Patent Application
20090146121
This invention relates to road safety barriers for use at the sides or central reservations of roads and motorways, and in particular these including a plurality of wire ropes interwoven and maintained under tension between supporting posts.
A known wire rope road safety barrier, described in EP 0 369 659 A1, includes two pairs of wire ropes, one pair of upper ropes supported in slots provided in a number of posts and lying generally parallel to one another, and a lower pair of ropes held in tension against and in contact with opposite side edge surfaces of posts. Each lower cable follows a sinuous path and passes to a different one of the two side surfaces of the same post. Although this safety barrier design added substantially to the containment capability over an earlier two wire rope barrier, it is now recognized that there are disadvantages associated with the parallel arrangement of the upper ropes because they have very little connectivity/cohesion with the posts. Consequently the upper ropes behave less stiffly and have less energy absorption capability than the (interwoven) lower ropes. Also because of the vertical rigidity of the posts there is a possibility of an errant vehicle straddling the safety barrier and receiving an upward thrust leading to overturning of the vehicle, if the posts fail to collapse in time.
It is desirable to achieve a degree of pre-tensioning of the interwoven wire ropes such that the integrity of the barrier is maintained during the mediate post-crash period. However, a consequence of the pre-tensioning is a tendency for the interwoven ropes to grip the posts so tightly that their combined frictional grip in the direction of the line of the barrier exceeds the elastic bending strength of the posts in that direction. This can lead to posts located some distance away from the vehicle impact zone being pulled over by the ropes towards the vehicle to the extent that they are permanently deformed.
It is an aim of the present invention to provide a road safety barrier which alleviates the aforementioned problems.
According to the present invention, there is provided a road safety barrier comprising four or more ropes supported by posts rigidly mounted on or in the ground, each rope being held in tension against the posts and following a sinuous path between the posts.
In embodiments of the invention, the tensioning of the ropes against the posts gives rise to a combined frictional resistance to displacement of the ropes relative to each post or at least some of the posts along the length of the safety barrier. The structure of each post and/or its/their mounting with respect to the ground defines a minimum bending yield strength in a direction along the length of the barrier. This minimum bending yield strength is advantageously greater than the bending moment resulting from the combined frictional resistance forces acting on the post.
Notwithstanding the above requirement it is highly desirable that all (or most) of the posts exhibit a preferential mode of collapse in a direction along the length of the safety barrier, relative to a transverse direction, so that they do not project from the line of the fence after an accident.
Embodiments of the present invention may provide an enhanced vehicle restraint capability relative to the four-wire rope fence described in EP 0 369 659 A1 particularly in cases involving larger and heavier vehicles. Further ropes may be interwoven between the posts to create a multi-rope barrier in order to achieve an increased containment capability although additional ropes to the minimum four are preferably added in pairs so the total number of ropes is even. This is so that the barrier has a more consistent resistance to vehicle penetration along its length. The ropes may be arranged in pairs at different heights on the posts or alternatively each rope may be at a different height from the others. In the latter case, the dispersion of the ropes allows the barrier to better accommodate a wide variety of vehicle types/heights and reduces the risk of rope redundancy in terms of vehicle capture.
Rope supports may be provided on the posts for vertically locating the ropes thereon while permitting longitudinal movement in the direction of the plane of the barrier. The rope supports may be formed integrally in the posts, possibly by way of longitudinally disposed notches. Alternatively the ropes may be supported on frangible supports such as rollers mounted on the posts.
The posts may have an asymmetrical cross-sectional profile such that the post presents the same profile to oncoming traffic on both sides of the barrier. This is, when the post is installed in the ground, rounded corners of the post are presented to oncoming traffic travelling in opposite directions on either side of the barrier. For example, the cross-sectional profile of the post may be of “S” or “Z”, preferably with rounded corners on the line of the bend so that a rounded corner is presented to oncoming traffic. The S-post is therefore to be preferred in the central reservation of dual carriageways where vehicles drive on the left-hand side of the road, whereas the Z-post is preferable in the near-side verges. The opposite choice would naturally prevail in right-hand drive countries.
Embodiments of the present invention are advantageous in that when a vehicle impacts the barrier, there is an enhanced vehicle containment/retardation capability and a reduced risk of post collapse or damage in the regions of the barrier up and downstream of the impact area.
The invention will now be further described by way of example with reference to the accompanying drawings, in which like reference numerals designate like elements, and in which:
a to 4c show a rope support which may be adopted in embodiments of the present invention;
d shows an alternative rope support which may be adopted in embodiments of the present invention;
In the arrangement shown in
The ropes 4, 5 of one pair are lying parallel to one another and supported within notches 8, 9 and 10 provided within respective posts 1, 2 and 3. The ropes 6,7 of the other pair are interwoven between the posts in the manner illustrated and supported in a vertical direction on the side of the posts by way of supports 11, 12 and 13. Each rope is maintained under tension so that the barrier provides an effective restraint to errant vehicles.
In the first embodiment of the present invention, as illustrated in
a to 4c show rope supports which may be advantageously adopted in the posts of the embodiments of
Alternatively, as illustrated in
Each of the ropes 4 to 7 is pre-tensioned by means of ground anchors at suitable intervals along the highway. The tension may be applied, for example, by temporary jacking means and adjustable rope anchorages, or by threaded end connectors and bottle screws (not shown). Intermediate tensioning means may be introduced to permit the end anchorages to be more widely separated.
During installation of the safety barrier, steps should be taken to ensure that the pre-tensioning of the wire ropes 4 to 7 is such that the tension is uniformly distributed along the barrier between the anchorage points.
In a preferred embodiment of the present invention, the yield strength of the posts in the longitudinal direction of the safety barrier exceeds the combined bending moments due to the normal frictional forces of the ropes on the posts under the expected tensions in the system. The significance of the post-rope frictional resistance and its bearing on the performance of the safety barrier will be explained in more detail below under the heading “Safety Barrier Crash Performance”.
The posts should be designed to be secured in the ground in a manner capable of resisting the (longitudinal and transverse) bending moments on the post prior to and during its collapse under vehicle impact conditions, having regard to the prevailing ground conditions.
The post cross-section may be of any size and shape which satisfies the above criteria, and may vary in dimensions along the length of the barrier to reflect differing requirements, e.g. curves in the highway and/or changing post spacing.
Examples of Possible Z-Post sections:
It may also vary in flexural stiffness along the length of the post to take account of the varying bending moment. The type of section will therefore preferably lend itself to being manufactured by processes which can readily accommodate changes in size and shape without incurring prohibitive costs for tooling and the like.
The posts shall be of such a cross-section that they not only provide the barrier with adequate resistance to vehicle penetration (transverse to the line of the barrier) but also have a preferential mode of collapse in the direction of the line of the barrier. This is achieved by making the second moment of area of the posts in the longitudinal direction (in the plane of the barrier) significantly less than its second moment of area in the transverse direction (normal to the barrier) as illustrated in the above table. In order to comply safely with this requirement it is expected that the depth of the post cross-section is preferably in the region of 2-3 times the width thereof.
The constructional design detail of the rope tendons is believed non-critical to the initial functionality of the barrier so long as the ultimate strength and axial stiffness of the ropes are correctly specified, in keeping with the expected (crash) performance of the barrier. However the 19 mm diameter 3×7(6/1) rope is commonly used at present in this application and is a suitable rope for use in barriers embodying the present invention. This type of rope is favored both for ease of manufacture/handling, and for its structural integrity when subjected to mechanical abrasion/abuse. In addition it is substantially torque balanced under load which facilitates pre-tensioning and avoids undesirable rotational displacements in service.
However to optimize the functionality of the barrier in the immediate post-crash period steps should be taken to minimize the loss in rope tension when the barrier is impacted by a vehicle. In addition to ensuring that the barrier is uniformly pre-tensioned along its length, the ropes should be pre-stretched at a tension equivalent to 50% of their breaking strength, to remove initial stretch and elevate the elastic limit of the wire rope. Typically such ropes will have a minimum breaking strength of 174 kN and an axial stiffness of at least 23 MN.
The level of pre-tension applied to the wire ropes during installation of the barrier maybe regarded as an important variable in determining the crash performance of the barrier, with particular regard to vehicle deceleration rates and the permissible level of penetration beyond the line of the barrier. Normally for effective containment the ropes will be pre-tensioned to a tension equal to at least 10% of their breaking strength, and preferably to a tension equivalent to about 15% of their breaking strength and even up to a level equivalent to about 20% of their breaking strength where other design and practical considerations allow.
Safety Barrier Crash Performance
The use of parallel top ropes in the prior art barrier illustrated in
Embodiments of the invention adopt interwoven ropes in place of the prior art parallel top rope arrangement. However, interwoven ropes are more difficult to pre-tension, because the angular deflection of the ropes creates a proportional increase in the frictional resistance to movement between them and the posts. Typically the ropes are deflected from the line of the barrier by 2-3 degrees, but at shorter post spacing the angular deflection increases rapidly and may reach 5 degrees or more. The effect of this on the frictional resistance between the ropes and the posts is illustrated in
This tensioning difficulty can be overcome by adopting an iterative tensioning procedure. The ropes may be tensioned up to or slightly beyond the desired level at the anchorage or tensioning points, and then the intervening posts (in the direction of the line of the fence) may be disturbed so as to promote rope slip and the re-distribution of the tension. This procedure is repeated to effect a progressive tensioning of the whole fence stage, up to the desired level.
Notwithstanding the effectiveness of this technique, the interwoven ropes suffer a significant loss in local tension when posts are collapsed by an impacting vehicle, as the angular (zigzag) deflection of the ropes is removed in the area of the collision.
This is admittedly a worst case scenario and in practice a considerable amount of these tension losses will be taken up by the undisturbed rope in the adjoining fence bays. Nevertheless the residual tension in the ropes will be significantly less than if they had not been interwoven. This emphasizes the need for effective pre-tensioning of the ropes to the recommended level, if a degree of barrier integrity is to be maintained in the immediate post-crash period.
A consequence of these effects is that the interwoven ropes will tend to grip the posts tightly such that their combined frictional grip in the direction of the line of the fence exceeds the elastic bending strength of the posts in that direction. When interwoven upper ropes are introduced, there is therefore the prospect of posts being pulled over by the ropes in positions not directly affected by an impacting vehicle. This presupposes that the rope displacements are sufficiently large to induce flexural yielding of the posts. Significantly the direction of this movement will be towards the colliding vehicle. Therefore, in accordance with a preferred aspect of the present invention, the posts are constructed and/or their attachment to the ground is such that the yield strength in bending of the posts (in the direction of the line of the fence) exceeds the combined bending moment of the rope frictional forces.
The move to a fully interwoven barrier system in accordance with the present invention further alleviates this problem. Embodiments may be provided with means for supporting the ropes, which are frangible at the posts. In the embodiment illustrated with reference to
Consider the case of a 4-rope interwoven barrier in which the ropes have a mean heist above ground level of 550 mm and posts at 2.4 m spacing, each having a depth of 100 mm. The resulting angular deviation of the ropes (in plan view relative to the line of the barrier) will be 2.38 degrees. If we assume for design purposes that each rope will see a tension of 50 kN, then it can be shown that the four ropes will generate a frictional grip on a post of 3.33 kN (taking the coefficient of friction to be 0.20). The effect of this force is to create a bending moment in the post which will reach a maximum of 1832 Nm (at the base of the post) before the ropes slip. The result of this bending moment in terms of maximum bending stress will vary with the strength and stiffness of the type of post selected as illustrated in the table below:
Comparison of Maximum Bending Stresses in Z-Posts at 2.4 m Centres:
[assumes 50 kN rope tension and 550 mm mean rope height]
With the Standard (100×32×6 mm) post it was found that the maximum bending stress greatly exceeded the yield strength of the post, which is 275 MPa [for Fe430A grade material]. The use of a larger (100×40×6.0 mm) post was therefore considered but the maximum bending stress still marginally exceeded the Fe430A yield strength. In this instance the problem could be solved by using a higher grade of steel post, e.g. Grade Fe510A which offers a yield strength of 355 MPa. A possible alternative solution would be to use a yet larger post such as the 120×50×6 mm section. Whilst this increases the angular deviation of the ropes and the bending moment slightly, the maximum bending stress falls to 224 MPa, well below the normal yield strength of 275 MPa.
Although intuition would suggest that post failure would be caused by direct impact of a colliding vehicle on the post, it appears that (for a pre-tensioned wire rope safety barrier) the mode of collapse of the posts is more generally attributable to the longitudinal components of the tensions in the ropes, as they are deflected by the ingress of the vehicle beyond the line of the barrier. The angular deflection of the ropes increases rapidly as the vehicle approaches the (first) post, up to the point at which the yield point of the post is reached, whereupon the ropes are released from the first post, to apply a similar progressive force (and bending moment) to the next post in line.
In an interwoven barrier, only the ropes that are on the upstream side of the post in question (i.e. lie between it and the oncoming vehicle) can act to pull it down. Hence, provision of an even number of ropes would render the barrier to a more consistent resistance to vehicle penetration along its length. Similar considerations apply to the selection of an optimum interweaving pattern for the ropes, if the ropes are not being paired at the same height.
It is noted that in embodiments of the present invention, the aforementioned problem of posts being pulled over is less apparent in the regions of the barrier close to the ends where the ropes are anchored to the ground. This is because at posts close to the barrier ends, the effective stiffness of the ropes increases due to the relatively short length thereof between the post in question and the anchorage point. Consequently, the ropes near the end positions of the barrier tend to deflect less under crash conditions relative to positions further away from the ends. As a result the frictional resistance of the ropes against the posts in these positions is less likely to deflect the post sufficient to cause yielding in bending. Therefore, posts near the anchorage ends of the barrier need not necessarily comply with the minimum bending yield strength of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0321757.7 | Sep 2003 | GB | national |
This application is a continuation of U.S. Ser. No. 10/924,240, filed Sep. 15, 2004, which claims priority to United Kingdom Patent Application No. 0321757.7, filed Sep. 17, 2003.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10942240 | Sep 2004 | US |
| Child | 12371735 | US |
