There is more than one historic iron bridge in Shropshire's Ironbridge Gorge. Dave Parker reports from Coalport's complex refurbishment.
Four weeks ago the two halves of a 68m span temporary steel truss bridge met each other high above the River Severn. Just below the new crossing lies a much older structure, the 1818 Coalport Bridge, officially scheduled as an ancient monument.
Soon the old bridge will be enshrouded in a web of scaffolding and enclosures as main contractor Dew Construction gets to grips with a tricky strengthening operation. But the temporary bridge is the key to the whole operation, as Shropshire County Council principal bridge engineer John Williams explains.
'We have to reduce the loads on the original structure as much as possible while we bond on steel strengthening plates. All construction loads will be taken by the temporary bridge, which will also permit pedestrian access across the river for most of the summer.'
Shropshire and the Borough of Telford & Wrekin, the crossing's joint owners, had been concerned about its condition for some time (see box). Severe weight and height restrictions were never totally effective - larger vehicles sneaked through, and the bridge was often loaded with multiple vehicles while drivers admired the view.
Four years ago the two councils commissioned consultant Gifford to investigate the feasibility of sympathetic and effective strengthening that would increase the bridge's safe capacity to 3t travelling in convoy.
'No more than this was needed because the local roads are narrow and twisty and HGVs are a rarity, ' Williams adds.
Gifford created a finite element model of the bridge and tested it by running a calibrated 2t vehicle over the structure during a 24 hour closure. More than 200 strain gauges installed by specialist Sandberg recorded the results - as well as six weeks' monitoring of normal traffic.
'The final analysis showed serious weaknesses in the raking struts between the arch ribs and the deck beams at the south pier - and in the bending capacity of the main ribs at midspan, ' says Gifford supervisor Bernard Bonfiglio.
'It also showed that if we replaced the existing nonstructural concrete deck with a structural slab spanning between the vertical struts we wouldn't have to strengthen the longitudinal deck beams.'
One rather chilling feature of the existing structure is that the deck plates are held onto the longitudinal beams only by pairs of folding timber wedges.
Static friction between the timber and the cast iron is the only transverse restraint on the deck.
Everyone involved with the project would have preferred to use carbon fibre reinforced plastic wrapping to strengthen the cast iron members, Bonfiglio reports. 'English Heritage liked the low visual impact of carbon fibre reinforcement, while the contractors appreciated the ease of application.
'The sand cast ribs were far from symmetrical - quite badly warped in fact - so a wrap looked like being much more practical than any other option.'
Unfortunately it soon became obvious that bonding on carbon fibre during the warmer summer months would store up all sorts of problems winter contractions of the bridge. The only long term option was to use a reinforcing material with a similar coefficient of thermal expansion.
Steel was the only possibility.
'This means each of the 1m long plates used at midspan would have to be machined out of 20mm thick steel to match the distortions in the ribs, ' Williams points out. 'They will be attached by epoxy adhesive, which gives us reasonable tolerance, but the distortions are quite large.'
One of the first tasks to be performed once SGB has finished erecting the access scaffolding hanging from the temporary truss will be a detailed and accurate survey of each rib's centre section, so production of the reinforcing plates can begin. Mabey Support Systems completed the truss on Monday 29 June, after carefully manoeuvring each half into position.
'There were unusual obstructions on each bank, ' reports Dew Construction contracts manager Angus Ridge.
In practice this meant that the two 4m wide by 2.6m deep sections would be launched on diverging lines and would be 1m apart at midpoint. With long temporary backspans each carrying the 25t of kentledge that made balanced cantilever launching possible, final alignment and link-up was a ticklish operation.
The backspans are now being dismantled and access ramps set up. Hydrodemolition of the existing concrete deck is scheduled to start in August and a new lightweight concrete structural deck is due to be placed some time in September.