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Severn Bridge corrosion revealed

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SEVERN BRIDGE suspension cable corrosion is worse than that found on the older Forth Road Bridge and more serious than the Highways Agency expected, it emerged this week.

Initial findings from intrusive investigations on the cables show the mid span section has suffered high levels of corrosion, which is affecting 50% of the cross section.

The results confirmed the need to fit a £20M dehumidification system to dry out the cables and halt deterioration.

Highways Agency senior bridge engineer Martin Lynch told NCE: 'The dehumidification system will be installed on the main span cables where corrosion is worst this summer.' He added: 'It offers an opportunity to monitor the success of the system before installing it on the whole bridge next year.' The dehumidification system, also known as dry air injection, works by blowing warm dry air though the gaps between the 8,322, 5mm diameter strands that make up each cable (see diagram).

Water ingress through cracks in the zinc-based sealant is believed to be the main cause of corrosion.

Lynch said he was 'comfortable the bridge could carry normal traffic loading'.

But load restrictions on vehicles over 7.5t to a single lane in each direction, as enforced last September (NCE 21 September 2006), will remain in place until the full testing and analysis has been completed to accurately calculate the bridge's resulting loading capacity.

The bridge is currently operated by the Severn River Crossing Group comprising John Laing, Vinci, Barclays and Bank of America UK. The concessionaire will hand the bridge back to the Highways Agency between 2016 and 2020, depending on traffic fl w. Responsibility for major works rests with the Highways Agency.

Dehumidifi ation systems are still regarded as experimental.

Their use has been limited to structures in Japan and recently on older bridges in Denmark and Sweden (see table). Despite this, Lynch is confident the system will halt corrosion on the Severn Bridge.

In the US, bridges suffering this type of corrosion have had cables resealed and the voids filked with oil. However, this is not considered a long-term solution as the oil will eventually leak to expose the strands.

Lynch and his team is now trying to find a long-term solution for the Severn Bridge.

'It will be an immense challenge to strengthen the bridge or replace the cables for a justifiable cost and without causing disruption to traffic, ' said Lynch. 'Cable replacement is a possible option and the cost will be looked at.' The Highways Agency said that no major structural work will be carried out until Severn River Crossing hands the bridge back.

Testing time for Humber Bridge

An intrusive inspection will be carried out on the Humber Bridge suspension cables this summer after a scoping study conducted last year revealed similar defects to those found on the Severn and Forth bridges.

Bridge master and engineer Peter Hill said: 'It's highly likely the remediation work will be needed, no matter how well the bridge is constructed it will still corrode.' The inspection will involve exposing the wires and driving a wedge between them to expose any damage.

Hill will install acoustic monitoring equipment to listen for any wires snapped or damaged during the inspection. If damage is found, dehumidification systems will be fitted between 2008 and 2010.

The Tamar bridge is not expected to be affected by this corrosion problem as its 49mm diameter strands within the cables have a smaller vulnerable surface area and twist along the length reducing void space.

Dehumidified bridges

Location Length Year opened

Akashi-Kaikyo Bridge Japan 3911m 1998

Ohnaruto Bridge Japan 1629m 1985

Shimotsui-Seto Bridge Japan 1400m 1988

Kita Bisan-Seto Bridge Japan 1538m 1985

Minami Bisan-Seto Bridge Japan 1648 1988

Innoshima Bridge Japan 1339 1983

Hakata-Ohshima Bridge Japan 840m 1988

Kurushima-Kaikyo Bridge (first, Japan 4015m 1999

Hüga Kusten Bridge Sweden 1800m 1997

Aquitaine Bridge France 1767m 1967

Ny Lillebµltsbro Denmark 1700m 1970

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