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Flawed design codes led to Thelwall bearing failure

BASIC FLAWS in design codes for roller bridge bearings are thought to be behind the failure of roller bearings on the M6 Thelwall Viaduct, the Highways Agency said this week.

Specially fabricated Italian bearings were installed in 1996 in a £27M renovation of the original 1963 structure by contractor Amec and consultant Pell Frischmann. These suffered apparent fatigue failure earlier this year and caused extensive traffic disruption on the M6 (NCE 25 July). The 1.4km long 36 span viaduct carries M6 traffic north and south as well as being the main Liverpool-Manchester link.

Main spans are made up from riveted steel plate girders braced by similar cross beams, with each pier head typically fitted with four roller bearings under the most heavily loaded areas of deck plus two sets of sliding bearings.

Roller bearings were chosen to minimise the transfer of lateral thermal and dynamic forces into the concrete piers, having an effective 'coefficient of friction' significantly lower than sliding bearings. The piers are said to be incapable of accepting significant lateral forces.

Investigations continue into whether the bearing failure was down to poor quality or slack design codes. 'Design codes for roller bearings only need consider a standard static test - with no fatigue analysis called for.

This needs to change because where high loads are concerned, roller bearings have something of a chequered history, ' said Highways Agency technical advisor, Derek Ives.

Failed bearings were due to be replaced by the end of November with a slide bearing, but this has now been delayed until next year. 'Jacking a riveted plate girder structure is very complicated. To do this we will need to greatly stiffen the cross girders.

Finding the right solution has been the major cause of delay, ' said Ives.

Because extra stress will be channelled into the piers, the slide bearing replacement will need constant monitoring and maintenance and is only seen as a temporary solution.

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