COMPLEX STRENGTHENING work must be carried out on the Thelwall viaduct before eight defective steel roller bearings can be replaced, Highways Agency consultants said last week.
Steel trusses supporting the viaduct deck, which is up to 30m above ground, will have to be beefed up with stiffeners before they are jacked up so bearings can be removed.
'The trusses are up to 6m deep and getting steel of that size up there is not possible. Instead we have had to go for partial height stiffeners, ' said Leslie Waud, structural design co-ordinator at viaduct managing agent Atkins. 'All this makes the design much more complex.'
Traffic jams and repairs on the 36 span 1.32km long viaduct carrying the M6 near Warrington are expected to remain until Easter (NCE last week).
The deck rests on 314 bearings, half of which are rocker bearings. The rest are cylindrical steel rollers of varying size up to 340mm diameter and 465mm long.
In 1996, Amec refurbished the viaduct, converting the jointed deck into a monolithic structure carrying one carriageway of the M6 motorway. Southbound traffic travels on a parallel viaduct completed in 1995.
But the new arrangement imposed different loads on the older structure. Roller bearings were chosen to minimise frictional loadings while new bracing was used to stiffen supporting trusses.
This bracing will make truss jacking more difficult, meaning vertical stiffeners are needed to help the steelwork take the high lifting forces.
The varying geometry of the trusses and the original 1960s riveted plate steel sections are making the task more difficult.
Each vertical stiffener will have to be individually designed, adding to the complexity of the work.
Two 1,000t jacks each lifting 800t will be required for the mammoth lift at each bearing, due to start next month.
Three roller bearings on the same pier and five others on different piers randomly along the bridge have failed.
Two distinct failure modes have emerged. The three failed bearings, near the viaduct's longest 102m span, developed longitudinal cracks parallel to the bearings' axis. Elsewhere, cracks have been transverse, parallel to the ends.