U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Ave. S.E.
Washington, D.C. 20590
July 10, 2009
In Reply Refer To: HSSD/CC-105
Mr. Dallas James
P.O. Box 303 177
Auckland 0751 New Zealand
Dear Mr. James:
This letter is in response to your request for the Federal Highway Administration (FHWA) acceptance of a roadside safety device for use on the National Highway System (NHS).
|Name of device:||Armorwire ATE-4|
|Type of device:||4-cable barrier end terminal|
|Test Level:||NCHRP Report 350 (modified) TL-3|
|Testing conducted by:||Holmes Solutions|
|Date of request:||January 15, 2009|
|Date of completed package:||April 21, 2009|
You requested that we find this device acceptable for use on the NHS under the provisions of National Cooperative Highway Research Program (NCHRP) Report 350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features.”
Roadside safety devices should meet the guidelines contained in the NCHRP Report 350." The FHWA Memorandum “Identifying Acceptable Highway Safety Features” of July 25, 1997, provides further guidance on crash testing requirements of longitudinal barriers.
The Armorwire Terminal End for 3-cable barrierswas the subject of FHWA Acceptance Letter CC-98 dated April 9, 2007. Your present request is for a similar terminal for 4-cable barriers. The ATE-4 terminal consists of the ground strut, trigger post, anchor cables, and other components as shown in the drawing enclosed for reference. A description of the major components follows.
The ground strut is fabricated from mild steel conforming to ASTM A36. The ground strut was 1.9 meters long and supported by three reinforced concrete foundations at approximately 900 mm spacing. The ground strut foundations consist of three 450 mm diameter concrete piles that are 1000 mm long.
The trigger post consists of a trigger release head, supported by two square section posts and fixed to the ground strut. The trigger release head is fabricated from hot rolled steel, galvanized after fabrication. The hot rolled steel complies with ASTM A36. The trigger release head is welded to two legs that are angled away from the release head. The head has five angled slots, two on each side and one on top, to accept the end fixing details of the cables from the wire rope barrier. A further central slot, open to the bottom of the head is provided to allow attachment of the two anchor cables. This head effectively joins the two cables that are attached to the ground strut to the four cables forming the barrier.
Two parallel 19 mm diameter by 2.0 m long wire rope anchor cables are fixed between the top of the trigger post and the opposite end of the ground strut. These pass through a tunnel section in the ground strut and are terminated, via a bolted connection, at the down stream end.
Wire Rope Barrier Cables
The four barrier cables were constructed using 19 mm 3 x 7 strand galvanized cable, prestretched by 35 percent. Cables are tensioned to a nominal 25 kN at an ambient temperature of 21 degrees Celsius. The cable breaking strain has an ultimate tensile capacity of 110 kN.
In the tested configuration the test article consisted of the ATE-4 terminal end attached to a 33 m length of Nucor Test Level 4(TL-4) cable barrier. All foundation elements were embedded in AASHTO “standard” soil. The wire ropes were attached to the posts at heights of 380, 690, 790, and 890 mm above ground level. The cables at 380 mm (bottom cable), and 890 mm (top cable) being attached to the impacted side of the post. The remaining two cables (at 690 mm and 790 mm height) were attached to the opposite side.
Comparing ATE-4 to the original ATE
The key element of the ATE-4 is the trigger post assembly and foundations. The trigger post forms a junction between the anchor cables and the barrier cables, where the cables are located in slots and held in place by the barrier tension. When a vehicle impacts on the end of the barrier, the two terminal cables projecting from the ground strut are required to release from the terminal head, allowing the trigger post to bend backward under the tension of the barrier cables and releases the barrier cables out of the top of the trigger head, thereby releasing the tension from the barrier. For impacts after the Length of Need (LON), the terminal end is required to have sufficient strength and stiffness to remain an integral part of the barrier, providing normal re-directive performance.
The basic operating principles of the ATE-4 and the original three cable terminal end design are the same. Both systems utilize an angled slotted hole, open at the bottom to attach the anchor cables. When impacted end on, the cables are forced downward out of the slot releasing the tension in the barrier. The means of attaching the cables from the barrier are different between the two systems. The original system used a central slot into which the three cables were located. The cables were provided with lateral support and were only capable of exiting the trigger head in a vertical direction. All cables were located on the centerline of the system and position directly in line with the anchor cables. The ATE-4 uses a series of four offset angled slots, located with two on either side of the impact head. The slots are open laterally thereby allowing the cables to be released sideways and also by prying or rotation of the connection detail. It is unclear if lateral deflection of the cables near the terminal post would cause sufficient rotation such as to dislodge the cables from their slots however it is considered unlikely that this would influence the gating length of the system. Both the original three wire ATE and the new ATE-4 have a LON of 8 m incorporating 4 line posts at 2 m centers.
The ATE-4 also uses a bolted trigger post detail, comprising of two sections. This provides the trigger post with greater lateral stability and strength than the original design. The original ATE used a single mild steel post, located in a cast-in socket. The alignment between the anchor cables and the barrier cables in the ATE-4 has less eccentricity than the original design, reducing the potential for imposed moments in the trigger head. This reduces the potential of the system to result in out-of-balance forces when impacted along the length of the barrier. The lateral location (side mounted) of the cables in the trigger increases the potential for lateral rotation of the head under an impact.
The ATE-4 has additional tolerance for the height of the cables in the wire rope barrier. Recent four cable barriers, particularly TL-4 cable barriers, tend to have higher cables that will have a greater propensity to ride over the hood of an 820C vehicle and cause damage to the A-pillar when struck in a reverse direction impact. For the purpose of this test, the NUCOR TL-4 cable barrier system was selected as the top two cables are set at 890 mm and 790 mm respectively from ground level.
Evaluation of NCHRP Report 350 Test Matrix
The NCHRP 350 calls for a series of seven tests to validate the crash worthiness of a gating terminal end. The original ATE terminal end was subjected to a reduced matrix of four tests. This testing program was established in consultation with the FHWA and was based on previous testing programs of other wire rope terminal end systems. The four tests completed were 3-30, 3-32, 3-35 and a modified 3-39. As is industry practice, test 3-39 was modified from using a 2000P to using an 820C test vehicle as it was felt this posed the greatest potential for occupancy harm.
The changes between the original three-wire ATE and the new ATE-4, described in the previous section, were considered to have little impact on the crashworthiness of the system and as such the system could be validated using a further reduced test matrix. The rational for the reduced testing matrix is provided below.
Tests 3-30 and 3-32
In end on impacts such as in tests 3-30 and 3-32, the anchor cables and the ground strut have remained unchanged between the ATE-4 and the original system. The washers joining the two anchor cable ends and the angle at which the washers sit in relation to the anchor cables are also unchanged. The failure mechanism, namely the anchor cables being forced out of the bottom of the slotted details, is identical to that already tested with the original ATE and therefore it is deemed unnecessary to retest the new design to tests 3-30 and 3-32.
Test 3-35 is used to evaluate the required length of need of the system. The original three cable ATE system was tested and successfully met the acceptance criteria for this test. The ATE-4 system has an additional cable which should make the barrier system stronger than the previously tested system. The new ATE-4 system attaches the barrier cables to the trigger post using slots located on the side of the trigger head. As with the original ATE, the new trigger post is set at an angle so that during any longitudinal barrier impacts, such as test 3-35, the cable end fittings are pulled down into their respective receptacles, removing any possibility of them releasing prematurely.
The cables release from the trigger head of the new ATE-4 laterally. This release mechanism has the potential to release a cable due to prying of the cable end fitting or rotation of the trigger head detail. These forms of release action could potentially be caused by a redirect impact close to the trigger post. Test 3-35 is used to evaluate the length of need (LON) of the system with an impact occurring some distance downstream from the trigger post. The LON of the new ATE-4 is 8 m, incorporating 4 cable barrier line posts at 2 m centers. This is an identical LON to the previously tested three-wire ATE system. It was considered unlikely than the forces induced by test 3-35 would cause prying of the cables or rotation of the trigger post head and therefore it was considered unnecessary to repeat this test on the modified ATE-4 system.
Test 3-39 Modified
The release mechanism and geometry of the four cables at the top of the trigger post has been altered in the new ATE-4 system. The cable end fittings are required to release laterally rather than vertically as in the original system. The influence of this design change can be evaluated using test 3-39. In addition, it was felt that the modified release mechanism of the trigger post increased the potential for the vehicle snagging when impacting from the reverse direction. In the reverse direction impact it was felt the vehicle had the potential to become wedged between the cables before impacting the trigger post. The cables must release at the trigger post in order for this test to be successful. It was concluded to undertake test 3-39 in a modified format, with an 820C test vehicle, as this presented the greatest likelihood of snagging or wedging the vehicle.
The critical impact point (CIP) was selected as being 4.0 meters upstream from the top of the trigger post, both as this is L/2 (the LON is at 8 meters from the trigger post) and because this CIP provides for the maximum penetration of the 820C into the barrier before impacting the trigger post. In addition, at this CIP the vehicle will be impacting the barrier before the cables transition down to the trigger post and therefore allows the greatest opportunity for the vehicle to become “wedged” between them. The modified test 3-39 was completed without the use of a dummy mass. This resulted in the test inertial mass of the vehicle being 816 kg.
The Armorflex 4-Wire terminal end performed adequately in a reverse direction impact test 3-39 modified as seen in the Test Summary which is enclosed for reference. The vehicle remained stable and the direction of travel was not significantly altered as a result of impact with the test article. The detached parts of the barrier and terminal end, although projected some distance as a result of the vehicle impact, followed a similar trajectory to the vehicle so were considered not to pose a significant hazard to other road users and pedestrians any greater than that of vehicle itself. No detached elements, fragments, or other debris showed potential for penetrating the occupant compartment. There were no deformations recorded in the occupant compartment. The vehicle remained upright during and after the collision period. Occupant risk factors were low, and the exit trajectory of the test vehicle did not impose on adjacent lanes and remained within the limits imposed by NCHRP 350.
Therefore, the ATE-4 device described above and detailed in the enclosed drawings is acceptable for use on the NHS under the range of conditions tested, when acceptable to a highway agency.
Please note the following standard provisions that apply to FHWA letters of acceptance:
David A. Nicol, P.E.