New standard unlocks potential of high strength steels to improve the safety of road restraint systems and lighting poles
Until 2011, the characteristics of road restraint systems in Europe were mainly defined through national regulations and standards. Many of these standards specified the design and materials that were to be used. Since the introduction of a new standard for road safety barriers (EN 1317) in January 2011, manufacturers are able to utilise new materials – including high strength steels – in their designs.
Like the EN 12767 standard for road infrastructure such as lighting poles (introduced in 2007), the EN 1317 standard is entirely performance based. This means designers are free to choose the materials – as long as these pass the tests defined in the relevant standards. For steel, minimum yield stress is usually set at 235 MPa.
25% reduction in weight per metre
Manufacturers of steel road safety systems have welcomed the innovative aspects of the standards. The rules enable them to replace structural steels such as S235JR with high strength steels (HSS) which are lighter per metre and better able to absorb crash energy. “Mieres Tubos prides itself on producing high-quality products. For that reason we have embraced the use of micro-alloyed HSS as a technical solution for our family of safe, competitive and sustainable safety barriers,” says Cristina Rodríguez, R&D Manager at Mieres Tubos – a major safety barrier manufacturer based in Spain.
The significant reduction in the overall weight of the pole or barrier per metre (up to 25% compared to those made with structural grades) reduces greenhouse gas emissions as less steel is required and more finished products can be transported to the installation site in one trip. Compared to other materials such as concrete, steel offers the best compromise between energy absorption and vehicle containment in road safety applications.
HSS properties easier to control
The mechanical properties of HSS grades are easier to control than those of structural grades such as S235JR. Quality is usually higher as most HSS grades are produced in Europe where controls are tighter. “Through several studies we have verified that the more controlled mechanical properties of these steels provide an optimal solution in terms of safety,” explains Cristina Rodríguez. “As a result, our barriers exhibit the same behaviour in an accident as they do in a crash test.”
One study of 200 coils carried out by Mieres Tubos found that the yield limit of S235JR can vary by up to 190 MPa. This means that an S235JR steel complying with the requirements of the EN 10025 standard for hot rolled structural steels may have up to 415 MPa of yield stress. This is 75% higher than the 235 MPa minimum specified in EN 1317.
For HSS, the variation was around 80 MPa – a significant improvement if the goal is to produce a safety barrier or lighting pole with consistent performances between test and production.
Cost-effective and long lasting
High performance coatings such as ArcelorMittal’s Magnelis® are also being utilised in conjunction with HSS. Composed of zinc with 3.5% aluminium and 3% magnesium, the Magnelis® coating lasts much longer than traditional hot dip galvanisation – the standard corrosion-protection mechanism for safety barriers and lighting poles.
When it comes to price, HSS safety barriers and lighting poles are more cost-effective than other materials. Their simple profile limits manufacturing operations, minimising production costs. Used in lower thicknesses, HSS guardrails require much less steel than structural steels thanks to their higher resistance.
The simplicity of HSS safety barriers means the same HSS post and beam can be used to create road restraint systems with different containment capacities (from level N2 to H2 for example). This enables manufacturers to maintain good price competitively. Barriers designed with HSS also require fewer components than those made with structural steels, further increasing the economic sustainability of HSS solutions.
However, the economic and environmental benefits of HSS safety barriers and lighting poles are far outweighed by their effectiveness at saving lives. Properly designed HSS barriers and poles absorb the crash energy of a vehicle and wrap around it to reduce momentum. There is less chance the vehicle will return to the road, injuring other motorists or the vehicle’s occupants. When combined with approved motorcyclist protection systems, even the most vulnerable road users are protected.