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Sliding scale

The UK Highways Agency claims it has a good handle on the cost of replacing all Thelwall viaduct's 136 roller bearings - an estimated $47M. But they will require onerous and costly maintenance.

'The replacement spherical slider bearings will require a lot more maintenance than rollers, which in theory should not need any. This is because over time the sliders' coefficient of friction will increase. And for this job we need that to be as low as possible to limit any lateral force directed into the piers, ' said Highways Agency principal structural engineer Tommy Dobson.

Should the forces increase, deck, pier and foundation strengthening may be required - and in a worst case scenario even further bearing replacements. The impact of the new bearings on the structure is now the subject of close scrutiny by the Highways Agency's managing, Atkins.

'A long term analysis of how the structure will respond is under way. This will decide whether pier and foundation strengthening will be required, ' confirmed Atkins project manager Lesley Waud.

Ideally analysis of the viaduct's structural response would be carried out before any new bearings are installed, but the urgency of the replacement gives engineers no time. Should any of the existing roller bearings fail completely, torsional forces transferred into the deck could see bracing or shear connector failure. Traffic has been reduced to single lane to reduce the likelihood of this happening.

The replacement spherical slider bearings consist of a low friction PTFE sheet sandwiched between thick steel plates, above which a chrome plated spherical segment sits in a PTFE lined, chrome plated housing. This arrangement allows horizontal and also a rocking movement, allowing the same degrees of freedom as the original rollers.

But sliders typically have a friction value of 3% to 4%, compared to just 0.2% for rollers.

The extra friction effectively increases lateral forces imposed on the piers and foundations by thermally induced movements in the deck. It also results in extra compression forces building up in the deck.

INFOPLUS Access analysis of how the bearings will be replaced at www. nceplus. co. uk/magazine

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