Engineering Design Consultants

Forth Road Bridge Main Cable Dehumidification

M G Bennett and Associates Ltd designed the first suspension bridge cable inspection gantries, used on the Forth Road Bridge. With this experience they were then appointed to design a moving gantry for the main cable dehumidification project. This would be used to facilitate the sealing of the main cable in order for it to be dried out. Whilst employed by Bennetts, David Price worked on the detailed design of this gantry as well as the air distribution network and plant room support steelwork.

The mobile structure needed to include a 20m long working platform for 8 people to travel along the main cable up to the top of the main towers. A distribution network and fittings was also developed for piping dehumidified air throughout the bridge. In addition, designs were provided for the supporting steelwork for the dehumidification plant rooms mounted on the bridge.

The gantry needed to be secured solely to the main cable in potential wind gusts of 120mph. To ensure safety and stability of the gantry, several dozen combinations of loads and supports were analysed using STAAD structural analysis package.

The moving gantry for the Forth Road Bridge is the first suspension bridge gantry of its type and is featured in the IStructE Publication "The operation and maintenance of bridge access gantries and runways".

An identical project was carried out on the Severn Bridge, also using M G Bennett and Associates, and using a similar gantry design for which David was responsible.

For more information regarding our movable structure design services or to discuss your movable structure requirements, please telephone 0114 399 0400 or email us.

Key Facts
  • Location:
    Forth Road Bridge, Scotland
  • Client:
    Forth Estuary Transport Authority