Abstract:
A retainer ( 10   a   , 10   b   , 10   c ) for a wire rope road safety barrier, includes a plurality of posts ( 14 ), for supporting one or more wire ropes ( 12 ) above the ground. The retainer includes an arm ( 16   a   , 16   b ) for embracing at least half of a circumference of the post whereby the retainer can be held on the post at a position along its length. The retainer also includes a frangible or yieldable tab ( 18 ) extending from the arm for retaining the wire rope against the post.

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     This invention relates to a road safety barrier having retainers for holding a wire rope or cable against a post or posts of the road safety barrier, and in particular but not exclusively to a retainer having a yieldable or frangible tab. 
     2. Discussion of Related Art 
     Conventional wire rope road safety barriers comprise a series of spaced apart posts that are provided with hooks or notches for supporting the wire rope which may weave sinuously around the posts or in parallel thereto. These road safety barriers typically consist of two or more ropes (normally two to five) along the side of the barrier and perhaps one or two ropes lying within slots positioned at the top of the posts. Wire rope safety barriers serve to deflect impacting vehicles back into the carriageway or to decelerate the impacting vehicle as it slides along the barrier. Posts within the impact zone tend to collapse on impact and as a consequence, the wire rope becomes detached from the posts. Conventional road safety barriers have the problem that the ropes on vehicle impact become detached from the barrier for a significant distance up and downstream of the impact area. The detachment arises as shock wave, sometimes referred to as ‘whip’, from the vehicle impact travels along the wire ropes of the barrier. Consequently, there is a significant degree of rope deflection or separation from this whip which leads to an unpredictable impact characteristic of the road safety barrier on vehicle impact. Over-deflection or separation of the ropes outside of the impact zone due to the whip reduces the effectiveness of the barrier&#39;s ability to restrain impacting vehicles. Moreover, the force of the whip itself can lead to unpredictable behaviour of the wire ropes on collapse which can compromise the ability of the barrier to restrain the vehicle. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to provide a retainer for holding a rope or cable against a post of a road safety barrier that serves to alleviate the aforementioned problem. In an alternative aspect, it is an aim of the present invention to provide a post for a road safety barrier that serves to improve the deflection characteristics of a wire rope on vehicle impact. 
     According to the present invention, there is provided a retainer for a wire rope road safety barrier comprising a plurality of posts for supporting one or more wire ropes above the ground, the retainer comprising an arm for embracing at least half of a circumference of the post whereby the retainer can be held on the post at a position along its length, and a frangible or yieldable tab extending from the arm for retaining the wire rope against the post. 
     In one embodiment, the arm may be in the form of a ring. In this case, the retainer can be placed over the post and slid along the longitudinal axis thereof into position such that the tab passes over the wire rope. Alternatively, the arm may be in the form of a split ring that can be positioned on the post from a transverse direction relative to the longitudinal axis of the post. 
     The frangible or yieldable tab is preferably configured to break or yield when subjected to a lateral force by the wire rope that exceeds a predetermined amount, thereby allowing the wire rope to deflect away from the post. The predetermined amount is inversely proportional to a lateral extent along the line of the road safety barrier of the impact zone that is subject to wire rope deflection. In other words, the force required to break or cause the tab to reach its yield point with respect to the arm is selected to resist separation of the wire rope from the road safety barrier posts such as to minimise the extent of the vehicle impact zone. It is therefore intended that the strength of the tab is greater than the force of the whip on vehicle impact, but not so strong that the tab will not break away from the arm in the region of contact between the impacting vehicle and the fence as the vehicle moves or slides along the road safety barrier. 
     In a preferred embodiment, the tab is configured to retain the wire rope snugly against the post. The tab may have a curved profile that corresponds to the curvature of the rope in order to provide a snug fit between the tab and the post. 
     According to the present invention, there is further provided a post for a wire rope road safety barrier comprising a cut-out for supporting the wire rope, wherein the cut-out is recessed within an outer periphery of the post and has a substantially flat portion that extends transversely to the longitudinal axis of the post and a curved portion extending from a part of the flat portion that is innermost relative to the outer periphery of the post to the outer periphery, wherein the substantially flat portion resists downward movement of the rope during vehicle impact whereas the curved portion permits upward movement thereof. This notch or cut-out profile also serves to provide a snug fit for the wire rope between the post and the tab. In this case, the tab bridges the notch to establish the snug fit of the wire rope between the tab, the curved and flat portions of the cut-out. The snug fit helps to secure the wire rope from moving as the shock wave (i.e. whip) arising from the vehicle impact is transmitted along the wire rope of the road safety barrier. The snug fit also assists in the transfer of the forces of the shock wave from the wire rope to the barrier post, thereby attenuating the distance of travel of the shock wave along the road safety barrier, which in turn, reduces the extent of separation of the wire rope from the posts. 
     According to the present invention, there is provided a wire rope road safety barrier comprising a plurality of posts as defined above supported in the ground along a roadside or a central reservation and at least one wire rope supported within a cut-out provided in respective ones of the posts, wherein the wire rope is retained captive within the cut-out by the tab of the retainer as defined above. 
     Embodiments of the present invention are advantageous in that they reduce the extent of separation of the wire rope from the road safety barrier posts outside the vehicle impact zone. Consequently, the integrity of the road safety barrier outside the immediate impact zone is maintained. The profile of the notch or cut-out is such as to resist downward movement of the wire rope but permit upward deflection of the wire rope outside the vehicle impact area. Embodiments of the present invention may include road safety barriers that have wire ropes woven sinuously about the posts or barriers having wire ropes that run parallel (i.e. non-woven) to the line of the barrier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic of a retainer embodying the present invention and is shown in use with a wire and a post according to another aspect of the present invention; 
         FIG. 2  is a schematic of a retainer according to an alternative embodiment of the present invention; 
         FIG. 3  shows a post according to yet another aspect with a notch for holding a wire rope; 
         FIG. 4  shows a side view of the post of  FIG. 3  wherein the wire rope is retained by a retainer according to an embodiment of the present invention; and 
         FIG. 5  shows a road safety barrier comprising a series of wire ropes and posts with an alternative arrangement of retainers. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a retainer  10   a , intended for holding a wire rope  12  or cable against a post  14  of a road safety barrier. The retainer  10   a  comprises at least one annular arm  16   a ,  16   b  having a diameter sufficiently greater than that of the post  14  to permit positioning of the retainer  10   a  over the post  14  into position so that a tab  18  clips over the wire rope  12  thereby securing it snugly against the post  14 . The wire rope  12  is held against the post  14  within a cut-away or notch  20  within the post  14 . Preferably, the retainer  10   a  is a plastics material, more preferably Nylon or High-Density Polyethylene (HDPE) although it may be appreciated that the retainer may be formed of any other suitable material including metal. 
     Within the embodiment shown in  FIG. 1  the arms  16   a ,  16   b  are substantially ‘C’ shaped such as to clip about part of the circumference of the post. The arm may therefore be in the form of a split-ring  22 . In this case, the arms  11   a ,  11   b  can be flexible enough to permit attachment of the retainer from the side of the post. 
     In an alternative embodiment, the retainer  10   b  is an annulus or ring as shown in  FIG. 2  and therefore the retainer arms  16   a ,  16   b  may be considered to be a continuous sleeve  16 . 
     Common to all embodiments of the invention, the retainer  10   a ,  10   b ,  10   c  comprises a clip or tab  18  that secures the wire rope  12  against the post  14 . The tab  18  extends perpendicularly from the plane of the arm or sleeve  16  and has an arcuate portion  30  for holding the wire rope  12  in place against a post  14 . In order to maximise the friction between the wire rope  12  and the tab  18 , the arcuate portion  30  is shaped to correspond to the curvature of the wire rope  12 . Additionally, in order to maximise the surface area of the wire rope  12  in contact with the post  14 , the arcuate portion  30  holds the rope  12  against the post  14  within a notch or cut-away  20  (FIG.  3 ). This provides a ‘snug-fit’ between the rope  12  and the post  14  and acts to minimise the pull on the ropes through the slots during impact by a vehicle against the post  14 . 
     The tab  18  of the retainer  10   a ,  10   b ,  10   c  is intended to be frangible or yieldable (for example at point  32 ) when subjected to a predetermined force. The point  32  and force at which the tab  18  yields or breaks is tailored to provide the required trade-off between holding the wire rope  12  snugly against the post  14  and releasing the wire rope  12  upon impact with a vehicle, the effect of which will be explained in detail below. 
       FIG. 3  shows a section of a post  14  in greater detail, with the wire rope  12  shown in phantom to allow the surface of the post  14  to be seen beneath the wire rope  12 . As may be seen, the post  14  holds the wire rope  12  within a notch or cutaway  20  portion of the post. The notch  20  is recessed within an outer surface of the post  14  and is preferably formed by making a single cut into the post  14  and pressing the surface of the post inwards to form a curved portion or indentation  40  and a substantially flat portion or base  42 . 
       FIG. 4  shows a cross-sectional view of the post  14  and wire rope  12 , with the retainer  10   a ,  10   b ,  10   c  shown in phantom. The arrangement of the wire rope  12  in relation to the retainer tab  18  and the post notch  20  is detailed. The wire rope  12  is held snugly due to its position against the base  42  and indented surface  40  of the post  14  and the arcuate surface  30  of the retainer  10   a ,  10   b ,  10   c.    
     In use, the wire rope  12  sits on the base  42  created by the cut into the post  14 . The base  42  prevents the wire rope  12  from falling downwardly towards the ground either in-situ, during impact with a vehicle, or in some instances after impact with a vehicle. During impact, the wire rope  12  will therefore preferentially move upwards riding along the indentation  40  and against the tab  18  of the retainer  10   a ,  10   b ,  10   c.    
     The additional advantages of providing an indentation  40  rather than a complete cutaway is that the wire rope  12  is held against the indented surface  40  which maximise the surface area and therefore the friction between the wire rope  12  and the post  14 . 
       FIG. 5  shows a series of wire ropes  12  held taught against a number of posts  14   a - c  by a plurality of retainers  10   c  to form a road safety barrier  50 .  FIG. 5  shows a further alternative retainer  10   c , where the arm or sleeve  16  of the retainers  10   c  extends along the length of the post  14  a greater distance than the embodiments shown in  FIGS. 1 and 2 . The arrangement between the retainers  10   a ,  10   b ,  10   c , wire ropes  12  and posts  14  are common to all embodiments. It may be appreciated that the number of wire ropes  12  and posts  14  may be chosen to provide the required strength of road safety barrier  50 . In addition, although shown with each wire rope  12  positioned on a single side of the posts  14   b  and  14   c  (and on opposing side of post  14   a ), the wire ropes  12  may be sinuously woven between the posts  14 , or the wire ropes  12  may be provided in parallel on the sides of the post  14 . 
     As highlighted above, during assembly of the road safety barrier  50 , the wire ropes  12  may be held in position against the posts  14  by the notches  20  within the posts. The retainer  10   a ,  10   b ,  10   c  may then be either slid over the post  14 , clipped around the post or fastened with using conventional nail or screw fixings. The wire ropes  12  may then be tightened to the correct tension. 
     During an impact between the road safety barrier  50  and a vehicle in the region of post  14   a , the post  14   a  begins to bend due to the impact of the vehicle. As the post  14   a  bends, the wire ropes  12  are tightened further due to an increase in the distance between the post  14   a  and its neighbouring posts  14   b ,  14   c  and the snug fit between the wire ropes  12  and the post  14  due to the retainers  10   a ,  10   b ,  10   c . This tightening of the wire ropes exerts a lateral force by the ropes against the tab  18  of the retainer  10   a ,  10   b ,  10   c . The wire ropes  12  act to dissipate the energy of the vehicle impact away from the impact point of post  14   a  and distribute the energy to the other posts  14   b ,  14   c , and further posts (not shown) along the road safety barrier  50 . However, the posts  14  are only able to assist in the energy dissipation of the impact if the wire ropes  12  are held in place against the post  14 . This is achieved by the retainers  10   a ,  10   b ,  10   c  that prevent the wire ropes  12  from being displaced from the posts  14  during the shockwave or whip induced within the wire rope  12  by the initial impact of a vehicle. This has the added effect of minimising or managing the impact zone created along the length of the road barrier  50  during an impact. By minimising this impact zone, the efficiency of the road barrier is improved and the structural integrity of neighbouring sections of the road barrier is maintained. 
     The direction of this lateral force of the wire ropes  12  is a result of the base  42  preventing the wire ropes  12  from moving downwards towards the ground, and the indentation surface  40  of the cut-away  20  that channels upward movement of the wire ropes  12  towards the frangible or yieldable break point  32  of the retainer tab  18 . As the yield stress of the tab  18  is reached, the tab  18  breaks (nominally at point  32 ), releasing the wire rope  12  from engagement with the post  14 . The yield stress of the tab  18  is selected along with the snugness of fit between the wire rope  12  and the posts  14 . If the yield stress of the tab  18  is too low, or the fit of the wire ropes  12  against the posts  14  is too loose, the ropes will tend to be released too early or will be ineffective at reducing the impact zone of the force from the impact. The impact force and whip will travel further down the line of the fence because the separation of the rope from the posts render the latter incapable of absorbing impact energy. Conversely, if the yield stress of the tab  18  is set too high, or the ropes are held too tightly against the posts, the wire ropes  12  will not be released from the posts. 
     The release of the wire ropes  12  from the posts  14  is necessary to minimise the impact zone and the extent of the road safety barrier  50  affected by an impact, or more pertinently, to ensure that the road safety barrier  50  provides a degree of give or movement during impact and does not act as a solid immoveable object. The primary advantage of this embodiment is that the release of the wire rope  12  from the post  14  is not instantaneous upon impact—rather the wire ropes  12  are held against the posts  14  for long enough to prevent the initial shockwave of the impact that travels along the wire ropes  12  (the ‘whip’ of the rope) from causing the wire ropes  12  to separate from a large number of posts  14  away of the impact point. As mentioned above, if the wire ropes  12  are separated from the posts  14   b ,  14   c , this prevents those posts  14   b ,  14   c  without wire ropes  12  from absorbing the force of the impact and reduces the effectiveness of the road safety barrier  50 . Instead, the engagement between the wire ropes  12  and the posts  14  by the retainer  10   a ,  10   b ,  10   c  holds the wire ropes  12  against the posts  14 , distributing the energy of the impact at post  14   a  to neighbouring posts  14   b ,  14   c . The wire ropes of these posts  14   b ,  14   c  are only released (via the frangible/yieldable tab  18 ) when the lateral force exceeds a predetermined amount and some of the energy from the impact has been absorbed by the posts  14   b ,  14   c.