Linkspans and ro-ro bridges

How does Linkspans and ro-ro bridges work, step by step?

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   Step 1: Define the operating envelope with the port operator

   Everything begins with what the linkspan has to do: the vessels it serves, their ramp geometry and freeboard range, the tidal range at the berth, the vehicle types and axle loads, the required loading rate and turnaround time, and the wind and wave conditions the operator expects to keep working in. The designer converts this into the movement envelope - how far the outer end has to travel, at what gradient limits vehicles can still be driven, and how quickly the bridge must adjust. Every later decision hangs off this envelope, so it is agreed and frozen before the structure is designed.

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   Step 2: Choose the type and fix the civil and mechanical interfaces

   The designer and the operator then choose between a pontoon-supported linkspan, which follows the tide passively, and a machinery-supported one, which is lifted and held. Pontoons are simple in principle but bring their own mooring, freeboard and maintenance issues; lifting machinery gives control but adds a substantial mechanical system. Once chosen, the interface schedule between the civil and mechanical packages is written: foundation loads, hinge and bearing details, embedded plates and pockets, service routes, control room provision and access for maintenance. This schedule is the project's most important document.

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   Step 3: Build the abutment and the marine foundations

   The shore abutment carries the hinge and takes the horizontal load of vehicles braking on the bridge, and it is usually a piled reinforced concrete structure built partly in the tidal zone. The support tower or pontoon anchor foundations are piled from marine plant in the ordinary way. Embedded items - hinge assemblies, holding-down bolt groups, ducts, drainage and earthing - are set with survey control and cast in, because their positional tolerance is set by the machine that will bolt to them and not by ordinary civil tolerances.

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   Step 4: Fabricate and trial-assemble the bridge and machinery

   The bridge deck steelwork and the mechanical assemblies are fabricated in a works, where welding, non-destructive testing and coating can be controlled. On most projects the machinery is trial-assembled and function-tested at the works before delivery, so that fit and operation problems are found where they are cheap to fix. Steelwork is coated to the marine specification with careful attention to the areas that will be inaccessible once erected. Delivery is planned as a marine transport operation with its own lift plan and sea fastenings.

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   Step 5: Erect the bridge and install the support system

   The bridge deck is delivered and lifted into place, landed on its hinge at the abutment and supported temporarily at its outer end while the permanent support is completed. Where a pontoon is used it is towed in, moored and connected; where lifting machinery is used the tower steelwork, cylinders or winches, ropes and sheaves are installed and aligned. Alignment tolerances for the mechanical elements are much tighter than for the structure around them, so the installation is surveyed and adjusted rather than simply bolted down.

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   Step 6: Install the controls, safety systems and traffic equipment

   Hydraulic power packs, control panels, position sensors, limit switches, interlocks and the operator control station are installed and wired, along with the traffic signals, barriers, edge protection, lighting and any vehicle detection. The safety logic - what prevents the bridge moving while vehicles are on it, what happens on power failure, how the bridge is operated manually in an emergency - is installed and then tested rather than assumed. Cable routes and terminations in a marine environment are detailed for salt and water ingress from the outset.

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   Step 7: Commission through the full movement range and against real vessels

   Commissioning runs the linkspan through its whole movement envelope, empty and then loaded, checking gradients, deflections, speeds, hydraulic pressures, alignment and the behaviour of every interlock and emergency function. Loaded trials with representative vehicle weights confirm the structural and mechanical behaviour under traffic. On most projects the final proving is done against an actual vessel at several states of tide, with the operator's crew involved, because that is the only test that resembles service.

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   Step 8: Train the operator and hand over the maintenance regime

   Handover of a linkspan is a machine handover. The operator's staff are trained in normal operation, in the emergency and manual procedures and in the daily checks. The maintenance regime is issued with intervals, lubrication schedules, wear limits, rope and hydraulic inspection requirements and the statutory examination arrangements for the lifting equipment. Spares and special tools are handed over with the operation and maintenance manuals, the as-built drawings and the full commissioning record.