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Kessock Gantry, Category 3 Check

The Kessock Bridge across the Beauly Firth in the Scottish Highlands is a 1,056m cable-stayed bridge with a 240m main span. The bridge was completed in 1982 and equipped with a moving access gantry suspended beneath the deck. The bridge operator, BEAR Scotland, identified that the access gantry was reaching the end of its safe working life and commissioned the design for a replacement gantry. Recent developments in gantry standards meant that the old design could not be replicated, necessitating a bespoke new structure.

Key facts

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Location

Kessock Bridge, Inverness, Scotland

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Client

BEAR Scotland

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In Partnership with

Riverside Automation

G J Fluid Power

Having extensive experience in both the design and verification of complex moving structures, Eadon Consulting were commissioned to conduct a full Category 3 Check on the design of the replacement mobile gantry.

Powered by an on-board generator, the proposed replacement gantry uses electrical drive motors for longitudinal and transverse travel. Hydraulic systems provide longitudinal levelling to accommodate the slope of the bridge and further hydraulic systems compensate for bridge camber and variations in deck width. Access under the deck, and up the sides of the main bridge beams, is achieved through a pair of bespoke hydraulic scissor lifts.

Eadon employed their customary holistic approach to validating the safety and serviceability of the design, conducting a comprehensive review of the through-life risks before zooming in on the technical detail. The Category 3 check included assessment of the control methodology and detailed checking of the electrical and hydraulic control systems. A comprehensive 3D clash check was also conducted to ensure safe movement of the gantry along the full length of the bridge.

Verification of the proposed design was conducted to BS EN 13001 and the IStructE “Purple Book” (The Operation and Maintenance of Bridge Access Gantries). The man-riding scissor lifts were also verified against the requirements of BS EN 280. A detailed ANSYS Finite Element Model was used to confirm stresses and check fatigue loading to the standards.