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Bearing up to the strain

Bridges Components

Is the recent spate of bearing failures on UK bridges indicative of a widespread structural problem, or just bad luck? Alan Sparks investigates.

Bridges are emblematic for civil engineering.

Their designers strive variously to achieve longer spans, thinner decks, cheaper construction.

Clients may demand architectural statements and pride themselves on commissioning structures that push engineering boundaries.

But all too often it seems, even as designers are stretching their limits, the crucial links between deck and supports are neglected - left to subcontractors to detail and supply. Designs are not properly checked, orders are placed with the cheapest supplier, and the bearings are installed with no performance warranty.

Bearings are a necessity on many structures, acting as a release valve for the sometimes extreme forces channelled through their joints. Low friction supports can be used to compensate for movement caused by expansion and contraction of the bridge deck, reducing bending moments in the columns and enabling leaner, sleeker structural members. Bridges have become steadily lighter over recent decades, requiring increased dependence on reliable moving joints.

But how much do structural engineers really know about the components they are staking their structure's integrity on?

It is possible to avoid bearings altogether by building integrated structures in which piers deflect with expansion and contraction of the deck. These are far from simple and call for precise design and construction, but some engineers believe it is worth the effort as it enables them to avoid potential bearing pitfalls - loss of performance resulting from wear, clogging with debris, corrosion, fatigue and ultimately failure. The vast majority of engineers, though, continue to rely on the quality of supplied components.

It is common practice for structural engineers who design the heavy engineering of a bridge to calculate the reaction, rotation and thermal movement at each bearing location. This is fed into a bearing schedule, which is then sent off to the bearing manufacturer to detail. These are almost always selected on price, say sources in the industry.

However, saving a few hundred pounds at the time of construction is a false economy if it leads to earlier replacement. As recent UK experience on Thelwall Viaduct or Orwell Bridge show, for example, replacement of failed bearings can be complicated, disruptive and expensive.

John Lane, a bridge consultant at researcher TRL, says the price for durable bearings may not just be cost but higher friction: there are suggestions that the ultra hard steel used for low friction roller bearings may be more susceptible to cracking. However, Lane argues that the cost of beefing up the rest of the structure to compensate would be peanuts compared with an unplanned replacement. It is not just the direct cost of replacement that needs to be reckoned if a bearing fails but the knock-on work on the deck and supports needed if the new bearings result in higher than original stresses in the rest of the structure.

It is recommended bearings are given a general inspection every two years following installation and a thorough inspection once every six years. But under most contracts the supplier will be liable for defects for only a three year period. It is extremely rare for bearings to be checked by the construction team before or during installation, says one manufacturer. When he recommended to an engineer that he check the quality of the bearings supplied, over half were rejected.

Selecting on price is a dangerous game, as recent UK bearing failures highlight. 'Cheaper European products have flooded the market and you have to look at what quality are you really getting for that price, ' warns Lane.

As a client the Highways Agency is backing a whole life cost approach to procurement and is tasking its contractors to do the same. However, Lane questions whether the approach is applied to bearing selection: 'In theory, building with whole life cost in mind should result in greater reliability. But in reality this is easy to hide behind and I have been led to believe that contractors still save money by buying in cheaper products.'

On most jobs, bearing manufacturers will come back with wildly varying tender prices.

'This is generally down to the way the bearings are made, ' says Atkins bridge engineer Mike Chubb. 'Aluminium castings are mass produced and provide a cheap off the shelf solution that may satisfy many applications.

Although fully machined steel alternatives naturally cost more, the quality and reliability is far higher.' Aluminium may offer improved corrosion resistance, but the softer material has higher friction properties compared to steel and is appreciably less durable.

Meanwhile, guidance on bearing design given by cobwebbed bridge design code BS4500 Part 9 is considered by many as vague at best. It has not been updated for a generation. Its replacement with a new Eurocode cannot come too soon, says Highways Agency structures design management group team leader, Awtar Jandu. 'This new code will consider bearing design in much more detail than its predecessor, including materials and dimensional considerations.'

But the new code will not address durability as a design issue, Jandu adds. In offshore engineering, the total distance travelled by a moving bearing is determined and considered as part of the design process, but in the civils industry design tables do not even mention the wear rates or duty cycles that determine endurance.

When movement bearings are selected, a value is assumed for friction efficiency. PTFE is popularly used between moving surfaces as it is initially very efficient. But there is little knowledge about the performance of PTFE over time. One major problem lies in the lack of suitable data gathering technology, reveals Jandu. 'We have no way of accurately measuring performance insitu during a routine inspection.'

Chubb confirms: 'Many bearings have been in service for over 25 years but little is known about how they wear - we are in the dark.'

Monitoring these key bridge components is a vital task, but those sent to inspect such structures can be unqualified for the task and too inexperienced to fully understand the real issues the bridge may face. 'For slider bearings, a friction value of 0.2% may be assumed upon installation. Once in service, if the structure shows no signs of distress then it is common practice to assume the bearing is still performing at a satisfactory efficiency, ' says Jandu.

Although Jandu insists bearing failures are few and far between, Lane argues that this could be down to redundancy in the rest of the bridge structure resulting from conservative design. More recent bridges, built under design and build contracts on which costs have been pruned back, are unlikely to offer the same robustness.

Wear on PTFE surfaces is far from straightforward. Common spherical slider bearings consist of a 3D curved face housed in a reciprocal unit, usually with a PTFE sheet sandwiched between. This is mounted on a PTFE slider, enabling three dimensional rotation and single direction movement. But where friction increases in the spherical contact, a concentrated load is applied to the slider, in turn threatening its performance.

Manufacturers claim few bridge inspectors on the UK's highways have this level of understanding.

Manufacturers are probably the best equipped to inspect and maintain bearings, argues Jandu.

'On key structures such as the QEII Bridge across the Thames at Dartford the Highways Agency has a deal with the bearing manufacturers. Instead of recommending a minimum maintenance regime, they are also responsible for the bearings' maintenance, inspection and repair. This arrangement could be extended for all major structures.'

Bearing manufacturers should also be more actively involved in early procurement discussions and be required to deliver products with performance guarantees, Jandu adds.

In the past the Highways Agency has avoided demanding guarantees as recovering damages in the event of failure was deemed too complicated - the variety of possible causes of failure was considered too great. But with modern structural monitoring, modelling and analysis tools it should be easier in future to determine what events and behaviours have led to failure.

Much of the UK's highways infrastructure was built in the 1970s and many structural bearings are coming towards the end of their 30 year service life.

Although many will remain in good working condition beyond this, the spate of recent failures suggests major challenges lie ahead.

While today's bridge engineers are bound by Construction Design & Management Regulation 13 to design with consideration for a structure's complete life cycle, older designs never took much account of maintenance, inspection and repair regimes. Nor did they allow for easy replacement, with bearings dowelled into place, making them almost impossible to remove without major surgery.

On older structures, says Chubb, there was often scant regard for waterproofing, protection from surface runoff and bird droppings, which can clog surfaces and accelerate corrosion. Exposed and uncared for, more bearing failures are not just a probability but a certainty, he predicts.

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