Road-Ferries for Congested Bridges
Stephen Salter, School of Engineering and Electronics, University of Edinburgh.
10 November 2004
The design specifications of many bridges were chosen with inaccurate knowledge of the growth of traffic density and axle loadings. This paper discusses ideas to increase the carrying capacity of long-span bridges and, at the same time, to extend their life.
If all the tricks to strengthen a bridge have already been tried it follows that increasing capacity and extending life can be achieved only by:
· Increasing vehicle speed.
· Reducing vehicle separation.
· Spreading local loads more evenly.
All three options can be exploited in the design of carrier vehicles which have been named ‘road-ferries’. Each road ferry might carry eight heavy goods vehicles or 30 cars. The weight of a road ferry designed for 44 tonne heavy goods vehicles is expected to be 15 to 20% of the payload weight but would be evenly spread. Ferry separation would be under control of the Bridgemaster's computer.
The techniques depend on new designs for mechanical power transmission developed originally for renewable energy, the reliable operation of electronics and advances in information technology.
DESIGN OBJECTIVES
· A substantial increase in carrying capacity to a level greater than that of any feasible system of approach roads.
· A substantial increase in fatigue life.
· Smooth change at any time to and from the present conventional mode of operation.
· Progressive introduction to keep pace with increased traffic demand.
· Use by vehicles with the heaviest EC-approved axle loading.
· Minimal modifications to the present bridge structure.
· The potential for disassembly into easily transported, exportable modules.
· The minimum reduction in crossing time.
· No reduction in vehicle safety.
· No increase of energy consumption.
· A capital cost substantially lower than that of a second bridge.
· A development time substantially less than that of the planning and construction of a second bridge.
DESIGN ASSUMPTIONS
· A general confidence in the growing reliability of electronics and information technology.
· Different designs for heavy goods vehicles and private cars implying segregation at the bridge approaches.
· The availability of land at both ends of the bridge for loading platforms.
· The acceptance of a shorter life, service interval and spare replacement cycles for the members of a road ferry fleet than for an entire bridge.
Initial sketch drawings of the hardware of road ferries are given in figures 1 to 3. Extensive parametric design equations which can be adapted for any input assumptions are available.
THE TECHNIQUES
To have reliable automated communications between road ferries allowing coherent velocity management and therefore much smaller separation than for independently-controlled vehicles.
Vehicles can be closely packed on road ferries and road ferries can be close to one another. This allows a much greater overall vehicle packing density combined with the high crossing speed of a single vehicle. The maximum possible gain is about 9 but as this would correspond to approach roads with 18 lanes in each direction it would far exceed any foreseeable requirements. For a given traffic flow, the faster the transit the lower the load on the bridge.
To fit road ferries with a large number of closely packed tyres with wide treads and much lower inflation pressures as shown in figure 1.
This reduces damage to the deck surface of the bridge. The estimated pressure reduction is 2 for heavy goods vehicles and 3.7 for private cars. The loads of an axle group, which are present spread over just one or two metres, will be spread evenly and so reduce bending moments in longitudinal support beams by a factor of nearly 2.
To choose the sum of the length of the road ferries and the separation between them to be a multiple of the distances between the vertical hangers.
This means that the tensions in the hangers are nearly constant, changing only with the differences between the weights of a set of heavy goods vehicles rather than going to zero after the passage of each axle group. Even if this ideal separation cannot be used during periods of intermediate traffic density, the number of load cycles for heavy goods vehicles will be reduced from one per axle group to one per ferry - a factor of about 16.
To fit the road ferries with soft 'intelligent' shock absorbers so that dynamic loads are reduced.
These shock absorbers can even respond to the known locations of incipient pot holes and thus off-load each wheel in turn as it goes over the trouble spot. This will make a road ferry look like a fastidious centipede on roller skates which is avoiding aphid droppings on a leaf.
To place the wheels of the road ferries at the edges of each carriageway as shown in figure 2 so that they run on surfaces rarely used by normal traffic.
Applying the load close to the edge of the carriageway will reduce the bending moments in cross beams. With no fore-and-aft load sharing the reduction in bending stress in the cross beams of the bridge structure would be about 5. However, if the load can also be spread fore and aft, the reduction is by a factor of nearly 20. This is shown in the following graphs.
The graph shows the load distribution and the resulting deck bending moments resulting from the worst case of two heavy goods vehicles driving side by side with triple axles each imposing 11.5 tonnes on to a two metre strip of bridge deck. The peak bending moment is 920 kNm at the centre of the 8 metre span.
Next we move the load out to 5 and 95% of the carriageway to leave a flat bending moment over most of the span of only 180 kNm a reduction by a factor of 5.
Then we spread the load which was being applied to a 2 metre strip to half the length of a heavy goods vehicle, say,
The force density drops to a drop of 3.75. The result is plotted to scale in the graph below with a peak bending moment of only 48 kNm, down by a factor of 19.
OPERATION
The present bridge carriageways would be unchanged and used just as at present for low traffic densities. At some traffic density chosen by the bridge operator, vehicles would be diverted to loading bays. The approach road would end with a sharp drop equal to the height of the deck of a road ferry. This allows vehicles to drive directly on to a ferry just as if a queue was approaching red lights. A similar arrangement at the far end of the bridge would allow vehicles to drive straight off.
The ferries should be wide enough, perhaps 7 metres, to span two lanes with the drive wheels on unused road surface. They would also be long enough to take two rows of four trucks, a length of about 60 metres depending on hanger spacing. This length means that conventional steering geometry will not be suitable. But if we can make each pair of drive wheels able to rotate about a vertical axis through the centre of their shaft then all wheels can perform a steering function according to the following modes.
· It will be possible to have separate centralising for the front and rear of the ferry.
· It will be possible for the ferry to move in a direction oblique to its length with no change of heading.
· If the rotation can be more than 90 degrees then the ferry can move crabwise.
· If each wheel axis can be directed to a common point then the vehicle will rotate about that point.
If we use the suspension in figure 3 to lift some fraction of the wheels off the ground then large changes in steering angle can be made with very little torque or tyre scrubbing. A ferry can move forwards and then immediately to one side with no rotation about a vertical axis, ideal for getting into tight parking spaces! A combination of these modes will allow movement along any path with very little deviation.
When a set of, say, 30 cars or 8 heavy goods vehicles has been loaded the ferry would accelerate away from the loading bay. Either electrical or hydraulic drives can be used. However acceleration will be best if there is some form of energy storage and the cycle life of present battery technology falls far below what can be achieved with gas/oil pressure accumulators. Engine efficiency and acceleration will be best if the engine drives a variable-displacement high-pressure oil pump which can send oil to a gas accumulator and to wheel motors built into each of the large number of drive wheels. The wheel motors must be able to free-wheel when the ferry is at its maximum speed so as to reduce oil flow. They must also be able to act as pumps during deceleration, sending energy back to the accumulator. It is also mechanically desirable if the wheel motors have shafts at both ends so that a tyre can be fitted at both ends. A good arrangement is to use a radial piston machine with displacement varied in digital steps by poppet valves rather than by the change of angle of a swash plate. This also allows several banks of machine to drive a common shaft.
ACCELERATION PROFILE
To understand the acceleration we have to understand the complementary behaviour of the engine and pressure accumulator. An engine will deliver its best power, economy and cleanliness at a fairly narrow range of speeds and torques. There is an absolute maximum oil flow rate when the engine is at maximum torque and the pump which is driven by the engine is at maximum displacement. The pressure of the oil which can be delivered by the pump is normally set by the load and ultimately by the strength of the piping. An engine which is rigidly coupled to a static load cannot deliver power because it cannot rotate. In small vehicles this mismatch is overcome with slipping clutches but the energy waste and high temperatures become more serious for heavy ones.
In contrast an accumulator can deliver oil at any rate while its pressure depends only on the history of in and out flows. If both pumps and motors can have intelligently-controlled variable-displacement then we can make every component work at its most comfortable operating condition.
We start with a stationary ferry and an accumulator charged to some fraction of its maximum pressure. The engine can immediately work at full power pumping oil into the accumulator. Passengers do not like sudden changes of acceleration so we command a small but rising fraction of wheel motor displacement. At the slow speeds there will be less oil used by the motors than is being supplied by the pump and so the accumulator pressure will rise. The engine can continue at its best operating point but the rising pressure means that we have to slowly reduce the displacement of the pump.
If we want to keep acceleration steady while the velocity increases, the displacement of all wheel motors must change by an amount which is affected by the changing accumulator pressure. The amount of oil being taken by the wheel motors will rise as a result of higher ferry velocity. Eventually the volume of oil used by wheel motors will exceed that delivered by the engine. The shortfall will be supplied by the accumulator, resulting in a fall in its pressure. This fall means that the accumulator can make less and less contribution. To keep the engine running at its optimum torque despite the falling pressure we increase the pump displacement. When all motors are at maximum displacement the acceleration has to fall.
In the middle of the trip the engine supplies just the power for wind resistance, tyre rolling drag and gradient with no flow in or out of the accumulator. With favourable winds or gradients the vehicle speed can be maintained by reducing the displacements of wheel motors.
Towards the end of the trip the engine power can be reduced to zero. The deceleration of the vehicle comes from the wheel motors acting as pumps to recharge the accumulator. Emergency braking can come from making wheel motors act as pumps to force oil through a pressure relief valve where it will be wasted as heat. The pressure limit will be set by the strength of pipes and motor bodies. Regulations may require additional braking with calliper disk brakes on each wheel. The limit is then set by the friction coefficient between tyre and road surface.
NAVIGATION
Road ferries could be driven by human pilots but if the safety and productivity are to be maximised we should design for entirely automated operation. The automatic steering of road ferries will be based on three sources of information. These are the position across the carriageway of the front and rear of the ferry and the distance it has travelled across the bridge. During the changeovers from conventional bridge traffic there will also be a need for information on the range and relative velocity of vehicles in front and behind. These data must be continuously collected with extreme reliability by a several parallel systems which must tolerate damage to the bridge or ferry structure, fog, rain, loss of power supplies, bridge vibration, impact with other vehicles, dirt, spilt cargo and oil. The systems must not impede normal function of conventional vehicles. They should also be sufficiently diverse that common mode failures will have a vanishingly low probability.
There are a number of possible solutions. Global positioning systems, even ones based on the differential carrier-phase technique, are not yet quite precise enough and can suffer if satellites are in unfavourable positions. However radio methods can make use of phase comparisons and pulse time differences of transmissions from sources on the bridge and local land points. Multiple receivers at all four corners and at points along the edges and even central points of the ferry will provide information with a high degree of redundancy with a high probability of a sufficient number surviving collisions.