Barrier terminals and crash cushions are developed to gradually decelerate an impacting vehicle to a stop or to suitably redirect it. Otherwise, untreated ends of barriers and fixed objects can cause severe accidents. A crashworthy end treatment is essential if a barrier terminates within the clear zone or other area where it is likely to be hit by an errant vehicle. Requirements for testing and performance are contained in NCHRP 350. Suitable devices must be able to perform under both head-on and side impacts, with no objects penetrating the passenger compartment or encroaching on other traffic. The vehicle should remain upright and not be redirected into adjacent traffic lanes. Occupant deceleration levels must be within target values. For longitudinal barriers that depend on the tensile strength of the elements, the end treatment must
6.12.1 Characteristics of End Treatments
Many types of end treatments are available. Table 6.8 provides a summary of characteristics and test levels for a number of them. A description follows. As indicated, many of the systems are proprietary.
The three-strand cable terminal is used at the ends of a three-cable barrier. In one version, the cable barrier is flared back at 4 ft (1.2 m) from the tangent barrier line. The three cable strands are turned down at 45° and anchored to a concrete block in the ground.
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TABLE 6.8 Crashworthy End Treatments
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The Wyoming Box Beam End Treatment (WYBET-350) is used with the box-beam barrier. It consists of a nosepiece welded to a box beam, which is inserted into a larger tube that contains a crushable fiberglass composite tube. The device is supported by a wood post. Crushing of the composite tube dissipates the energy. It may be installed parallel to the roadway or flared out at a maximum rate of 1:10.
A barrier anchored in backslope is sometimes used in areas of a roadway cut section, or where the road is transitioning from cut to fill. A W-beam guardrail thus anchored has been successfully crash-tested to TL-3. This type of anchor can provide full shielding, eliminate the possibility of an end-on impact with the barrier terminal, and minimize the likelihood of the vehicle passing behind the rail. According to the AASHTO Roadside Design Guide, key design considerations include the following:
(1) maintaining a uniform rail height relative to the roadway grade until the barrier
crosses the ditch flow line, (2) using a flare rate within the clear zone that is appropriate for the design speed, (3) adding a rubrail for W-beam guardrail installations, and (4) using an anchor that is capable of developing the full tensile strength of the W-beam rail. Also, the foreslopes on the approach should be no greater than 1:4. If a barrier cannot be terminated in a backslope without violating any of these principles, a different type of end treatment may be more appropriate.
The eccentric loader terminal (ELT) evolved from efforts to improve the breakaway cable terminal (BCT). The resulting device has a fabricated steel lever nose inside a section of corrugated steel pipe. A strut between the steel tube foundations for the two end posts enables these posts to act together to resist impact loads. Holes are drilled in the next four posts, one hole at ground line and one below ground, to make them break away. A blockout is added to the second post to increase curvature near the end of the rail, thus reducing rail column strength and reducing the likelihood of rail penetration. The end post is offset 4 ft (1.2 m).
The Slotted Rail Terminal (SRT-350) is a proprietary, flared, non-energy-absorbing terminal, designed to break away when impacted end-on. There are two versions, one with an offset of 4 ft (1.2 m) and another with an offset of 3 ft (0.9 m). They consist of a curved W-beam rail element in which longitudinal slots have been cut at specific locations. This reduces dynamic buckling strength to an acceptable level and controls buckling location, so that the yaw of an impacting vehicle and the potential for secondary impacts with the bent rail are minimized. Rail tension is developed through a cable anchor system. A traversable area must be provided behind the terminal since it is designed to break away when impacted, allowing the vehicle to travel behind the guardrail.
The REGENT is a proprietary energy-absorbing end treatment. It is a flared W-beam terminal that consists of a slider head assembly, a cable anchor/strut and yoke assembly, modified W-beam rail panels, and special weakened wood posts. The post offsets correspond to those of the BCT, except that the REGENT uses more posts to minimize deflection and the posts are of unique design. The modified rail elements are partially crushed at two locations to induce predictable kinks in the rail in end-on hits while maintaining most of the rail’s bending strength. A traversable area must be provided behind the terminal since it is designed to break away when impacted, allowing the vehicle to travel behind the guardrail.
The Vermont low-speed, W-beam guardrail end terminal is a nonproprietary end treatment for use on roadways where impact speeds do not exceed 45 mi/h (70 km/h). It consists of a 12.5-ft (3.8-m) W-beam rail section that is shop-bent to a 16-ft (4.9-m) radius and mounted on W6 X 9 W150 X 14 steel posts with steel blocks. An anchor consisting of a steel rod and buried concrete block is attached to the rail at the third post from the end.
The Flared Energy-Absorbing Terminal (FLEAT) is a proprietary energy-absorbing end treatment that consists of an impact head installed at the end of a modified W-beam rail, a guide tube assembly, a breakaway cable anchor assembly, and a series of weakened posts. The posts may be wood or of a welded-steel breakaway design. The kinetic energy of a crash is absorbed by the head sliding along the rail element while bending it. The flattened rail exits the head on the traffic side and coils into a tight loop. Tension in the rail is developed through the cable anchor system. The terminal has been tested successfully to TL-3 and TL-2, with a total length of 37.5 ft (11.4 m) and to 25 ft (7.62 m), respectively. The TL-3 terminal can be installed with an offset from 2.5 to 4 ft (0.76 to 1.2 m), and the TL-2 terminal with an offset from 1.7 to 2.7 ft (0.51 to 0.81 m). A traversable area must be provided behind the terminal since it is designed to break away when impacted, allowing the vehicle to travel behind the guardrail.
The Beam-Eating Steel Terminal (BEST) is a proprietary energy-absorbing end treatment with an impact head mounted on the end of a wood post W-beam guardrail system. Kinetic energy is absorbed by the head, which contains three teeth that slide along the rail and cut it into four relatively flat widths. These widths are subsequently bent out of the path of the impacting vehicle. A cable provides anchorage for downstream impacts, and a quick release attachment allows the W-beam to feed into the impact head during end-on impacts. No flare is required, but to position the impact head entirely outside the shoulder, a 1:50 flare may be desirable.
The Extruder Terminal (ET-2000) is a proprietary energy-absorbing end treatment, with an extruder head installed over the end of a standard W-beam guardrail element. Kinetic energy is absorbed by the head sliding along the rail element while flattening it and bending it away from the traffic. The extruder head includes a squeezing section and a bending section. The W-beam is fed through the squeezing section, which reshapes the rail into a flat section. Next, the bending section bends the rail around a small radius and directs it to the side, away from the vehicle. A cable provides anchorage for downstream impacts, and a quick release attachment allows the W-beam to feed into the impact head during end-on impacts. No flare is required, but to position the impact head entirely outside the shoulder, a 1:50 flare may be desirable. Either breakaway timber posts or hinged breakaway steel posts may be used with this terminal.
The Sequential Kinking Terminal (SKT-350) is a proprietary energy-absorbing end treatment that consists of an impact head mounted over the end of a modified W-beam guardrail. The modification consists of punching three slots in the valley of the rail at specific locations. The impact head is forced rearward, bending the W-beam rail against the deflector plate, and absorbing the kinetic energy. A “kinker” beam in the head causes short segments of the rail to kink sequentially and bend away from the impacting vehicle. A cable anchorage system is provided to develop the tensile strength of the rail. No flare is required, but some offset is recommended to locate the edge of the impact head farther from the traveled way. Either breakaway timber posts or hinged breakaway steel posts may be used with this terminal.
The QuadTrend-350 is a proprietary unidirectional end treatment for direct attachment to a vertical concrete barrier or vertical concrete bridge parapet. Additional transition guardrail sections are not needed. It employs sand-filled, energy-absorbing containers that are sacrificial and must be replaced following impact. Many of the other parts can be reused. A concrete pad is required.
The Narrow Energy-Absorbing Terminal (NEAT) is a proprietary, narrow, nonredirective, energy-absorbing terminal. The NEAT is an aluminum cartridge, designed to shield the approach end of a portable concrete safety-shape barrier or a Quickchange® moveable barrier system.
A sloped concrete end treatment is sometimes used to terminate a concrete barrier, although this end tapering treatment has not met the crash-testing criteria of NCHRP
Report 350. This treatment should only be considered for locations where traffic speeds are low, 40 mi/h (60 km/h) or less, and limited space precludes the use of a tested end treatment. Other possible applications include locations where the barrier is flared out beyond the clear zone or where end-on impacts are not likely to occur. Recommended length of the taper is 20 ft (6 m) with 30 to 40 ft (9 to 12m) desirable. The height of the end of the taper should be no greater than 4 in (100 mm).
