ANTI ^. VEHICLE BARRIER FIELD OFTHE INVENTION

The invention relates to the field of perimeter security structures, such as fences, counter-observation screens and anti-vehicle barriers. In particular, the invention relates to an improved construction for an energy absorbing device for use in such an anti-vehicle barrier. BACKGROUND TO THE INVENTION

Military and similar installations tend to be increasingly coming under attack via weapons delivered either in or on motor vehicles. For example, in some parts of the world, it has become common practice to attack military or civilian compounds by attempting to drive motor vehicles laden with explosives or the like through the perimeter barriers of such compounds in order to detonate said explosives at the most adjacent and potential damaging point inside the compound. Accordingly, in order to maintain the security of such compounds, it has become necessary to devise means to prevent such motor vehicles from entering the compound area, in order to preserve the maximum 'distance between the potential detonation point and the assets and personnel to be protected.

Whilst there are a number of avenues available to achieve prevention of access of said vehicles to the compound, such as tyre puncturing mechanisms and moats or ditches, one particular avenue which has been explored for preventing access of these vehicles to said compounds is to provide an energy- absorbing barrier or fence which is capable of arresting the motion of the vehicle at the perimeter of the compound. For example, the perimeter fencing system sold under the trade mark

"IMPASSE" by Ameristar Fence Products of 1555 North Mingo Road, Tulsa, Oklahoma 74116, United States of America, involves the placement of a fence which has been reinforced by one or more horizontally positioned roll-formed beams, said beams housing tensioned cables as additional reinforcement. While such systems are mostly effective in preventing the access of vehicles to target compounds, these kinds of systems have particular disadvantages in certain situations. One disadvantage of this system is that a single collision is likely to destroy the system, rendering it potentially impotent

until the entire structure can be replaced or repaired. Another disadvantage is that these kinds of systems are better suited to permanent installations, as they are relatively time-consuming to install, and require a number of persons with specialised equipment to be successfully installed. These types of prior art systems are accordingly not suitable for use with temporary or mobile compounds.

A further disadvantage of this kind of system is that it is not possible to modify the level of force required to defeat the system without completely reinforcing or redoubling the structural components of the system. Therefore, it is an object of the invention to provide an anti-vehicle barrier which is effective, but which is relatively simple to handle, install, reinforce or repair. SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided an anti-vehicle barrier, said barrier including one or more vehicle contact members spanning <the path of the vehicle" to be repelled, said vehicle contact members being engaged with one or more - energy absorption devices, said energy absorption devices being in turn anchored to the ground or other fixed point, wherein said energy absorption device is a deformable structure constructed substantially from steel rods and/or bars.

An anti-vehicle barrier according to the above definition has a number of advantages over the prior art. One advantage is that there are relatively few components which are likely to be permanently damaged by a collision with a vehicle. This is facilitated by the use of a specific energy absorbing device to dissipate the energy of the colliding vehicle, whilst maintaining the integrity of the other components of the system, such as the vehicle contact members and the structures used to support said members.

The principle of operation of the barrier is that the energy absorber is designed to deform plastically, to reduce the initial stress in the vehicle contact members to a point below its breaking strain, and to progressively deform to absorb the kinetic energy of the vehicle, thereby allowing the vehicle contact members to bring the vehicle to a stationary state within a limited distance. '

Preferably, said energy absorption device is a rectangular truss structure, constructed from a plurality of overlapping longitudinally and laterally arranged steel rods and/or bars, which overlap, and are affixed together, at least at the vertices of the rectangle. The particular design of the energy absorbing device described above allows further particular advantages. One such advantage is that such a structure can be readily assembled and disassembled by a single person, and in its disassembled state maybe transported by a single person. Another advantage of this kind of structure is that the energy absorption capability and strain behaviour can be modified simply by replacing the stesl components with ones of different size and cross-sectional dimensions. A further advantage of this kind of structure is that it has been demonstrated to deform in a predictable manner under strain sufficient to produce plastic deformation.

Advantageously, said trass is engaged with both the vehicle contact members and with the ground (or other fixed point) from a position approximately halfway along the length of the longer sides of said rectangle. It has been found that this anchoring position on the> truss provides the most effective level of predictable deformation during energy absorption in a collision situation.

More preferably, the vehicle contact members are one or more steel cables, and the connection of said cables to said truss is effected by way of an adjustable cable end fitting. This kind of arrangement allows the cable to be retained after a successful vehicle collision engagement, without the need to cut or re-terminate the cable.

A particular advantage of using steel cables as the vehicle contact members is that they undergo substantial elastic deformation following vehicle impact, meaning that the tensile forces applied to the energy absorption device are applied progressively, rather than instantly. This helps to preserve the integrity of the linking structures used to connect the cables, the energy absorption device and the ground anchor. Where said vehicle contact members are two or more steel cables, it is preferred that said cables are arranged to span the path of the vehicle to be repelled at different heights, and that each cable is engaged with at least one energy absorption device as defined above.

Such an arrangement allows the energy absorption capacity of the anti- vehicle barrier system to be at least doubled, without the need to reinforce or strengthen all of the components of the system.

According to another aspect of the invention, there is provided an energy absorption device for an anti-vehicle barrier as defined above.

Now will be described, by way of a specific, non-limiting example, a preferred embodiment of the invention. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows a schematic plan view of an energy absorption device, as installed in an anti-vehicle barrier according to the invention.

Figure 2 depicts an end elevation of the energy absorption device of Figure 1.

Figure 3 depicts an elevation of the energy absorption device of Figure 1 : Figure 4 depicts a plan view of a left-hand end of an anti-vehicle barrier arrangement according to the invention.

Figure 5 depicts an elevation of the anti-vehicle barrier -arrangement of Figure 4.

Figure 6 shows a performance curve of energy absorbed by an energy absorption device according to the invention verse deflection of the members of said energy absorption device, both for an anti-vehicle barrier featuring a single energy absorption device, and for an anti-vehicle barrier featuring twin energy absorption devices according to the invention.

Figure 7 shows a comparative energy curve, relating the kinetic energy of a vehicle with its velocity, for three different vehicle masses. Figure 8 depicts a plan view an anti-vehicle barrier spanning installed to protect a 50 meter length of perimeter in its entirety.

Figure 9 depicts an elevation view of the anti-vehicle barrier of Figure 8. Figure 10 depicts one end of an anti-vehicle barrier featuring twin energy absorption devices according to the invention. Figure 11 depicts two computer generated views of a simulated collision with an anti-vehicle barrier according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The advantages of the invention will be further apparent from the following description of an anti-vehicle barrier system, constructed and installed according to the invention. First will be described the particularly advantageous embodiment of an energy absorption device according to the invention, which forms a critical feature of the anti-vehicle barrier system overall.

Turning first to Figures 1 , 2 and 3, there are shown three different views of an energy absorption device 5 as it forms part of an anti-vehicle security barrier. The device is in the form of a rectangular truss 10, made up of longer longitudinal members (15, 20), which are connected by shorter lateral members (25, 30). In the embodiment shown, the longitudinal members (15, 20) are each constructed from four steel slats: two of said slats-in an "upper" position 35 'and 'two of said slats in a "lower" position 40, the upper and lower slat pairs (35M0) being separated by. laterally arranged spacers, which comprise thershorter lateral members (25, 30). The spacers (25, 30) are steel tubes of approximately square profile. ■

^• It is. preferred that the slats (35, 40) are each approximatsly_r2,000 mm long, by 100 mm wide and approximately 3 mm thick, and formed from- structural steel.

It will be noted that the upper and lower slat pairs (35, 40) are bolted together by bolts (45, 45', 50, 50') passing through said spacers (25, 30). They are further bolted together via four .further bolts (55, 55', 60, 60', 65, 65', 70, 70') along the length of each of the longitudinal members (15, 20), each of which passes through a steel spacer (75, 80, 85, 90).

In Figures 1 , 2 and 3, the truss 10 is shown connected both to a ground anchor (partly shown) 95 on one side and to twin steel cables (100, 100") which act as vehicle eontact members in the anti-vehicle security barrier as a whole. On both sides, the connection is effected via D-shaped flange members (105, 105', 110, 110'), attached respectively to the lower surface 115 of the upper slat pairs 35 and the upper surface 120 of the lower slat pairs 40.

In the case of the connection to the ground anchor 95, a steel retaining sleeve 125, of approximately square profile, is welded to the facing surfaces of

the D-shaped flanges (105, 110) attached to the relevant longitudinal member 15. This retaining sleeve 125 is positioned approximately centrally in said flanges.

Connection of the ground anchor 95 to the energy absorption device 5 is achieved via insertion of a securing tongue member 130 into said retaining sleeve 125, which is secured therein by means of a cross-bolt 135. The securing tongue member 125 is in turn connected to an interlocking pair of D-bolts 140, 145 which are flanged to a ground anchor 95.

In the case of the connection to the steel cables (100, 100'), two retaining sleeves (150, 155) are located in a spaced apart manner approximately centrally in between the facing surfaces of the relevant D-shaped flanges (105, 105'). The cables (100, 100') are inserted in these retaining sleeves (150, 155), and are anchored in a screw-adjustable manner to the ends of said sleeves on the inner- side of the truss 10.

Turning to Figures 4 and 5, there is depicted one end of an anti-vehicle' barrier system accordihg to the invention. The system consists of two 'cables (105, 105') suspended at an appropriate ^" height (approximately 0.5 m and 1>iO>m above the ground, respectively) across in front of a perimeter fence (not shown) • to be protected. The cables, (si 05, 105') are anchored at one end to an energy _* absorption device 5, as described above. They are positioned at appropriate ^■ heights above the ground via a three-legged post system 200.

Said post system 200 consists of left and right bracing posts (205, 210) which are anchored at a lower end to the ground, and which meet at an upper end, thereby to form a triangular arrangement. A third bracing post 215 extends rearwardly towards the perimeter fence from the pinnacle of said triangle 220 and is in turn anchored at a ground engaging end 225 securely in the ground.

It is preferred that the ground-engaging ends of the left and right bracing posts (205, 210) are spaced apart at approximately 1,000 mm. It is also preferred that the rearwardly projecting bracing post 215 engages the ground at approximately 1 ,500 mm from the plane of the left and right bracing posts (205, 210). One of said cables 105" passes through a cable-holder located at the pinnacle of the triangle 220, whilst the second, lower, cable 105 passes through cable retaining means located approximately half-way up the height of the left and right bracing members (205, 210).

The cables (105, 105') then extend, via one or more further post arrangements to an anchor point (not shown). The energy absorption device 5 to which the two cables (105, 105') are connected is located in a below-ground pit 230, preferably filled with gravel. This helps to disguise the nature of the energy absorption device 5. Said energy absorption device 5 is also connected to a secure ground-engaging anchor 95, as described above.

The exact nature of the ground-engaging anchor 95 which hold both the energy absorption device 5 and the cable support post arrangement 200 securely to the ground will vary depending on the ground conditions on site. Persons skilled in the art will readily be able to determine the most appropriate anchoring device for the particular situation.

Turning now to Figure 8 and 9, there are also plan and elevation views of a section of anti-vehicle security barrier 300φrotecting, in its entirety, a">50-m length of perimeter fence (not shown). Six thr-ee-legged cable support posts (200) are spaced at approximately 10 m intervals along the-50 m section. The cables (105, 105') are- anchored securely to energy 'absorption devices' (5, 5') at each end* of the barrier. Also partly shown in the figures is>one end of a second anti-vehicle security barrier 305 according to the invention.

Turning to Figure 10, there is shown one end of a different embodiment of an anti-vehicle security barrier 400 according to the invention. In this particular embodiment, the barrier has been designed to absorb the impact of a vehicle having approximately twice the kinetic energy as compared with the system of

Figures 8 and 9. This has been achieved by connecting the cables (405, 410) to two separate energy absorption devices (415, 420) at each end of the protected barrier. This is achieved by placing two energy absorption devices (415, 420) in a spaced apart manner in two separate gravel-filled pits (425, 430) adjacent to the end of the barrier 400.

Now will be described, with generic reference to the barriers described above, how the system works in practice. When a vehicle travelling toward the barrier impacts the cables, the cables become tensioned. Said cables deform elastically, and in turn transfer a tensile force to the energy absorbing device. This force is applied at the opposing connection points to the cable/cables and the ground anchor. The tensile force tends to pull the longitudinal members

apart, which causes first elastic, then plastic, deformation in said members. This plastic deformation of said members requires a large energy input, which is derived from the kinetic energy of the vehicle impacting the cables, and transferred via the tensile force applied to and by said cables. Provided none of the individual components undergo catastrophic failure, the tensioning of the cable, and the concomitant deformation of the energy absorption device, progressively removes kinetic energy from the vehicle, thereby slowing the vehicle to a stop.

To illustrate the above process, turning to Figure 11 , there are shown two simulated schematic views of an anti-vehicle security barrier according to the- invention, at two different points in time as a simulated collision occurs. In the * embodiment shown, a single cable is suspended between two simple cable Support posts, and is anchored -at each end to a simulated energy absorption ■ device',' similar in principle to that described above. • Both energy absorption ^devices are in turn anchored to a theoretically immovable point %f> ,

!Vi- ϋ' "The "system, shown on the right hand side of trmdiagram ¹ is ¹ shown in a idormant'statθ, readyto repel a colliding vehicle. In the system shown on the left ■.hand side, a simulated vehicle collision has occurred. The vehioletiaβ displaced the cable' a small distance to the left. This has tensioned the cable; which in turn has caused the longitudinal members of the energy absorption devices to be pulled apart from one another, thereby deforming the energy absorption device, which in turn absorbs the kinetic energy of the colliding vehicle.

It is possible to predict the energy absorption performance of the anti- vehicle security barrier according to the invention, due to its particularly simple design, Turning to Figure 6, there is shown a graph of system performance, in terms of energy absorbed (in Joules) versus millimetres of deflection of the longitudinal members of the energy absorption devices incorporated in said system. For example, it can be shown that a barrier system as shown in Figures 8 and 9 is theoretically capable of absorbing 200,000 Joules from a colliding vehicle, via a deflection of 2,200 mm of the longitudinal members away from one another with respect to their original positions. Turning to Figure 7, this level of energy absorption would equate to removing all of the kinetic energy from a 1 ,500 kg vehicle travelling at 60 krn/h, or a 6,000 kg vehicle travelling at 30 km/h.

Of course, it is possible to significantly change the energy absorption performance of the energy absorption devices by changing the width, thickness and/or length of the slats which make up the longitudinal members. For example, using slats of 30 mm thickness, as opposed to 20 mm thickness, will significantly increase the level of energy absorbed per unit of deflection of the longitudinal members.

It will be appreciated by those skilled in the art that the above described example is merely one way in which the inventive concept may be put into practice. Other embodiments of the invention may equally be envisioned which though physically different in construction ^• from that described above, will nevertheless deliver the advantages of the present invention without departing from the spirit or scope of the invention.