Brunel's Royal Albert Bridge is the only rail link between Cornwall and the rest of Britain. Refurbishing it is a complex engineering project.
Dubbed 'Brunel's masterpiece', the 151-year-old Royal Albert Bridge spans the River Tamar between Plymouth and Saltash, carrying around 30 trains a day. Now, under the most complex refurbishment plan in its history, the structure will be strengthened, restored and repainted over the next three years. Network Rail, which is responsible for the bridge, will soon be inviting tenders for the work.
'We have come to a stage now where we need to do a major overhaul of the bridge,' says Peter Haigh, structures management engineer for Network Rail. 'Six or seven years ago we replaced the ballasted timber deck with longitudinal timbers to address a problem that we had perceived previously which was a slight lateral sway on the bridge when 25-tonne axle load clay trains would go over. Now it is time to completely renovate the structure.'
'The project has actually been running for a couple of years with investigations into the paint system, analysis of the structure and feasibility studies,' says Mike Palmer, Network Rail's project manager for the refurbishment job.
The consultants employed to uncover the bridge's painting history found up to 30 coats in certain places. 'The bridge used to be painted on a cycle where they would paint one-sixth of the bridge every year, so in theory every six years it got a new coat of paint, explains George Lawlor, project manager for Aecom, Network Rail's consultants. 'We are now coming to the end of the detailed design part of the work and the next phase will be to go out to tender. We hope to have that all completed by the autumn.'
Part of the works will involve removing the lower diagonal braces, which were added in the 1970s. 'This was the result of analysis carried out at the time using perspex models loaded with weights,' says Lawlor. 'It was not until about 10 or 15 years ago that we had an actual idea of how the structure worked with modern computing techniques, because it is a highly redundant structure and the load paths are numerous, so trying to consider how the bridge worked when a load went across was too difficult for a long time.
'In the 1970s after analysis they added the lower diagonal braces. We have now discovered that some of the members are bent. On analysis you can see that they are not behaving in the way they were intended to, so they are fairly redundant in their current condition. We are taking those off and reproducing their tension with some strengthening work elsewhere, such as the hanger tops. These cracked every 20-25 years in some locations as a result of trains going over the bridge. We are strengthening those areas to address this cracking issue.'
There is virtually no welding on the bridge as welding wrought iron is plagued with difficulties. 'We have tried to weld it in the past with varying degrees of success,' explains Lawlor, 'so by and large we are bolting new metal wherever we need to introduce new material. We are using tension control bolts with domed heads, that will appear from one side to look like rivets. That's more of a heritage consideration than a mechanical consideration, although they have very good properties and they are actually fairly quick and easy to fit.'
The programme calls for up to 30 coats of paint to be removed from over 20,000 square metres - about three football pitches. The bridge will then be re-painted using a more effective three-coat system. The existing paints have quite a high lead content - up to 13 per cent in places - so the surface will be taken back to bare metal.
'The programme we have developed for the implementation shows that we think the actual works can be done in two years, although there is a lot of float in that,' states Palmer. 'However, we are hoping that when we get the tenders back, the contractors will be able to scale that down to 15-18 months.'
The bridge will be painted with a glass fleck epoxy system that was developed for the offshore industry. Network Rail has used this successfully on the Forth Rail Bridge. Goose grey has been chosen to be compatible with the colour of the bridge when it was listed Grade 1 in 1952 by English Heritage.
'The paint system will have a 25 year life and hopefully we will have a bridge that is maintained and painted to give us a reliable railway crossing of the Tamar,' concludes Haigh.