Engineers are walking on eggshells to bring a major UK viaduct back from the brink. Mark Hansford reports.
To look at it, few people would realise that something major is happening to the UK's Tinsley viaduct. It is a rather unremarkable twin deck box girder bridge carrying the M1 and A631 past the Meadowhall retail park on the outskirts of Sheffield.
Fewer people still would know that work to strengthen the viaduct is being carried out with the most extreme caution, with every move liable to trigger rapid structural deterioration.
So what has prompted such drastic action?
Built in 1968 at a cost of US$9.4M, Tinsley faced its first bout of strengthening in the late 1970s when the Merrison Committee demanded the addition of inclined raking struts following the collapses in 1970 of the Milford Haven bridge in Wales and the Westgate bridge in Australia.
With the work completed in 1980, problems resurfaced in the late 1990s when the bridge was assessed for compliance with the EU directive requiring all structures supporting main roads to be capable of carrying 40t vehicles. It failed the assessment spectacularly - a 7.5t weight limit was imposed, and traffic was reduced to two lanes in each direction.
Consultant Owen Williams carried out the assessment. It was then retained to work up a strengthening solution - the only option with replacement today costing $314M in construction costs and $2.2bn in traffic delays and disruption during the construction period.
Two years of finite element analysis later and Owen Williams was ready with a detailed package of work. This included the installation of 2,500t of steel, 3,500t of reinforced concrete, 100km of welds, 53,000 bolts, 75,000 shear connectors, 155,000m 2of paint and 4km of parapets.
The innovative two stage contract was let to Nuttall in January 2002. Cleveland Bridge was brought in as steelwork subcontractor, and immediately recognised that success or failure hinged on the strengthening of 34 load-bearing diaphragms that transfer load from the crossboxes on the lower deck into the viaduct's 17 piers.
'Each diaphragm carries 3,000t, and the challenge was to create a steel block capable of transferring this load from the upper diaphragm into the rocker on the beam, ' explains Cleveland project manager Andy McGhee.
With a maximum allowable tolerance of the steel blocks of 0.25mm over 60% of the surface area and 0.75mm over the remaining 40%, traditional survey methods were never going to be enough.
Instead, a 3D laser scanner was used to give the position in space of the existing diaphragms and welds to an accuracy of 0.1mm. 'This information was then downloaded into drilling machines and we are achieving tolerances of less than 0.25mm over 100% of the area, ' says McGhee. 'You simply could not have done this 10 years ago.'
Once machined, the blocks, along with all equipment and workers, have to enter the box girders through holes around 1m by 0.6m wide in the sides.
In many cases this is simply too small, and the box girders must be strengthened locally with reinforced concrete slabs to allow the holes to be enlarged.
A further complication is the perilous state of the structure.
All work to the cross boxes must be carried out at night when the road is closed and traffic loads are removed. Six hour windows are provided five nights a week, Sunday to Thursday.
'But this is such a tight window you are always worried that you cannot finish before the road must reopen, ' says McGhee.
This fear was vividly realised last November, when the road had to be shut for 36 hours after cracks began propagating out of control in pier 17 at the end of one six-hour possession.
'By Monday morning it was clear we needed the 36 hour closure to get on top of it, ' says Owen Williams project manager John Evans. 'If we hadn't moved fast the bridge could have been closed for weeks, ' adds Evans.
Unplanned closure of the motorway was one of the risks the UK Highways Agency was keen to transfer to the contractor, so the move will prove costly; proving that engineers on Tinsley really are walking on eggshells.