Road safety barrier
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.
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'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.
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.
The invention will now be described by way of example, with reference to the
accompanying drawings, in which:
Figure 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;
Figure 2 is a schematic of a retainer according to an alternative embodiment of
the present invention;
Figure 3 shows a post according to yet another aspect with a notch for holding
a wire rope;
Figure 4 shows a side view of the post of figure 3 wherein the wire rope is
retained by a retainer according to an embodiment of the present invention; and
Figure 5 shows a road safety barrier comprising a series of wire ropes and
posts with an alternative arrangement of retainers.
Figure 1 shows a retainer 10a, intended for holding a wire rope 12 or cable
against a post 14 of a road safety barrier. The retainer 0a comprises at least one
annular arm 16a, 16b having a diameter sufficiently greater than that of the post 4 to
permit positioning of the retainer 10a 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 1 is held against the post 14 within a cut-away or notch 20 within the post 14.
Preferably, the retainer 10a 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 Figure 1 the arms 16a, 16b 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 1a, l b can be
flexible enough to permit attachment of the retainer from the side of the post.
In an alternative embodiment, the retainer 10b is an annulus or ring as shown
in figure 2 and therefore the retainer arms 16a, 16b may be considered to be a
continuous sleeve 16.
Common to all embodiments of the invention, the retainer 10a, 10b, 10c
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 4. 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 (figure
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 8 of the retainer 10a, 10b, 10c 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 2 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.
Figure 3 shows a section of a post 14 in greater detail, with the wire rope 2
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 4
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.
Figure 4 shows a cross-sectional view of the post 14 and wire rope 12, with
the retainer 10a, 10b, 10c 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 10a, 10b, 10c.
In use, the wire rope sits on the base 42 created by the cut into the post 14.
The base 42 prevents the wire rope 2 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 8 of the retainer 10a, 10b, 10c.
The additional advantages of providing an indentation 40 rather than a
complete cutaway is that the wire rope 1 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.
Figure 5 shows a series of wire ropes 12 held taught against a number of posts
14a-c by a plurality of retainers 10c to form a road safety barrier 50. Figure 5 shows a
further alternative retainer 10c, where the arm or sleeve 16 of the retainers 10c
extends along the length of the post 14 a greater distance than the embodiments
shown in figures 1 and 2. The arrangement between the retainers 10a, 10b, 10c, 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 14b and 14c (and on opposing side of post
14a), the wire ropes 12 may be sinuously woven between the posts 14, or the wire
ropes 2 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 10a, 10b, 10c may then be either slid over the post 1 , 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 14a, the post 14a begins to bend due to the impact of the vehicle. As
the post 14a bends, the wire ropes are tightened further due to an increase in the
distance between the post 14a and its neighbouring posts 14b, 14c and the snug fit
between the wire ropes 12 and the post 14 due to the retainers 10a, 10b, 10c. This
tightening of the wire ropes exerts a lateral force by the ropes against the tab 18 of the
retainer 10a, 10b, 10c. The wire ropes 12 act to dissipate the energy of the vehicle
impact away from the impact point of post 14a and distribute the energy to the other
posts 14b, 14c, 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 10a, 10b, 10c that prevent the wire ropes 2 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 1 from moving downwards towards the ground, and the
indentation surface 40 of the cut-away 20 that channels upward movement of the wire
ropes 2 towards the frangible or yieldable break point 32 of the retainer tab 8. 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 1 and the
posts 14. If the yield stress of the tab 18 is too low, or the fit of the wire ropes 1
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 1 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 1 from the posts 4 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 2 from the post 14
is not instantaneous upon impact - rather the wire ropes 12 are held against the posts
4 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 14b, 14c, this prevents those posts 14b, 14c
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 4 by the retainer 10a, 10b, 10c holds the wire ropes 12 against
the posts 14, distributing the energy of the impact at post 14a to neighbouring posts
14b, 14c. The wire ropes of these posts 14b, 14c 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 14b, 14c.

Claims:
1. 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.
2. A retainer according to claim 1, wherein the arm in the form of a ring.
3. A retainer according to claim 1, wherein the arm is in the form of a split ring
that can be positioned on the post from a direction transverse to the longitudinal axis
of the post.
4 . A retainer according to claim 1, claim 2 or claim 3, wherein the frangible or
yieldable tab is 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.
5 . A retainer according to claim 4, wherein said 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.
6 . A retainer according to any one of the preceding claims wherein the tab is
configured to retain the wire rope snugly against the post.
7. 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 .
8. A wire rope road safety barrier comprising a plurality of posts as defined in
claim 7 supported in the ground along a roadside or 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 in any one of claims 1 to 6.
9 . A wire rope safety barrier according to claim 8, wherein the tab is frangible or
yieldable such as to permit separation of the rope from the barrier within a
predetermined impact zone.