was also seen as a threat to river-trade because irrigators continued pumping water out of the river during dry seasons reducing water depth further when it was already too low for safe navigation.

Swan Hill Vertical-Lift-Span Bridge
Swan Hill Vertical-Lift-Span Bridge

The boom time had faded by the 1890s. In one year the Port of Echuca recorded 74 boats leaving the port after recording 240 in 1874. The reduced wool clip during the 1880s drought and the 1890s national economic crisis made life difficult for everybody, but river trade was also badly affected by competition from the growing network of railway lines offering a lower-cost and more reliable service to pastoralists than was possible from paddle-steamers limited by water depth in rivers. Motor vehicles were also threatening the river-trade, though by 1895 the major roads had lost their former importance.

The railways of the Colony for the most part follow the direction of the main roads, and attract to themselves nearly all the through traffic. The tendency now is to make the roads act as feeders to the railway, by converging the traffic from the outlying districts towards convenient stations along the line.

But the river-trade was far from dead and paddle-steamers continued carrying freight and passengers, especially to places such as Mildura and the lower Murray River which did not have a railway connection with Melbourne until 1903. Tourist trips became more popular, often linking rail heads with paddle-steamers; one example was a tourist trip from Melbourne to Echuca by rail, paddle-steamer from Echuca to Morgan, train to Adelaide then return to Melbourne by coastal vessel.

A particularly prolonged drought from the late 1890s until 1902 so badly disrupted irrigation and river traffic that the states met to discuss ‘drought proofing’ the Murray. By 1915 they had agreed to build a series of weirs and locks to manage the flow of the river for navigation and irrigation. But river traffic was declining and in 1924 the agreement was amended to give higher priority to irrigation requirements than to navigation. In 1934 the agreement was further altered to provide for only 14 locks instead of the original 26. Paddle-streamers assisted in construction by carrying cement, crushed granite and other building material to the construction sites.

After Locks 1 to 11 were completed in the late 1930s the Murray was navigable in years of normal rainfall from the Mouth to 100 kilometres upstream of Mildura. But the river trade had dwindled to insignificance; the Port of Echuca had so few movements that record keeping ceased in 1910. The last profitable riverboat trading area to Echuca was on the Edward River and the Lower Murrumbidgee to Balranald. Railways from Victoria tapped this trade when lines were constructed to Moulamein in 1925, Balranald in 1926 and Stoney Crossing on the Wakool in 1928.

Virticle-Lift-Span Railway Bridge
Paddle Steamer passing Balranald Bridge in the 1890s

The last commercial river-boat left Bourke in 1931. There was still some activity around Murray Bridge with bagged wheat carried in the 1920s and 1930s and local movement of milk from dairies along the river to the milk factory downstream of Murray Bridge wharf. Around Echuca there was some local logging-related river activity until the mid-1950s, but the river trade as a whole had ended decades before then.

In summary, the river trade grew from a very small beginning into a substantial enterprise because it provided better transport service, at lower cost, than bullock wagons. Pastoralists growing wool on properties along the Murray and Darling Rivers relied on paddle-steamers to move their wool clip and the paddle-steamers relied on the income from carrying wool to remain profitable. Despite problems with variable water levels in the rivers, and difficulties caused by competition between the three colonial governments concerned, the river trade thrived while paddle-steamers provided the best service available to pastoralists.

In the late 1850s, South Australia planned to funnel the Murray River trade down the Murray River and associated River systems to Goolwa from the Goldfields of Victoria. This was foiled by the entrepreneurs in Melbourne connected to the Black Ball shipping line, and the Bright family, who started to develop the “Melbourne Mount Alexander and Murray River Railway” to Bendigo, Echuca and beyond. They were unable to borrow capital, so the Victorian Government took over and built the railway to Bendigo and Echuca, establishing the major port of Echuca, turning the River trade around, so that the river boats sailed down the Darling and the Murrumbidgee to the Murray, then upstream to Echuca. This then set a pattern of funnelling the goods of inland Australia down to the Port of Melbourne.

NSW Government tried to retrieve the situation by building railways out to places like Wagga Wagga, in an attempt to capture the trade. The expanding rail network took business previously handled by river traffic and soon paddle-steamer operators were going out of business. However, once road transport took hold, the trade continued to funnel goods from inland NSW and southern Queensland down the Newell Highway, on to the Port of Melbourne. But the growing railway network offered a more reliable, faster service to pastoralists and to the general community. Paddle-steamers replaced bullock drays because of better service and the paddle-steamer was replaced by the railway because the train offered still better service.

Mulwala Bridge
Mulwala Bridge just after completion in 1924, with operator at the top of the tower.

As a postscript from the 1970s a new generation of tourist cruise-boats came into operation ensuring the continuation of the navigation tradition on the Murray River. These include completely new purpose-built vessels and restored paddle-steamers. The oldest paddle steamer still in regular use on the Murray River is the P.S. Adelaide which was built in 1866.

Description of Vertical-Lift-Span Bridge:

Vertical lift span bridges are movable bridges which rise vertically and remain horizontal throughout operation. The first generation of vertical lift span bridges in Australia are of particular interest as there is a fascinating evolution in designs, with a number of distinguished Australian engineers contributing to the body of knowledge of each subset.

Characteristic components of vertical lift bridges include towers supporting a sheave at each corner of the opening span, counterweights to minimise the force required for operation and subsequent ropes or chains which pass from the counterweight over the sheaves and attach onto the lift span.

There are a number of advantages for vertical lift bridges noted by Hovey (1926) and they include: — When the bridge deck is high above water level a low lift will allow the passage of vessels.

  • There is the ability to partly raise the bridge for smaller vessels. 
  • If the bridge is built on an alluvial soil where the channel is likely to shift, an allowance for changing the movable span can be introduced by adding towers to adjacent spans and moving the mechanism when required. 
  • Parallel bridges can be readily added to cater for growing vehicular traffic.
  • Minimal obstruction of the waterway.

The disadvantages of the vertical lift span bridge include the limited headway depending upon tower height and the difficulties encountered when there is a need to maintain, repair or renew components.

Vertical lift bridges are categorised by the arrangement of the span drive machinery and superstructure geometry. There are four representative sub-types of vertical lift bridges including the wire rope span drive, tower drive, connected tower drive and lastly the pit drive (table bridge).

Tower Drive Vertical-Lift-Span:

Span drive vertical lift bridges are typically a balanced vertical lift system. The lift span ends are attached to wire ropes that pass over the sheaves mounted at the tops of the towers. The ropes then attach to counterweights at the opposite end of the rope. The counterweights typically balance the weight of the lift span, however for large lift bridges the magnitude of the counterweight ropes create a significant weight differential, as the ropes pass from one side of the sheave to the other. This differential is often balanced by an auxiliary system to mitigate power requirements for the operation of the span. The principle types of span drive vertical bridges include wire rope span drive and rack and pinion span drive.

Virticle-Lift-Span Railway Bridge - 02
Wire Rope Drive& Vertical-Lift-Span Bridge

The defining feature of a span drive bridge is the wire rope drive mounted on the lift span that hauls the span upward or downward. The figure above shows the directions of rope travel for the lift span being lowered. The noteworthy advantage of this type of drive is the mitigation of longitudinal skewing, the continuous connection of haul ropes prevents one end of the lift span rising faster than the other, which would result in jamming the span. In the figure below, the hoist drum is located at mid-span, however span drive bridges exist for which the primary drive machinery is at mid-span and the secondary machinery and hoist drums are located at the ends of the span.

Rack and Pinion Vertical-Lift-Span Drive:

Rack and pinion span drive In place of haul ropes to operate the drive machinery, rack and pinions are another common drive system. These bridges are still balanced throughout operation. The primary driving mechanism is located at the centre of the lift span and the secondary machinery is located at each end of the span. Operation is achieved through pinions engaging racks that are mounted vertically on the towers, with rotation of the pinions raising or lowering the span. This type of drive has been known to result in significant longitudinal skewing of the lift span.

Tower Drive Vertical-Lift-Span:

Tower drive vertical lift bridges are balanced bridges which have drive machinery located at either the top or base of the towers. The two basic types include traction drive and winch drive bridges. The characteristic feature of tower drive vertical lifts is that the machinery in one tower is mechanically independent of that in the other tower.

This often results in differential rising of the span ends, thus requiring the implementation of mechanical or electrical controls to limit this differential. The principle types of tower drive vertical bridges include wire rope traction drive and winch drive.

Wire Rope Traction Drive:

The figure below presents a schematic of a tower drive vertical lift bridge with a traction drive. Drive machinery located at the top of each tower rotates the counterweight sheaves. The forces necessary to raise the span are then transmitted from the sheaves to the counterweight ropes by friction. It is noteworthy that the force necessary to raise the lift span at each end may differ due to unequal machinery friction, differential span loading and the temporary effects of rain or ice.

Virticle-Lift-Span Railway Bridge - 03
Tower Drive Vertical-Lift-Span Bridge

Winch drive Winch drives are another variant of tower drive bridges. The lift span is balanced in an identical fashion to previously noted bridges, however the power to operate the span is provided by winch drives that are commonly mounted in the base of the tower. Haul ropes are connected to either the lift span framing or to the counterweights, with the rotation of the winch recoiling the wire ropes thus raising the span. As with the traction drive variation, because the mechanical machinery on one side of the channel is independent of that on the other side, skewing has to be controlled (WisDOT, 2011).

Connected Tower Drive:

Connected tower drive vertical lift span bridges are those bridges with have longitudinal and lateral framing which connects the tops of the towers and also supports machinery and access walkways (Figure below). As with other vertical lift bridges the operation is balanced by counterweights. However, since this type of bridge is only suitable for short spans with a moderate lift, the counterweight ropes do not require an auxiliary counterweight system. Span drive machinery is mounted on the top connecting structure. Various machinery arrangements exist with different bridges positioning mechanisms at mid span or adjacent to a sheave. Driving power is provided by pinions that engage curved racks fastened to the counterweight sheaves. 

Virticle-Lift-Span Railway Bridge - 04
Connected Tower Drive Vertical-Lift-Span Bridge

The force necessary to move or hold the lift span is transmitted between the sheaves and the counterweight ropes by friction. Alternate driving mechanisms with wire ropes connecting the sheaves, to transfer rotation to each end, of the bridge also exist.

As all the span drive machinery is directly connected skewing of the lift span in either the longitudinal or transverse direction is mitigated. However, minor skewing due to differential counterweight rope stretch, different sheave diameters, and rope slip in the counterweight sheave grooves may cause skewing over time.

Pit Dive Vertical-Lift-Span or Table Bridge:

Table bridges are those bridges which remain vertical during operation and are powered by hydraulic cylinders that are largely hidden from view when the bridge is in the closed position (Figure below). It is noteworthy that table bridges have also been referred to as pit drive vertical lift bridges due to the location of the driving mechanism (WisDOT, 2011). The defining feature of table bridges is the lack in visual presence of mechanical components and supporting superstructure. Select table bridges are still fitted with supporting counterweights and sheaves to reduce the effective weight that must be raised; however these features appear to be less frequent as this style of bridge evolved.

Virticle-Lift-Span Railway Bridge - 05
Table Bridge / Pit Drive Vertical-Lift-Span Bridge

Table bridges are constructed when the cost of the greater lifting capacity required, compared to a balanced bridge, is estimated to be less than the cost of counterweights or for aesthetic constraints. The hydraulic cylinder drives are most suitable for providing the power for operation because large actuating forces can be produced using smaller electric motors than would be required for mechanical drive. There is also less friction between the prime mover and the point of force application in a hydraulic system (WisDOT).

LEGO Mindstorms based ‘Vertical-Lift-Span Railway Bridge’:

My LEGO Mindstorms NXT controlled Connected Tower Drive, Vertical-Lift-Span Railway Bridge is built entirely from LEGO. The observant may notice the Railway Track Piece Ends which allow for a flush junction between the Lift-Span and the ends of non-moving Track. The LEGO Purist will shoot me down in flames for this approach, but it makes for a neater outcome, plus I recycled the Railway Track Piece Ends from some badly worn and useless Curved Railway Track Pieces.

Each end of the span is winched up on cables to a separate Winding Drum for each corner. The for Winding Drums are all driven from a very early edition of a PowerFunctions Medium Motor which wouldn’t pull the skin off your custard, but thanks to the gearing arrangement with a  final gear ratio is 1:40 or, the speed is decreased by a factor of 40 times, with an increase torque factor of 40 as well. The Winches will aslo quite happily run at a gear Ratio of 1:5, but the Bridge Span moves far too fast as to be almost comical.

[youtube]ar0Gsj1CnU4[/youtube] Vertical-Lift-Span Railway Bridge with single Lane Service Road.

At this stage I am using a LEGO Mindstorms NXT Brick to control the Vertical-Lift-Span Railway Bridge, along with three Touch Sensors. A standard NXT Touch Sensor to start the Winches moving, and two RCX Type Touch Sensors as Limit Switches. There is a lower Limit Switches to sense when the Bridge Span is in the down position, and a second Limit Switches to sense when the Bridge Span has been fully lifted up.

It is actually a waste of a LEGO Mindstorms NXT Brick for this purpose, so when I integrated the Vertical-Lift-Span Railway Bridge into my Railway Layout, I will use a $5 Arduino Microcontroller instead. Also when using an Arduino, I can simply add a $0.05, 940nm Infra-red LED  to the circuit which will allow me to broadcast PowerFunctions Commands to the LEGO Trains. Not only will the Arduino control the Vertical-Lift-Span Railway Bridge, it will also Stop and Start the Trains when the reach the Bridge. Hopefully avoiding the Engine Driver from having a rough trip into the river as the Bridge Span has been, or is being lifted upwards.



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