Forth Bridge, UK

SectorSector: Bridges

CostCost: £140,000,000

ClientClient: Network Rail

locationLocation: Scotland, UK

project DetailsProject Details

The historic Forth Rail Bridge spans the Firth of Forth north of Edinburgh. At 121 years old, it was the first large span bridge to be built using steel and broke all records for length and height. It is the second longest cantilever ever built and is a Grade A Listed structure. "Painting the Forth Bridge" is a colloquial expression for a never-ending task, coined on belief that repainting was required and commenced immediately upon completion of the previous repaint due to the sheer size and scale of the bridge. Overall, it is one of the undisputed wonders of the Victorian age. It spans a total length of 2,528.7 metres and is owned by Network Rail Infrastructure Limited.

 

The project, completed in December 2011, saw the culmination of 15-years of work. Network Rail and Pell Frischmann Consulting Engineers Ltd, were responsible for all the civil and structural engineering elements of the bridge's restoration. Starting in 1995, we carried out an integrity study and condition survey, hazard report and structural analysis of the primary bridge members following a HMRI review that the past maintenance had not been robust enough to prevent significant deterioration of structural members. This analysis included:

(i)                  The overall condition;

(ii)                Structural assessment of the bridge;

(iii)               An action plan for necessary refurbishment;

(iv)              Preparation of a maintenance plan.

(v)                Monitoring of bearing movements

 

Load Modelling

Pell Frischmann surveyed the members, using rope access where required, on a selective basis to identify the condition of the paintwork and level of corrosion. It was determined that this survey would be representative of the worst condition. Coupon plates (fabricated from replaced members of the original structure) were weighed and attached to the bridge, along the length and at different heights. These were removed at pre-determined intervals to be cleaned and re-weighed to monitor corrosion loss, this being undertaken over a 4yr period with selective checks on a 6 monthly cycle. This data was used to prioritise the proposed restoration works.

 

Three dimensional computer models were developed using the original drawings and data obtained from the survey and used to determine member stresses for dead, wind and real train loading cases. Our finite element analysis confirmed that the original design basis was sound, that all original members were adequate and was used to confirm the required repair works for corroded members. Automatic bearing lubrication was installed.

 

Re-Painting

The new maintenance programme we developed moved away from the previous “care and maintain” approach to providing a comprehensive solution for restoring the bridge. We have worked with Balfour Beatty's in their 10-year commission for the completion of the repainting works, the priority being to remove the existing paint system back to bare metal and replace with a modern, high-tech glass flake epoxy system, applied in a three-part coating method that has been tried and tested on north-sea oil platforms, but not before used on bridge infrastructure. Independent trials carried out by Network Rail verified that the top coat will last for at least 20 years - putting an end to the myth that painting the bridge is a never ending task.

 

After 16-years of engineering by Pell Frischmann for Network Rail and 10-years construction by Balfour Beatty, the bridge has been completely recoated - the first time in a single operation since its construction, requiring 4,500,000 working hours for member strengthening and re-painting, 240,000 litres of paint sprayed onto 255,000m2 surface area and hand applied stripe coat to 6,500,000 rivets and sharp edges. This project is one of the greatest achievements for civil engineering and heritage restoration.