Engineering marvel's makeover
Publication: Contract Journal, 17 September 2008
Journalist: Paul Thompson
Factfile
Project: Clifton Suspension Bridge Maintenance Programme
Value: Overall £1m; Drainage and surfacing £675,000
Client: The Clifton Suspension Bridge Trust
Engineer: Flint & Neil Partnership
Main contractor: Dean & Dyball Civil Engineering
One of the world's most famous bridges is being spruced up. Paul Thompson reports from Bristol, where local landmark the Clifton Suspension Bridge, is being resurfaced.
For almost 150 years, people have stopped and stared at the Clifton Suspension Bridge. Spanning across the gorge worn by the River Avon as it passes through Bristol, the spectacular structure became an engineering landmark almost as soon as it was completed in 1864.
Recognised around the globe as a symbol of engineering excellence, its image greets passengers as they arrive at Bristol International Airport a few miles south of the city. Even Concorde dipped its wings at the Grade I listed structure when it made its final flight from its birthplace in 2003.
But while it may be one of the great masterpieces of the engineering world, even the Clifton Suspension Bridge needs a facelift every now and then.
Local contractor Dean & Dyball is spearheading the first phase of a £1m maintenance programme in a £675,000 contract for client the Clifton Suspension Bridge Trust. It will resurface the existing carriageway across the length of the bridge and install a new drainage system to help wick water away from its main longitudinal beams.
“The existing surface has rutted quite badly over the years and rainwater tends to build up in these ruts. As cars run through these pools the water is sprayed onto the main girders; it then runs down to the joints below the deck causing corrosion,” says Dean & Dyball project manager Damien Fensome.
Wrought iron beams
The main 840mm-deep wrought iron beams span the full 214m across the Avon Gorge as the bridge stretches from the Clifton side on the east, to the western abutment on the Leigh Woods side. Wrought iron cross girders span between each of the main beams, but the deck structure itself is largely timber.
Huge 5m-long, 250mm-wide and 1125mm-deep Baltic Pine timber baulks run longitudinally along the bridge and are topped with 250mm-wide, 50mm-thick transverse planking which runs the width of the carriageway. This wooden decking is then covered with a nominal 35mm-thick mastic asphalt wearing course – although Fensome admits this thickness can vary markedly in places.
The full scope of Dean & Dyball’s work includes replacing the articulation span at each end of the bridge, as well as the first two bays of the bridge itself. This has forced some overnight road closures while the site investigation work is carried out.
Whole chunks of bridge deck have been carefully cut through and lifted out to allow a full inspection by Fensome and the team of specialists from the client’s structural engineer Flint & Neil Partnership.
“Until those inspections have been carried out, you are not entirely sure what it is you are dealing with,” says Fensome.
In all likelihood though, the replacement spans will be built offsite then disassembled and rebuilt in-situ during a weekend closure, of which five are anticipated.
“It will probably take two weekends for the articulation spans then another two weekends to strip the surfacing off. We will need another weekend closure to lay the new surface too,” he says.
But working on a listed structure of such importance is not as easy as it sounds. There are strict loading limits that cannot be exceeded and Fensome will have to juggle his plant and staff if these are not to be breached.
“The weight restrictions allow us to use plant weighing up to a maximum of 4t, as long as the overall gross weight limit is not exceeded. We are also able to use one piece of plant weighing as much as 8t, but cannot have any other loading on the bridge at all when we are using it,” he explains.
Large planer
That 8t extension does mean, however, that the surfacing gang will be able to use a large planer capable of trimming a 500mm-wide strip of surfacing rather than a 4t machine, which is limited to a 350mm strip. It is a welcome boost to efficiency, but the loading restraints are forcing Fensom and his site team down another route in a bit to clear the deck.
Because the 8t machine is being used to scrape-off the top 25mm of wearing course, no other plant can be used on the bridge at the same time. Loose planings will be left on the deck ready to be collected after the planer has completed the work.
Symmetrical loading
But even then, direct loading to the bridge must remain symmetrical. Loads applied on one side of the bridge must be replicated by similar loading patterns on the other. As the mastic asphalt surfacing is a uniformly distributed load with an estimated weight of 100t, the planings cannot even be swept up into a large stockpile – they must be removed from where they lie.
“It makes things a bit awkward,” says Fensome. “If I lift 20m of planings on one side, I have to mirror that on the other.”
He estimates that trimming the first 25mm from the asphalt layer should be completed by lunchtime on the Saturday of a weekend possession. The remaining 10mm will be chipped away by hand to reduce the risk of the planer cutting through the mastic asphalt and damaging the timber deck structure.
A layer of felt and galvanised wire mesh will then be attached to the timber deck. These act as a breather membrane and reinforcement to the bottom of the new surfacing, which will be applied in two layers.
“The first 10mm layer acts as waterproofing and the second 25mm layer will bring the surface up to level. It is hand applied and has to be carried out over one closure. The waterproofing layer cannot be trafficked and mastic asphalt has to be laid perfectly dry if its integrity is not to be compromised,” Fensome adds. “We will certainly be watching the weather forecast closely.”
Work on the scheme is expected to be complete by the end of September, when drivers of the 12,000 vehicles that use the bridge each day will be able to enjoy a smoother ride across the river.
Drainage from the deck down
As part of the project, the Dean & Dyball team is replacing the antiquated drainage system along the length of the deck.
A glass fibre reinforced polymer guttering system will be glued onto the bridge’s main girder draining surface water through the deck and straight into the river 76m below.
This will help prevent water running down the road clogging the bridge bearings with debris as it flows through them. This debris can hold water, rotting the timber and corroding the steel, weakening the bridge.
All photography courtesy of Paul Thompson.