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Step change in barrier design

CONCTRETE

After years of relative inactivity in concrete motorway safety barriers design, two have rolled off the drawing board. Paul Thompson reports

MOTORWAY barriers are designed to prevent 'crossover' accidents, where vehicles stray across the central reservation into the path of oncoming traffic.

Last week the Highways Agency approved a new concrete safety barrier that industry body Britpave claims will dramatically increase the safety of motorway users thanks to its ability to contain vehicles weighing up to 13 tonnes.

The new Step Barrier, developed in Holland, has been tested in accordance with the latest European standards EN 1317 to containment level H2, meaning it is capable of restraining runaway vehicles as large as buses, coaches and light trucks.

Currently existing steel and concrete central reservation barriers are only certified to level N2, which ensures the system is capable of restraining a car of 1.5 tonnes travelling at 80 km/h. Britpave claims this is insufficient for modern demands.

'The concrete step barrier provides greatly superior inherent containment levels, ' says Britpave director David Jones.'Standard steel and wire rope barriers can only offer containment up to N2 compared with the much higher H2 level provided by the step barrier.'

The Step Barrier is slipformed using reinforced concrete, and slows vehicles by relying on the friction between the vehicle and the barrier base.

The initial impact occurs between the step at the barrier base and the tyres and suspension system.Only heavy collisions will result in the passenger cage impacting the barrier.

The new barriers will also reduce the amount of maintenance required compared with steel barriers, and, as one length of Step Barrier provides protection to both carriageways, Britpave claims the system will reduce the overall installation costs.

An additional benefit of the Step Barrier is that it can be produced to incorporate roadside drainage, lighting column anchorage and sections for super-elevated carriageways.

An alternative barrier NOT TO BE outdone by the Dutch, a London-based civil and structural engineer, Iain Thomson, has drafted and patented a precast concrete system that he believes will help cut costs and save lives.

'I have been working on the new barrier for more than two years, but I first thought about the need for a better system when I read about vehicles rebounding from other barriers, ' he says.

Mr Thomson claims another shortfall of the traditional mass-concrete barriers is that larger vehicles can topple over if they hit the barrier.His system prevents this by clever profiling of a gravel trap, akin to those used to slow runaway Formula One cars, running alongside the barrier.This, coupled with the sloping side of the barrier face itself, directs the wheel of the vehicle down into the gravel and ensures there is sufficient force working against the wheel to stop the vehicle toppling.

'The key point is the gravel-filled trough; the angle on the barrier directs the vehicle into it, ' says Mr Thomson.'A normal barrier slows a vehicle through the friction between it and the barrier.This design slows the vehicle through the action of the gravel, which also helps reduce damage to the vehicle.'

Mr Thomson's design could be manufactured in precast concrete units to speed up installation times on site or built using slipformed reinforced concrete techniques.

Two sections would be able to site street lighting columns and provide a channel for cables to run the length of the system (see diagram).

'The basic barrier and theory behind it are very simple and it can serve ancillary purposes, such as providing the drainage for the highway, a base for lamp standards and a route for utility cables, ' he explains.