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Out of joint

Three years after an expensive and disruptive refurbishment contractors are back at work on the Avonmouth Bridge. Mark Hansford explains why.

Regular travellers to the south west of England will not have forgotten the five year traffic nightmare that was the £125M Avonmouth Bridge strengthening project. So, just three years on, the reappearance of cones and a contraflow later this month is likely to cause dismay.

Unfortunately for motorists, Avonmouth Bridge is a high maintenance structure. The reason lies in its rare Demag expansion joint.

As the only longitudinal movement joint on the 1.4km long steel and concrete box structure, the Demag joint must absorb up to 850mm of expansion under serviceability limits and a massive 1.22m at ultimate limit state.

The principle of the joint is fairly simple - a tongue plate fixed to the main span deck overlaps a link plate, or train, which is tied back to the approach span via an anchor plate (see diagram).

This link plate sits on a slide track mounted on a curved wedge-shaped slide box.

The joint is not unique - the Highways Agency has similar joints on the M6 Tinsley Viaduct and the original Severn Crossing - and their main features are pretty common.

'But there are many differences in the detail, ' insists Highways Agency principal bridge engineer Martin Lynch. Not least of these is access.

'The joint is located in an unfavourable position for monitoring, ' says Lynch. He is not kidding. The joint is located on pier 10, 30m in the air between the River Avon and the Severn Beach railway line.

'Gantries cannot traverse over the joint, so you cannot access it from underneath. The box is only 500m deep, so you can't get between the slide rails to inspect in service, ' he adds.

This makes preventative maintenance hard to plan, a problem compounded by the fact that since the strengthening works and the addition of an extra lane in either direction, traffic flows have increased by about 30%. This has significantly increased loading and the accumulation of road dirt in the joint.

The Agency admits that maintenance records are poor, but they seem to have indicated a need for major refurbishment on a five yearly cycle - mainly driven by noise.

'The way it deteriorated in the past was that with the load and the dirt, the shoes on the link plates simply wore away and grooves were made in the slide rails. We had to take the slide rail out to repair the grooves and then send the link plate away for new shoes, ' he explains.

During the 1995-2001 strengthening works the Agency took the opportunity to try to improve the Demag joints, replacing the original hinge cum shoes with separate hinges and phosphor bronze Glycodur sliding bearings. Stainless steel rails also replaced the steel slide track.

Glycodur bearings have a copper-plated steel base and a 0.2mm to 0.4mm sintered tin bronze layer. The key characteristic is then a polyoxymethylene (POM) top layer, which is solidly joined to the sintered bronze. This surface layer is up to 0.4mm thick and features pockets for lubrication grease, improving the bearing's performance even when there are misalignments.

Unfortunately, while the harder stainless steel rails have made maintenance easier, the wear on the phosphor bronze bearings has been faster than expected. Once these wear down the hinges themselves start wearing down, leading to breakages.

'The performance of the modifications has been a bit disappointing and the wear got further than we would wish, ' says Lynch. 'One of the snags with the modifications is that it's difficult to predict how long we've got once the bearing starts to wear.'

'So we are going to have to beef things up a bit to make the time between maintenance longer, ' he adds.

Ahead of this month's work, the Agency experimented with little success in 2003 with thicker profile shoulders on the phosphor bronze wearing blocks and with stainless steel wearing blocks. The latter would work fine, but would rapidly wear out the stainless steel slide track.

So the Agency came up with a new plan. This month eight link plates covering lanes one and two - they get the biggest battering from HGVs - are being worked on.

As it is not possible to have a hinge and bearing shoe in the same position, two will be installed, one each side of the hinge.

The hinges themselves will be fixed with screw studs and welded. The shoes will be stainless steel, so to slow wear to the slide track a stainless steel hard facing will be applied to the five slide tracks that carry the four trains sitting under lane one.

It is applying this hard facing that takes time, and has demanded a five week closure of lanes one and two. Each rail must be lifted out, the hard facing applied, and then ground back to match the profile of the link plates.

All must be done in daytime because of the noise - 300 properties are in earshot.

'It's a belt and braces approach, ' says Lynch. And really the best the Agency can do.

'Doing a more thorough replacement would mean going down to one carriageway, and that is not an option with the diversion routes available, ' says Lynch.

The only trunk road diversion available for Birmingham to Exeter traffic is via the M40, A34 and A303 - a long detour.

'So we are in the predicament of making modifications rather than wholesale changes.'

But the Agency hopes that this will be the last time major modifications are called for.

Four of the eight link plates being worked on will be new, with the old ones taken out, refurbished and kept on permanent standby.

This will make future maintenance significantly faster.

The link plates on lanes one and two southbound and three and four northbound will all need work in the next five years, but with the spares on standby this can be done in an overnight closure. The rails will then need maintenance on a five year cycle, but the Agency aims to do this in a planned maintenance regime.

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