Bearing failure is causing massive disruption on the M6 Thelwall viaduct. Alan Sparks looks at the latest threat to the nation's transport infrastructure.
STRUCTURAL BEARING manufacturers are warning that design codes for high strength bridge bearings are not strong enough to guarantee quality. When these bridge bearings fail, vital trade links can choke up and the cost to the economy can be immense, as the M6 Thelwall viaduct maintenance works currently demonstrates (News last week).
All the viaduct's roller bearings are to be replaced early next year, even though they are only six years old. Investigations continue as to whether codes, installation, or metallurgical quality was to blame for the apparent fatigue failure of the hardened stainless steel units.
'Installing bearings that meet bridge design code BS5400 does not guarantee a quality product, ' says Matthew Dronfield, director of bearing manufacturer Ekspan.
In theory, bearings should last for the whole of a bridge's design life. 'But in reality replacement is generally expected after around 30 years, ' says FaberMaunsell bridges and special structures director Jolyon Gill. When bearing replacement is needed, it is often fraught with difficulty, requiring complicated jacking procedures, as with Thelwall.
Roller bearings are more extensively used across the rail network, where the type has been installed for many decades.
But loadings here have not increased as dramatically as on the highways network.
'Savings made on cheaper bearing selection form a false economy, ' says Dronfield. 'On one job I know of, an extra £500 spent on bearings 15 years ago would almost certainly have avoided a complex £250,000 replacement scheme.
The importance of long life bearings is often overlooked.
Bearings are the forgotten heroes of the UK's infrastructure.'
Dronfield warns that an even greater threat to a bearing's longevity is introduced during construction. 'We have come across bearings that have been installed for 30 years, and yet are still in their transit cases - so unable even to do their job.
'Local authority engineers need to ask themselves, do they really know what condition their bridge bearings are in, and what are the real costs faced by the economy should these fail?'
Highways Agency spokesman Peter Morgan said, 'Roller bearings, such as those used at Thelwall, are present in around 250 bridges across England's road network. But it is unlikely Thelwall could be symptomatic of a common problem as each bridge will have unique bearings - there is not a common offthe-shelf design.'
Owen Williams engineer Henry Ramsey explains, 'There is such a variety in bearings around the UK that BS5400 does not relate to many of the older bearings that we come across.'
Gill explains that, despite their popularity in bridges built over the past 30 years, 'because of inherent weaknesses, roller bearings are rarely used today in the UK for new large bridges'.
Older bridges often have mild steel roller bearings that are less likely to suffer fatigue failure than the hard brittle steels often used for modern rollers. If some contamination does occur, hardened surface materials can quickly generate a 'stress razor' - where a hinge or crack can take hold. Much like the cracking found in high strength rail, says Ramsey.
However, code writers are beginning to wake up to the issue of quality, which has been neglected in the past, says Dronfield. 'The new Eurocodes will have a consideration for life cycle analysis. Although this still falls short of guaranteeing quality, it is a positive move in the right direction.'
INFOPLUS www. ekspan. com