| Brifen's advantage |
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| Written by Paul Hansen | |
Brifen's advantageWith the growing number of wire rope fences available in the market place (Brifen only has three different wire rope fences to suit the various standards used in Australia), there is a trend in some states for the main road departments to push responsibility for selection and certification of wire rope safety barrier product down-stream. This raises the question what to look for when assessing the performance of a wire rope safety barrier.In the past when there was only one Standard and only a couple of products, the various state main road authorities generally published a table or a list of suitable variations of the products, and Contractors adopted an attitude that if the product was on the list or in the table, then it was approved. The trend now is towards having the contractor verify the manufacturer’s claims about their product, and having the contractor obtain certified documents to prove the claims. The contractor now has to know the product, the product performance with respect to their site, and the manufacturer. Further the manufacturer now has to be involved and has to provide detailed advice. The manufacturer’s advice needed is not just on the fence product supply, but the material properties, the installation procedures, the maintenance procedures, the OHS issues, the design for various site conditions and various traffic conditions. Some state main roads departments are talking about having the manufacturer sign off on the final product in place. However, the primary problem is selection of barrier to meet the Specification. Selection of barrier generally is about performance. It is not uncommon these days that the client wants a barrier designed that will give a maximum two metre deflection when impacted with a 2000kg high centre of gravity type vehicle at 25° angle of impact. This is where a lot of wire rope barrier systems run into trouble, proving that level of performance. The problem being that most barrier systems test over short lengths of fence of around 60m to 100m length. 
In 2007 the FHWA (the US Federal Highway Administration) noted in an approval letter for another wire rope fence system that “Although the barrier performed well under ideal test impact conditions . . . the dynamic deflection of the barrier is likely to . . . when distances between anchorages exceed the 100 m test length”. Simply put, wire rope fences increase in deflection as the fence length increases. Indeed the deflection of wire rope fences using only straight parallel ropes (that is, wire rope fences with no inter-woven ropes) is very much related to the length of the fence. This is a fact long recognised in the UK where the Highways Agency require manufacturers to test fences over both 100m and 500m length for approval in the UK. However, wire rope fences with 2 or more ropes inter-woven between posts have a substantial mechanical advantage due to the friction of the weaving ropes acting on the posts. This is a two-fold advantage since energy (from the impacting vehicle) is dissipated both in having to stretch effectively shorter lengths of rope, and in deforming the posts. So how does this mechanical advantage influence deflection performance? Again simply put, the energy of the impact is confined to a short length of fence (typically of about 60-70 posts either side of the impact point). Therefore even though a fence is greater length than this zone of influence typically of 300m-500m (it depends on post spacing and the number of woven ropes) the deflection does not increase, but plateaus irrespective of the long fence length. The characteristic curves of woven and non-woven wire rope fence systems are well established by work from Prof. Ian Howard and others at the University of Sheffield in the 1990’s. A new study from a soon to be published paper in the USA, plots identical characteristic curves based on finite element analyses of two three (3) rope systems available in the USA (neither system is used in Australia). The curves are calibrated to the NCHRP350 TL3 test results for each fence system. The resultant curves are plotted in the graphic. The FEA curves calculated are identical to the characteristic curves generated numerically by Howard et al 10 years ago. Since 1984 all Brifen fence systems have been tested at both 100 metre lengths and at longer lengths (up to 626 metres in some cases). Brifen has extensive information on how their fence systems work and have the Engineering staff with over 50 years cumulative knowledge of wire rope fence, with modern computer facilities, to provide expert advise. . . and that is in Australia. Beyond Australia is the world wide network of Brifen expertise in Europe and North America, Africa and Asia . . . building Brifen systems since 1972. 36 years and thousands of kilometres of experience at your disposal. |
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| Last Updated ( Sunday, 06 April 2008 ) |
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