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Insight | Bridge strike blues

Bridge strike warning

At around five per day, bridge strikes are so common that they rarely make national news. When a lorry crashed into a pedestrian bridge over the M20 last year, the resulting media attention was an exception to the rule.

But despite the severity of the crash, which caused a partial collapse of the bridge and chaos on the motorway, collisions with road bridges are relatively rare. A staggering 90% of strikes affect rail bridges: the Office of Rail and Road found that in 2015-16, 1,753 bridges suffered collisions, costing Network Rail around £12.7M in compensation.

So why are rail bridges more likely to suffer? According to Aecom bridges and structures European market director John Longthorne, our Victorian bridges are partly to blame.

“If you look at the infrastructure in the UK from a bridge stock point of view, it’s quite aged,” says Longthorne. Many older bridges, designed well before high vehicles were common on our roads, do not meet modern headroom clearance standards of 5.3m.

Network Rail uses a traffic light system to determine the severity of a bridge strike, ranging from low-risk green bridges through to double amber, amber and red. If a red bridge is hit, trains are stopped and engineers are sent out to assess the damage straight away as the bridge is at high risk of lifting or sliding out of place.

New bridges are subject to rigorous testing to ensure impacts from vehicle collisions do not cause structural damage. But as replacing the country’s old, low bridge stock would be far too costly, engineers are tasked with mitigating rather than solving the problem.

As Network Rail bridge strike champion Mark Wheel explains, the engineer’s role is about improving the environment around the bridge, rather than making changes to the bridge itself.

“If there’s an existing bridge there at the moment, we’ll try to move any driver distractions, make sure that the signage is correct, and the bridge is highly visible,” he says.

For high-risk bridges, steel beams, known as collision protection beams, are in some cases attached to the side of the bridge to absorb the impact from a potential strike. But as Wheel explains, it is too expensive to roll out across the network.

“On frequently struck sites it’s something we consider…but they’re not the cheapest of things because they have to be quite substantial structures so that they don’t fail,” he says.

More technology-driven solutions are used too. Detector systems have been installed at some bridges to identify at-risk vehicles and warn drivers off using a set of LED lights.

Given that engineers have a limited role to play in preventing bridge strikes, Wheel argues that the responsibility falls on drivers.

“The fundamental thing for me is that the lorry drivers and operators need to be aware of the vehicle’s height,” he says, highlighting SatNav systems which warn drivers approaching low bridges.

In 2014 a government report for the Bridge Strike Prevention Group found that several footbridges and pipe gantries had been damaged by drivers’ carelessness over the height of their loads; the lorry which struck the M20 footbridge last year was carrying a digger.

Wheel is hoping to reduce bridge strikes through an awareness campaign starting in October.

“What I’m after trying to do is get a cultural change in the haulage industry, so that every time they set out they know exactly the height of their vehicle and they know which route they’re going,” he says.

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