Improvements in or relating to safety fences

Abstract: A tensioned wire cable safety fence in which two lower cables (17,18) are interwoven through a row of posts (6,7,8), one cable (17) passing the posts on the opposite sides to the other cable (18). The lower cables are tensioned after interweaving. Upper cables (15,16) are positioned in slots formed in the top of each post (6.7,8) and tensioned.

Description: This invention relates to safety fences designed to redirect or prevent passage of vehicles over prohibited ground and is particularly, but not exclusively, applicable to safety fences used on the sides of roads or central reservations of high-speed carriageways, roads or motorways, or embankments. It is known that safety fences are available consisting of a number of spaced upright posts to which are clamped a number of tensioned horizontal wire ropes. It has been found that these known wire rope fences may be satisfactory when a vehicle approaches a fence at a relatively large angle of impact exceeding 20° whereas at small angles of impact below approximately 10° the vehicle may tend to spin or roll off the fence with consequent danger to the occupants of the vehicle. It is believed that one of the factors contributing to this hazard is the fact that the ropes are normally clamped to the steel posts by means of 'U' bolts or other heavy attachment devices which are strong enough to withstand the collision loading. The disadvantage of clamping wire ropes to posts is overcome by the invention described in UK Patent No. 1,103,873 (see preamble of claim 1) in which the tension cables are positioned as a slack fit in vertical slots in posts fixed into the ground. Tensioned cables act as a continuous beam to redirect a colliding vehicle smoothly back on to the roadway. UK Patent 1,103,873 provides for a plurality of ropes supported either in slots in the top of the post or supported in brackets on either side of the post such that the cables are parallel to each other. The testing of safety fence constructions in accordance with this patent has shown that the penetration is greater than that permitted in certain circumstances. It has also been shown that the release of the cables from the slots, whether in the posts or in the brackets, caused by the post deflection, may give rise to a situation that the cables are released too quickly or too far ahead of an impact point. This led to cables going slack too far ahead of impact and insufficient restraint for the vehicle and a danger that the vehicle will run over cable or cables lying on the ground. U.K. Patent No. 1120959 discloses a wire cable fence having tubular thin wall posts which flatten locally when impacted by a vehicle to enhance bending in any direction of impact. A single pair of wire cables are interwoven through the posts by connections which fracture automatically when the force on the connections exceed a given value. There is no upper level of wire cable restraint to cooperate with the single pair of wire cables. The fence according to the present invention is an improvement over the fence described in U.K. Patent No. 1120959 in that the interwoven cable is not connected to the posts but is free to slide relative to the posts so that the increasing tension produced by vehicle impact on the cables delays post bending. Furthermore, the posts in the present invention advantageously have an asymmetric cross section in order to enhance post bending in the plane of the fence and provide more resistance to bending in other directions. The posts for all the wire rope fences previously referred to apart from UK 1120959 which are tubular have a main web and at least one flange with a cross section, such an 'I' section, with the main web of the section extending in the direction of the cables, but there are no lower interwoven cables to provide additional restraint. The post therefore has a weak axis in the direction of the fence, such that it can be more easily run down but the fence offers insufficient restraint for present day motorways. It is believed that the correct juxtaposition of tension cables and posts in the wire rope safety fence according to the invention met the objectives and the tests laid down by the Ministry of Transport at that time. For over 16 years such a wire rope safety fence has been used on the Pennine Section of the M62 motorway and has proved to have had considerable advantages over standard type central reservation barriers in particular they have prevented build-up of drifting snow. It is an object of the present invention to overcome the disadvantages of the cable safety fence system described in U.K. Patent No. 1103873 and U.K. No. 1120959 to produce better control of the action of a vehicle during impact. Accordingly the present invention provides a cable safety fence of the kind comprising upper and lower cables supported in a number of posts (6,7,8) in which the upper cables (15,16) are held under tension in slots (9,10,11) in posts (6,7,8) being arranged so as to permit the upper cables (15,16) to be displaced from the slots (9,10,11) and separated from support from at least one post (6,7,8) under impact, characterised in that the fence further comprises a pair of lower cables (17,18) being held in tension against and in contact with opposite side edge surfaces of posts, each lower cable following a sinuous path and passing to a different one of the two side surfaces of the same post, whereby the lower pair of cables (17,18) is free to slide relative to the post edge surfaces on impact by the vehicle before the said at least one post is bent to the ground towards a following post subsequent to the release of the upper cables (15,16) from at least one post, and that each post has a weaker axis of bending which axis is so orientated to the fence that the said edges of the post surfaces in the preferred direction of bending are substantially in the line of the fence. Preferably the posts have a cable safety fence as claimed in Claim 1, characterised in that the posts (6,7,8) have a cross-sectional shape such that a rounded corner is offered to the direction of the traffic, and may be formed by bending steel sheet into profiles which are of "S" or "Z" cross-section and have rounded corners on the line of the bend. Location means may be provided on each side of the post for the lower cables permitting relative motion between the posts and the cables. These locating means may be grooves formed in the post or other suitable abutments, rings or hooks. All cables are anchored to a suitable anchoring point and tensioned to between 1,000 and 5,000 KgF. The height of the ropes above the carriageway shall be for the lower crossed ropes 450mm to 500mm and determined by the position of the said Location means, and for the upper ropes 575mm to 615mm, the preferred height being 495mm and 585mm respectively. The posts may be located in the ground either as a driven post, i.e. a post having a plate welded to its lower section to prevent overturning on impact, or a concrete footing which prevents overturning of the post and allows the post to bend during impact. The concrete footing may either be of a pre-cast design having an internal socket or opening to receive the post and thus to enable the height of the post to be set accurately on installation. Such preformed footings overcome the problem of soil ground and the difficulty of ensuring that the post is installed properly to the right depth and with the required strength of the concrete infill. In addition, when it is necessary to replace the posts, because of vehicle impact, the impacted posts can be readily withdrawn, the replacement posts inserted immediately, thus facilitating re-erection of the damaged barrier in a very short period of time. Due to the design of the safety fence, one fence on the central reservation will serve both carriageways. After an impact, repair is speedy and economic, requiring damaged supports to be removed and new one inserted, the wire ropes being re-located and possibly re-tensioned, but not necessarily replaced. Repair work could be carried out from either carriageway. The posts may be of 'S' or 'Z' section and may be formed from pressed sheet steel of a thickness between 3mm and 7mm and adapt to deflect or distort under impact from a vehicle. The bending moment at yield of the post should be less than 6,000 Nm in its weakest plane. The invention may be performed in various ways and a number of possible embodiments will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a cable support post disclosed in UK Patent No. 1,103,873; Figure 2 shows a 4-cable safety fence with the lower cables woven between the posts; Figure 3 shows diagrammatically the method of weaving the lower cables around the erected posts; Figure 4 shows typical pre-cast footings for the posts; Figure 5 shows a quick-release system to provide emergency access; Figure 6 shows a cable safety fence system attached at one end to a known corrugated tension beam barrier, and Figure 7 which shows how a cable safety fence may be used in parallel or as a first barrier with a corrugated tension beam barrier forming a second or final barrier. As will be seen from Figure 1, the cable supporting post 1 has a slot 2 in the top thereof; an upper cable 3 is indicated in position at the bottom of the slot 2. A lower cable 4 is positioned at the bottom of a slot formed in a bracket 5 attached to the post 1. The upper rope 3 and the lower rope 4 are parallel to each other and with this form of cable support, the deflection of the fence under impact is greater than now required by the Ministry of Transport. In addition, bending of the post caused by impact may release the lower and upper cables from their respective slots more or less simultaneously and thus lead to the cables being released too quickly or too far ahead from the impact point 6 causing lowering of the cable, reducing restraint further ahead and increasing the likelihood of vehicles passing over the cables. Figures 2 and 3 relate to the present invention. A number of posts are inserted into the ground (not shown) either into recesses in pre-cast footings or by any other suitable means. Suitable pre-cast footings are shown in Figure 4. Other post retention means to be inserted into the ground may be used, for example, cast or pressed steel hollow tube-like structures, having a plate welded to its lower end to prevent overturning on impact, are alternatives but are not illustrated and nor described. The posts 6, 7 and 8 have respectively slots 9, 10 and 11 formed in their upper ends. The slots are parallel-sided slots and parallel to the longitudinal edges of the posts. Location means 12, 13 and 14 are attached to the posts. Similar location means on the other side of the posts are provided but not illustrated. Two wire ropes 15 and 16 placed on top of one another are placed in position into slots 9, 10 and 11 and anchored to the ground and tensioned, as will be described in more detail with reference to Figure 3. The posts 6, 7 and 8 are made from steel pressings and have an 'S' or 'Z' cross-section such that a rounded corner on the line of the bend is offered to the direction of the traffic and not a sharp edge. Such a design of post permits bending along the weak axis but does not involve a solid restriction when a vehicle hits the post at 90° since the post will twist slightly and bend on the preferred weak axis. Lower ropes 17,18 are woven through the posts such that the lower ropes cross as indicated at 20. Generally speaking, crossing before and after each post provides better restraint and delays the release of the lower cables from the post until after the initial bending of the post by impact has released the upper ropes. This delay may be very significant in providing maximum restraint while limited damage to the vehicle. Figure 3 shows very diagrammatically the posts 9,19 and 11 and the lower cables 17,18. Cable 18 is drawn off drum 21 so as to pass the side of the post 9 before crossing over the opposite side of post 10 and then again to the opposite side of post 11. Cable 17 is drawn off the drum 22, passes along the opposite side of post 9 as compared with cable 18 and so on in sequence so that the cables cross as indicated at 20. The height of the ropes above the carriageway are, for the lower ropes between 450mm and 500mm, and for the upper ropes between 575mm and 615mm which, it is believed, will be suitable for restraining a typically mixed traffic flow associated with motorways. The height of the lower cables is controlled by abutments, grooves or hooks attached to the sides of the posts and are arranged so that the cable can slide along the edge of the post when positioning the ropes and when tensioning. Figure 4 shows cross-sections of suitable pre-cast footings which are suitable for wire cable safety fences and enable quick replacement of damaged posts. Furthermore, as compared with the posts used for tensioned beam barrier posts for cable wire fences according to the present invention, require bending above ground on impact. Posts for corrugated tensioned beam barriers are often just driven into soft ground since no bending is required on impact with the beam barrier which is just pulled out of the ground and/or fractured by impact. Figure 5 shows a typical quick-release mechanism which can be utilised to join all four of the cables in a 4-wire system such that they can be disconnected to provide easy access in the case of accidents. Figure 6 shows how a wire rope fencing system may be attached to the ends of a corrugated tensioned beam barrier 26. This enables replacement of a tensioned beam barrier when damaged or extensions of motorway where it has been decided to take advantage of the tensioned wire cable safety fence without incurring the costs of replacing the tensioned beam barrier already in place. Figure 7 shows how a tensioned wire cable safety fence 27 may be placed in parallel with a tensioned beam barrier 28 so that vehicles leaving the carriageway into the central reservation will first be restrained by the cable safety fence and secondly by the final barrier formed by the tensioned beam barrier. In a similar manner, wire cable safety fences may be positioned to restrain vehicles from other road furniture, lighting and road signs, for example. All four ropes are anchored to a suitable anchoring point and tensioned between 1,000 and 5,000 KgF. This is not described in detail and is similar to the wire rope safety fence system in UK Patent No. 1,103,873. The manner in which the ropes are anchored, how the anchorages are staggered along the length of the fence and how the cables are restrained by tethering wires when they are severed, is well known from the practice of wire fences that are already in use.

Claims: A cable safety fence of the kind comprising upper and lower cables supported in a number of posts (6,7,8) in which the upper cables (15,16) are held under tension in slots (9,10,11) in posts (6,7,8) being arranged so as to permit the said upper cables (15,16) to be displaced from the slots (9,10,11) and separated from support from at least one post (6,7,8) under impact, characterised in that the fence further comprises a pair of lower cables (17,18) being held in tension against and in contact with opposite side edge surfaces of posts, each lower cable following a sinuous path and passing to a different one of the two side edge surfaces of the same post, whereby the lower pair of cables (17,18) is free to slide relative to the post edge surfaces on impact by the vehicle before the said at least one post is bent to the ground towards a following post subsequent to the release of the upper cables (15,16) from the at least one post, and that each post has a weaker axis of bending which axis is so orientated to the fence that the said edges of the post surfaces in the preferred direction of bending are substantially in the line of the fence. A cable safety fence as claimed in Claim 1, characterised in that the posts (6,7,8) have a cross-sectional shape such that a rounded corner is offered to the direction of the traffic. A cable safety fence as claimed in Claim 2, in which the posts (6,7,8) are formed by bending steel sheet into profiles which are of "S" or "Z" cross-section and have rounded corners on the line of the bend. A cable fence as claimed in Claim 3, characterised in that each post (6,7,8) has a concrete footing (Figure 4) which allows the post to bend at its lower end about the footing. A cable safety fence as claimed in Claim 4, characterised in that the concrete footing is pre-cast and has an internal socket or opening to receive the post and thus to enable the height of the post to be set accurately on installation and simplify replacement of damaged posts. A cable safety fence as claimed in any of the Claims 1 to 5, characterised in that the upper cable or cables (15,16) is set at a height of between 575mm and 615mm and the lower cables (17,18) are set at a height of between 450mm and 500mm. A cable safety fence as claimed in any preceding Claim, characterised in that all cables (15,16,17,18) are tensioned to between 1,000 and 5,000 KgF. A cable safety fence as claimed in any of Claims 1 to 7, wherein the posts are provided with location means to determine the height of the lower cables.

Inventors: STACEY ANDREW GORDON, STACEY, ANDREW GORDON
Applicants: BRIDON ROPES LTD, BRIDON ROPES LIMITED
Application year: 1989
Main classification: E01F15
IPC codes: E01F15, E04H17