The 50 year old Severn Bridge has recently undergone a third round of investigative surgery on its two main suspension cables after concerns that they had corroded surfaced in the early part of the last decade.
While the results are only preliminary, the team of specialists from Aecom is seeing positive signs that extensive works to protect the steel cables from corrosion is paying off.
In 2004, investigations on six major parallel-strand suspension bridges in the United States identified high levels of corrosion in the steel which made up the main cables. The problem lay in their structure.
Modern bridges generally use locked strand coils where the outer rings of the cables are made up of a series of Z cross-section wires which fit tightly into one another and form a barrier to water. However, parallel strand cables simply comprise wires with circular cross-sections bundled together. This leaves voids between the wires, making them vulnerable to water ingress and consequently corrosion.
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After the corrosion problem discovered in the US, the English Department for Transport (DfT) started to inspect its own three parallel strand bridges, the Forth Road Bridge, the Severn Bridge and the Humber bridge. Initial inspections were carried out on the Forth Road Bridge near Edinburgh in 2004, followed by inspections of the Severn Bridge in 2006, and the younger Humber Bridge in 2009.
The first inspection of the 1.6km long Severn Bridge confirmed all fears. It was found that a number of the parallel strands within the main cable had corroded to an unacceptable level. In an attempt to control the situation, weight limits were placed on the bridge.
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In response, Highways England’s predecessor the Highways Agency installed a new dehumidification system to dry out the cables and arrest any further damage to the steel in 2008. To create an airtight seal around the 511mm diameter cable, it wrapped it in a polyurethane wrap before blowing air into five key points along its length.
The relative humidity of the air leaving the polyeurethane wrap was compared to that of the injected air and, over a period of time this balanced, giving the team confidence that the moisture within the cable had dried up.
“If you get below 40% relative humidity then you arrest corrosion. So the object is to ensure that the atmosphere inside the cable is less than this,” says Highways England structures team leader Mark Maynard. “At the start, the relative humidity of the air coming out was very high compared to the air going in.
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“Since then the numbers have been almost identical, which gives us confidence that the cable is dry as it’s not picking up any more moisture in between.”
To further the effectiveness of the new system, the dry air was also passed through a vapour phase inhibitor.
“The dry air is pumped through what could only be described as a tea bag-like structure which is called a vapour phase inhibitor,” says Maynard. “The bag creates a chemical reaction which produces the ions from that flow through and bond themselves to the cable and inhibit corrosion. It’s a belt and braces approach.”
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In 2010, a follow up inspection found that the cables had stabilised, giving Highways England assurance that the dehumidification system was preventing further corrosion of the steel.
But the problem was not over. The corrosion which had already occurred could not be ignored and so a third inspection was carried out by contractor American Bridge and consultant Aecom over the summer this year.
The inspections themselves were akin to open heart surgery. Each of the cables was first stripped of its white protective polyethylene sheath, the original red lead paint was then removed to expose the 8,322 individual 5mm diameter parallel high-tensile steel strands which make up the overall main cable.
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When the individual wires were exposed, wedges were hammered into the centre of the cable at eight points around the circumference. This allowed the team to separate out the strands and inspect their surface for damage and corrosion.
“They drive wedges into the cable to inspect a face in the middle of the cable,” says Maynard. “They do this by hammering a wedge in at the right point. From the top it’s relatively easy, but driving them in from the bottom requires significant manpower.”
From the wedge inspections, cross-sections of the cables were produced, modelling the condition of the cable and the number of broken wires. By carrying out a statistical analysis of the gathered data, the condition and therefore the residual strength of a whole cable could then be calculated.
While the wedges opened up the cable the team was able to repair broken or corroded strands. Where a broken wires were identified, a section 6m long was cut out and replaced by feeding both ends into turnbuckles. These were then tensioned to return the wire to its original state.
After the inspection was carried out, the strands were re-compacted and wound tightly together with a 3.78mm diameter wire wrap, before the white polyethylene wrap was reinstated.
The inspections were largely carried out in the central sections of the suspended cable as this is where the most moisture was expected to have collected and consequently where the worst corrosion was anticipated. However, higher level cables were also checked to ensure that this was the case.
Despite the fact that the majority of the work was carried out on the lower areas of the cables, the work was complicated as all access to the cables was restricted to the public footpath and cycleway on either side of the bridge deck.
Unlike the Forth Road Bridge whose deck is supported by a large truss, the Severn Bridge’s deck structure is more streamlined. Its main deck is a enclosed steel box girder with a lightweight cantilevered foot and cycle path on either side. The cantilever limited the weight of the scaffiolding the team could use to access the work sites.
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“The scaffold which we have used for the works might seem simple to build but it’s been complicated because it has to withstand the wind loading on the bridge,” says Maynard. “It therefore has to have kentledge on the bridge to hold it down, but that puts the load on a thin cantilever.
“So we can’t put the maximum required kentledge to hold it down in all wind conditions because of the effects it would have on the permanent structure.”
Because of this, the work was subject to wind restrictions, which if exceeded meant the scaffolding had to be partially dismantled causing delays in proceeding with the inspection.
To mitigate the risk of a vehicle crashing into the scaffolding, the team closed the outer lanes of the deck to traffic and installed a specialist safety barrier. But even this was not a simple task.
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“Because you’ve got scaffold on the structure, you’ve got to protect it from the traffic,” says Maynard. “But because of the sensitive nature of the structure and the limited weight capacity of the structure, we had to go for a steel barrier as a concrete one was too heavy.
“Normally steel barriers are bolted down to the ground, but in this case that wasn’t possible [to avoid damaging the deck] ,so as far as we know this was the first example of a Varioguard barrier using a small anchorage, 3t concrete anchors at the ends of it.”
Inspecting the cables higher up was also a challenge. Each area has had to be inspected from a 30m long, high level scaffolding system which was hoisted up to wrap around the cable.
The sensors listen all the time and record when they get the right signal, which they have correlated to a wire break.
Highways England structures team leader Mark Maynard
“The high level scaffold system was fabricated in two halves, erected together on either side of the hangers and then hoisted into position up to 200m above the bridge deck,” says Maynard.
In conjunction with the cable inspection work, the team also carried out a detailed analysis of the actual loading on the bridge.
By embedding sensors in the road surface, the team has been able to compare the actual loads on the bridge to the revised capacity of the cables, based on the corrosion of the cables. This helps the team to assess how much traffic can safely pass over the structure.
In early 2008 the team also installed an acoustic monitoring system which is sensitive enough to listen out for wire breaks which might occur within the cables. Sensors were placed on every second hanger and then calibrated to pick up the sound of a wire break.
“The sensors listen all the time and record when they get the right signal, which they have correlated to a wire break,” explains Maynard. “When they have a potential wire break, a trained person looks at the signal and will confirm it.
“It’s another mechanism by which you can monitor the health of your structure.”
To calibrate the system and check the sensors are still working, they are tested on a regular basis by breaking a pencil lead next to them as it produces the same signal as a wire break.
Future inspections will be planned based on the results of most recent tests, but for now, the Severn Bridge has been given a new lease of life, hopefully for decades to come.