It may sound a simple path to market but this MASH standard fence is actually a fourth generation Brifen fence and the design in effect represents 40 years of development.

A wire rope safety fence has two key functions (a) safe containment and redirection of errant vehicles, and (b) small predictable deflection in standard impact scenarios. The first requirement is a no brainer and obvious, the second requirement is harder criteria to achieve if the deflection performance is critical, since deflection performance is governed by several criteria, including (but not exclusively) the fence length, the post spacing, the post design, the number of ropes, the tension of the ropes, and the interaction between the ropes and post.

For the first time there is an assessment standard and the wire rope safety fence products now are to be are to tested under standard conditions of fence length and rope tension. The MASH standard of product assessment requires all fences be tested over a length of 600 feet (183 metres) and the ropes be tensioned at approximately 15kN (depends on the manufacturers recommendation for a 40°C day). There are some other standard requirements to try and get all products being assessed on a level playing field.

There are other changes in the new MASH document that will bring in significant change. These concern the weights of the vehicles. In Australia the specifications call for fence designs either to NCHRP350 TL3 or TL4. The standard NCHRP350 has now been superseded by MASH09. Refer the following table.

The severity of the American MASH TL4 test now exceeds the severity of the European EN1317 TB41 10,000kg truck test (at 75kph speed) by 44%. So the new MASH TL4 fences will require to be significantly better performing than the old NCHRP350 TL4 fences.

It will be interesting to see the types of fences each manufacturer designs. By Christmas we should have some idea what the market leaders are looking for in their designs. However such is the increase in impact severity it may be 12 months before the new fences are available.

It is important to look at the origins of the high tension wire rope fences systems. The Americans have been using low tensioned wire rope systems since the 1930’s. However these systems are now being replaced by the better performing high tension wire rope fences. Brifen was the first of the high tension WRSF systems.

Brifen WRSF was developed by the UK Department of Transport for the motorway projects evolving in England in the 1960’s. Wire rope barriers were considered to be both an efficient use of material, and, a forgiving elastic product to cushion vehicle impact then safely redirect the vehicle.

In the 1960’s some 48 full scale crash tests were carried out to assess critical aspects of WRSB design. These included

Wire rope : Height and Tension and type of connectors

Post : Shape, Size and Spacing

Post foundation : Soil plates, concrete

Anchor : Shape and interaction with wire rope

Fence : Length

Vehicle : Impact geometry

The first Brifen fences were constructed in 1972 on the M62 Motorway in England. The Brifen fences at that time used just 2 straight ropes. These 2 rope fences are still constructed today in the Middle East and parts of Africa.

In the 1980’s the design of the Brifen system was revisited in an effort to reduce deflection to about 1.5 metres and to create a fence with predictable and reproducible deflections independent of fence length. Having settled on specific post details and a post spacing of 2.4 metres, and having settled on developing a barrier with 4 wire ropes, the Department of Transport engineers set to work to find the optimal use of the wire rope to meet their needs. A total of 17 full scale tests were carried out initially trying four straight ropes then trying a combination of weaves. All the testing was carried out 626 metre test lengths, the minimum test length required by the DOT.

Ultimately the combination of two straight ropes and two inter-weaving ropes met the deflection requirement of 1.5 metres. This arrangement was further tested in non-standard tests such as the Verge Test (November 1994) and the 200 metre radius Convex test (conducted by the RTA in Sydney in September 2003). In 1995 the arrangement of wire ropes was subjected to further testing in both England and Sweden at 3.2 metre post spacing.

However the European standards with testing based on the 1500kg sedan vehicle and an impact energy of 81.9kJ was hardly representative of the USA fleet where 40% of vehicles are 2000kg pick-up trucks with an impact energy of 137.8kJ. Not only did the standard American test represent a 60% increase in impacting energy but the test involved vehicles with higher centre-ofgravity and larger wheels. It was deemed that higher rope heights were required. This taller fence for the USA market was developed and tested in 1993.

By the end of the 1990’s Highway Engineers focus was shifting again and looking at the truck issue. On both sides of the Atlantic (and in Australia) there were calls for wire rope fences tested to either the European standard EN1317 for a 10 tonne truck or the American standard NCHRP350 for am 8 tonne truck. This demand lead to the evolution of a new series of Brifen fences that meet both standards.

Brifen looked to a Swedish fence that had been used for some years with considerable success along batters on 1V:3H slopes, known as the Swedish slope fence. This fence was crash tested to European standards. In November 2004 with some minor modifications the fence was successfully tested to the American standards and in 2005 to the European standards. In 2006 it was further tested on batter slopes to assess the optimum location in cross median impacts in symmetrical V-shaped median situations. This Brifen TL4 fence is a highly successful fence and dominates the Brifen fence range in the USA and Canada.

However when MASH09 was introduced with such large increases in impact severities, the Brifen management decided to go to first principles. They started with a blank sheet. They utilised computer modelling initially and then again after Phase 1 development testing and again after Phase 2 development testing. Fifty years ago the developers had to crash test but they only tested cars. They had no finite element tools to plug in. Now the vehicles are trucks and very expensive. Indeed doing a test is very expensive with high speed camera gear, laser tools for measurement and a lack of old airports (abandoned airports are great of crash testing and very cheap to use). Now finite element simulations are a much to cut costs and time from concept to implementation.

For further information on Brifen wire rope safety fences contact Paul Hansen of Brifen Australia (02) 9631 8833.