Serious corrosion threatened to reduce the life span of the Humber Bridge. Declan Lynch visits the bridge to see how it was averted.
The Humber Bridge across the Humber Estuary on the east coast of England has the longest suspended central span in Britain, and had the longest central span in the world when completed.
However, as with the Severn Bridge in Wales and the Forth Road Bridge in Scotland, engineers discovered significant corrosion in the two huge suspension cables spanning the bridge.
Since 2007 client Humber Bridge Board has been working with consultant Aecom and contractor C Spencer to develop and implement a dehumidification system in the cables to reduce and prevent corrosion.
The project finished a year ago and one year ahead of schedule, so recently NCE visited the bridge to see how well the dehumidification system is working.
Humber Bridge Board design and major projects manager John Cooper is pleased with the progress made.
“Oiling cables is a messy business. The dehumidification system is cleaner and more effective”
Aecom transportation technical director Mark Bulmer
“Previously you could only inspect the outside of the cable,” said Cooper. “So you never knew what the exact condition within the cable was.”
The Humber Bridge Board was prompted to further examine the cables on its bridge following the discovery of serious corrosion in the cables of the Severn and Forth road bridges in 2004.
In 2007 Aecom began a scoping exercise to decide what had to be done to the Humber bridge cables.
In 2009, inspections on suspension cables revealed that although there were very few broken wires, widespread corrosion, particularly at the low points of the cables, where water collects, had occurred.
“It was on the same deterioration path as the Forth bridge…albeit 17 years earlier in the bridge’s life,” says Aecom transportation technical director Mark Bulmer.
The cables support a 1,410m long main span, a 530m long side span at the southern end and a 280m long side span at the northern end. Each cable contains 14,948 galvanised steel wires 4.98mm in diameter, with the shorter north side span containing an additional 800 wires.
Voids take up up to 20% of the cross sectional diameter of the cables and water can get into these spaces during the construction process. While the Humber Bridge cables were continually monitored and painted, internal water was causing corrosion, which had not been anticipated when the bridge was designed. During the construction process engineers used red-lead paste and wrapping to seal the cables and at the time it was believed this would provide adequate waterproofing over the bridge’s lifespan.
“We used off the shelf equipment wherever possible”
Aecom transportation technical director Mark Bulmer
“We never envisaged installing this system,” says Cooper. “But when the results came back showing cable breaks we knew we must intervene.”
To prevent corrosion within the suspension cables the options are oiling the cables, or install a dehumidification system. This involves filling the cable casing with oil to fill the voids.
“Oiling cables is a messy process,” says Bulmer. “The dehumidification system is cleaner and more effective.”
The Humber Bridge Board appointed Aecom to design and specify the dehumidification system in June 2009, and contractor C Spencer was selected in the following December.
The system blows dry air into the cables via injection sleeves which are wrapped around the cable casings. The dry air absorbs moisture before it exits the sleeves via exhaust sleeves.
C Spencer installed seven injection sleeves on the steel cables and 10 exhaust sleeves using a gantry attached the suspension cables. Work started at the highest point of each cable and moved down to the lowest point, to stop water being trapped.
The sleeves are connected to four plant rooms within the bridge deck - two on the main span and one on each of the main spans - which blow air into the cables.
“We used off the shelf equipment wherever possible,” adds Bulmer.
Installing the dehumidification system was expected to take two years, but good weather and previous experience allowed C Spencer to achieve early completion of the work.
“Our experience from working on the Severn and Forth bridges really helped,” says C Spencer engineer Antony Goose. “Everything seemed to go our way.”
With the system in place, engineers began drying the cables by injecting air at pressures of up to 3,000Pa with a relative humidity of 10%. Relative humidity is the amount of water vapour in the air. Before the drying took place the relative humidity was at about 90%, a level where corrosion will take place.
It took about two months to reduce the relative humidity in the cables from about 90% to the targeted level of 40%.
At 40% relative humidity, there is no corrosion, explains Bulmer.
The dehumidification will remain in continuous operation, although engineers hope to reduce fan speeds to save energy.
In addition to the dehumidification, the bridge board let a contract to Physical Acoustics to install acoustic monitoring to monitor the cables.
The monitoring system detects the sound of the small cable strands snapping. This is an occasional occurrence, but the frequency is an indication of the extent of corrosion.
The dehumidification system cost £14M, and the acoustic monitoring was a further £1M.
“It will help preserve the life of the bridge,” adds Cooper. The 6.5M motorists using the bridge each year will doubtless agree.