| Up to the Challenge... |
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| Written by BRIFEN Australia | |
Up to the Challenge...Building on its success throughout Australia and internationally, Brifen’s high performance TL4 Wire Rope Safety Fence (WRSF) continues to go from strength to strength, following a number of major fence installations along the newly upgraded Bruce Highway at Caboolture north of Brisbane. Incorporating 40 new verge and median fences, totalling some 29,000 metres in length, the new Bruce Highway fences represent the latest chapter in Brifen’s remarkable Australian success story. Interestingly, while the majority of safety barrier installations tend to focus on one primary safety issue – such as protecting vehicles from a steep batter verge, preventing collisions with objects near the carriageway, preventing ‘run off road’ incidents, or preventing median crossover incidents - the safety barrier challenge along the newly upgraded Bruce Highway at Caboolture must surely be considered as ‘...one with the lot’. Together with the challenge of steep non-traversable batters (with slopes of up to 1V:3H) along the outside edge of sections of both carriageways, the Bruce Highway design also incorporates a deep ‘V-shaped’ median with batter slopes of up to 1V:3H for drainage requirements. In addition, the fact that the major project section between Uhlmann Road and Caboolture involves widening the highway from four to six lanes in a transport corridor with a limited width, meant that in several sections the large concrete noise abatement walls had to be constructed in close proximity to the carriageway. As such, the design also called for the installation of protective barrier fencing to prevent vehicular impacts with the concrete noise abatement walls along both the north- and south-bound carriageways. The Bruce Highway upgrade projects represent the final stages of the Federal Government’s $362 million commitment to upgrade the highway from four to six lanes from the Gateway Bridge through to Caboolture, some 40 kilometres north of Brisbane. The works at Caboolture incorporate two separate contracts; the $8.5 million Boundary Road to Uhlmann Road contract – which was awarded to CMC - and the $183 million Uhlmann Road to Caboolture section, which was awarded to Leighton Contractors. 
Stretching some 9.8 kilometres, th e Boundary Road to Uhlmann Road contract section incorporated widening the median shoulder on both the north- and south-bound lanes of the existing 6-lane highway, together with the installation of 18 Brifen TL4 WRSF sections.
The larger Uhlmann Road to Caboolture contract section incorporated an upgrade of the existing 4-lane highway to a motorway standard 6-lane highway, including: upgrades to bridges, interchanges and improvements bicycle and pedestrian infrastructure, together with the installation of noise abatement walls and a total of 22 new Brifen TL4 WRSF sections along the verge and median of both carriageways. Needless to say, when it came to selecting an appropriate safety barrier system for the Bruce Highway upgrade projects, engineers and designers not only focused on ‘containment capabilities’ and performance in sloping batter installations, they also placed a significant emphasis on the amount of fence deflection during an impact. Indeed, for the fences that were to be constructed adjacent to the sound walls, engineers specified a maximum fence deflection of 2 metres – a critical factor in preventing an errant vehicle from impacting the sound walls. In order to meet these strict design and performance challenges, the project’s engneers selected the high performance Brifen TL4 Wire Rope Safety Fence (WRSF) for both the verge and median installations, including those adjacent to the sound walls. Highly regarded for its ability to ‘capture’ an errant vehicle and bring it safely to a stop in both verge and median installations (including locations with steep sloping batters) Brifen fences have been directly credited with reducing the severity of literally thousands of vehicular impacts - saving numerous lives and delivering a significant reduction in both the level and seriousness of the associated injuries throughout Australia and internationally. Not surprisingly, Brifen WRSF’s ‘life-saving’ performance in the field, and perhaps most importantly, its ability to minimise fence deflection during an impact, both played a major role in its selection for the Bruce Highway upgrade projects. Another key factor in Brifen WRSF being specified for use along the Bruce Highway upgrade sections was its performance along sloped batters and in ‘V-Shaped’ median installations. 
Brifen WRSF has been used extensively on non-traversable 1V:3H slopes for almost 10 years – particularly in Sweden where the Brifen 4-woven rope ‘Slope Fence’ was developed. Batter slopes flatter than 1V:10H do not require attention, with V-shaped medians designed on these slopes posing no additional hazard to motorists.
Medians designed with slopes flatter than 1V:10H are ideal in low rainfall areas. Batter slopes between 1V:10H and 1V:4H are considered traversable - although in V-shaped medians this becomes somewhat bumpy at the batter slopes increase past 1V:6H. Batter slopes greater than 1V:4H are considered non-traversable. The high concentration of rainfall and severe weather conditions prevalent throughout the Caboolture region required well designed medians to minimise road hazards. Whilst the steep batter slopes (up to 1V:3H) were an essential part of the hydraulic design, they created a hazard for vehicles that needed to be addressed for both run-off road accidents and cross median accidents. Interestingly, unlike the majority of American safety barrier installations along ‘ V-Shaped’ medians, where fences are constructed "in the ditch", the preference in Australia is to construct the barrier along the median verge, thereby preventing the errant vehicle from entering the ditch. This is predominantly due to the fact that whilst it is easy to predict where a vehicle will initially "land" in the ditch, the trajectory of the vehicle and the height of trajectory from the initial impact point are unknown and dependent on so many variables, including: the shape of the median, the slope of the median batter, condition of the median surface (vegetated, saturated soils, etc.), the type of vehicle, and the condition of the vehicle suspension to name but a few. Not surprisingly, the success of the Brifen ‘Slope Fence’ in the field over the past decade, together with results of full-scale crash testing, and the extensive computer modelled Finite Element (FE) simulations in the USA by the NCAC (National Crash Analysis Centre) division of the FHWA (U.S. Department of Transportation Federal Highway Administration) provided clear evidence of Brifen WRSF’s outstanding performance capabilities in non-traversable batter applications. With that in mind, the Bruce Highway upgrades were designed with Brifen TL4 fences installed along the median verge of both the north- and south-bound carriageways, to prevent errant vehicles from entering the ‘V-Shaped’ median. The Benefits of Maximising Rope-Post InteractionWhilst there are clearly a number of contributing factors (including: post design, post height, the location of the wire ropes, end anchor design and installation location), the key to Brifen WRSF’s outstanding performance capabilities during an impact undoubtedly lies within its unique patented ‘woven rope’ design that ensures the maximum benefit of rope-post interaction. The combination of the inter-woven lower ropes and the linear ‘upper’ rope delivers two energy absorbing actions, namely: • the linear stretching of the upper rope; and • the mechanical action of the lower woven ropes acting on the steel post and using the deformation of the posts to absorb impact energy into the fence system. In short, rather than being designed as a simple system with a primary reliance on the wire rope, Brifen WRSF incorporates a complex system of multiple-interactions between the wire rope and posts. 
Working together in tandem, these two energy absorbing actions result in a significant reduction in total fence deflection and fence damage during an impact. The complex interactions of rope and post ensures rapid transfer of the vehicle kinetic energy into the fence and then into the ground.
Indeed, Brifen’s patented system combines the best attributes of both wire rope configurations: the straight rope to provide the initial resistance to the impact; and the woven ropes to both localise and control the impact by making most efficient use of the supporting posts. The benefits of incorporating woven ropes into the fence design is best realised on longer lengths of fence. As the fences length increases, the fence anchors have reducing influence on the fence performance. For short fence lengths (say 50 metres to 150 metres) the end anchors tend to do the bulk of the “work” However as fence lengths increase, it becomes very important to confine the influence of the impact locally to the area of the impact. To achieve this efficiently the rope and posts have to interact as a team. Interestingly, the extensive regime of crash testing and computer modelling undertaken of Brifen’s ‘woven’ rope design also uncovered another extremely important performance characteristic; viz: the increased rope-post interaction achieved by the ‘woven’ rope resulted in the deflection reaching a ‘plateau’ for woven rope fences with lengths greater than approximately 300m (depending on the post spacing). This ‘active region’ results from the fact that whilst the load on the parallel (top) rope is spread over the entire length of the fence through to the anchors at each end of the fence, the woven ropes ‘localise’ the tension (as a result of work having to be done to break the rope friction with the fence posts). This friction, or mechanical action, allows the stresses to dissipate over roughly 70 fence posts on either side of the impact point. The benefits of this localised ‘active region’ are clearly evidenced in the maximum deflection readings from the ‘full-scale’ Brifen crash tests and computer crash modelling - particularly for fences over 450 metres in length, and for impact speeds ranging from 80 – 110 kph. As a result of the lower interwoven ropes, rather than the maximum deflection continuing to increase in relation to the length of the fence, the deflection actually ‘levels out’ for fence lengths greater than 140 times the post spacing distance. Understandably, the size (number of posts/bays) of the ‘active region’, together with the total fence deflection, reduces for impacts of less than 100kph. Highlighting the Importance of Fence Length in Reducing DeflectionTogether with the benefits of the woven rope design, another major contributing factor in Brifen WRSF’s ability to minimise fence deflection during an impact, lies within the design of the finished fence sections – in terms of both post spacing and the total length of each fence. 
The important role that the overall length of the fence (spacing between end anchors) plays in minimising deflection during an impact was a major feature of a detailed research White Paper by Dhafer Marzougui of the US-based FHWA/NHTSA National Crash Analysis Center at The George Washington University entitled: “Effects End-Anchor Spacing and Initial Tension on Cable Barrier Deflection.”
The White Paper is the result of an exhaustive 12-month study to investigate the influence of different installation configurations and design parameters on the safety performance of high-tension cable barrier systems. The installation configurations investigated in the study included end-anchor spacing (installation length) and the initial cable tension (static tension), whilst design parameters examined in the study included cable weaving effects (weaved and parallel systems) and the number of cables (three and four-cable systems). 
The study utilised available full-scale crash testing data with sophisticated Finite Element (FE) computer simulations. Models were created for two types of proprietary cable systems: one a woven configuration that uses the post and ropes to interact in a complex fashion and the other a parallel rope configuration with the posts simply supporting the ropes; in both three and four-cable versions. For each system, was modelled with a range of anchor spacings (100m, 200m, 300m, 500m, and 1000m) and two cable tensions (15 kN, and 24 kN) were developed. The White Paper concluded: “…simulation results indicate that in impacts with cable barriers, the maximum dynamic deflection is significantly affected by the end-anchor spacing. Greater end-anchor spacing leads to increased barrier deflections.”
Importantly, it also found that: “…the results also indicate that the effects of end - anchor spacings are different for different types of barrier systems. The deflection of weaved cable barrier systems, which provide higher friction between the posts and cables, reaches a maximum at end-anchor spacings of 300m. The parallel system, on the other hand, shows continuing increase in deflection beyond 1000m anchor spacings. These results lead to the conclusion that correction factors should be used to convert deflections observed in crash tests to those expected in the field where actual anchor spacings are much greater than the system lengths tested. These correction factors must account for the expected field installation lengths as well as the type of barrier system. For the weaved cable barrier simulated, dynamic deflection does not increase for installation lengths greater than 300m. However, for parallel systems, deflections will continue to increase for installations of over 1000m in length.” The White Paper also concluded that “…the simulations had demonstrated that four-cable systems have lower dynamic defections compared to three-cable systems”, and as such, “…increasing the number of cables leads to a higher number of cables engaging the vehicle during the impact and increases the chance of capturing and redirecting the vehicle.” In order to meet the Bruce Highway project’s strict maximum deflection requirements of 2 metres during a TL3 rated impact (2,000kg vehicle impacting at 120km/h), the Brifen fences constructed in areas adjacent to the sound abatement walls were constructed with a reduced post spacing of 2 metres, a maximum fence section length of 410 metres and an intermediate anchor spacing of 137 metres. These fences also incorporate a specially designed intermediate anchor that was developed by Brifen UK and LB International (Brifen WRSF’s exclusive Australian distributor) specifically for these sections. 
The reminder of the Brifen fences along the project – in both verge and centre median locations – were constructed with the standard 3.2 metre spacing and a maximum fence length of 1,000 metres. The fences have been constructed in an overlapping format to ensure maximum protection along the entire length of the fence. For further information, please contact Mr Paul Hansen, Managing Director, LB International Pty Ltd, Ph: (02) 9631 8833, Fax: (02) 9688 4503 or Email: This email address is being protected from spam bots, you need Javascript enabled to view it Download pdf version of this article (1.1MB) Highway_Engineering_in_Australia.pdf (Right click and Save as). This article has been published in September / October 2009 issue of Highway Engineering in Australia. |
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| Last Updated ( Sunday, 03 January 2010 ) |
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