Engineering longer lives for bridges

Friday, 9 January, 2015
Bridges are an integral part of today's road and rail transport network, but maintaining them puts significant strain on public finances. The EU-funded project Long Life Bridges has found a way to keep them safe at lower cost. It has also built a prototype of a device designed to extend the lifetime of bridge cables.
The railway bridge over the river Ljungan at Alby, Sweden.

While investing in the safety of our road and rail bridges is important, highly expensive repairs, or even replacement, may not always be necessary. The construction may have deteriorated, but the damage might be limited to a non-critical part of the bridge, making it perfectly safe to keep it in service without extensive works.

Cutting back on maintenance requires a better understanding of the behaviour of bridges when subjected to real-life dynamic conditions – taking into account design, traffic load and maintenance history, for example.

The EU-funded project Long Life Bridges is developing new models to calculate the true safety of bridges in a more accurate way and aims to help public authorities evaluate whether to maintain, repair or rebuild a specific bridge.

The models would remove the need for traditionally highly conservative risk assessments, rendering certain investments unnecessary in the short term.

“Bridges will last longer, and therefore roads will be cheaper,” says Eugene O'Brien, who coordinates the project on behalf of the project’s leader, Irish civil engineering consultancy Roughan & O’Donovan (ROD). “It will cost the taxpayer less to develop a good transport network. And the transport network is one of the fundamental building blocks of the economy.”

Rich exchange between industry and academia

In their quest, the project team is putting particular emphasis on exchanges between industry and academia. Thanks to the project, academic partners from Aalborg University in Denmark and the Swedish Royal Institute of Technology are getting exposure to the commercial world and practising real-life engineering. Meanwhile ROD and French engineering company PHIMECA have the opportunity to work with some of the leading bridge researchers in Europe.

As part of this exchange, ROD was able to send employees to Denmark and Sweden. ROD’s senior bridge engineer Alisa Hayrapetova, for instance, spent 12 months in Denmark, working on load models for long-span cable stayed bridges. Looking at patterns of driver behaviour and traffic congestion on a bridge, she found that the most critical load combination does not occur when the road is fully jammed. More critical is when the traffic is still moving, but constantly stopping and starting.

The Long Life Bridges project partners are now using such studies to develop new models that give a much more accurate calculation of the total weight a bridge could reasonably be expected to experience in its lifetime. This, in turn, makes it possible to assess true safety more accurately.

Dampening vibrations

The project partners have also developed an innovative way to reduce potentially harmful vibrations in supporting cables, caused – for example – by a train passing over the bridge.

Normally, to prevent such vibrations a device known as a ‘tuned mass damper’ is mounted in a bridge to stabilise it. This usually comprises a plumb weight, springs or fluid tuned to one specific frequency. When a train passes by, the damper reduces the structure’s dynamic response and, therefore, mechanical vibration.

But with the traditional tuned mass damper, the bridge then continues to vibrate at the same frequency when there is no train, potentially having a negative impact on structural integrity. The Life Long Bridges innovative solution is a damper that can be tuned to two frequencies.

“It is a very simple idea, really,” explains O'Brien. “One will disrupt the vibrations when the train is present, and one when it is not present. This is a prototype, and it works beautifully well. It kills nearly all the vibrations, and it will hugely extend the safe working life of the cables.” The same principle could also be applied cable bridges carrying road traffic, he adds.

The project, which ends in August 2015, continues the work to refine its assessment models.

Long Life Bridges
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