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Evaluation of Alternate Cable Anchor Designs and Three-Cable Guardrail Adjacent to Steep Slope

REPORT NUMBER

TRP-03-155-05

AUTHORS

Ryan Nelson, Dean Sicking, Ronald Faller, John Reid, John Rohde, Karla Lechtenberg (Polivka), Jason Hascall

PUBLICATION DATE

2005-02-03

ABSTRACT

The three-cable guardrail system is most commonly used to protect motorists from roadside slopes. Due to the fact that the system is limited to sites where relatively large barrier displacement is acceptable, concern arises when the barrier system must be placed close to steep roadside slopes. An additional concern with regard to the use of cable guardrail is that there only exists one approved non-proprietary end terminal which utilizes a large, cast-in-place, reinforced concrete anchor block. The current terminal and anchor design has proven to be quite costly when compared to the entire system. The objectives of the study were: (1) to evaluate three cable anchor designs in order to develop a more cost-effective anchor alternative and (2) to evaluate South Dakota's three-cable barrier system placed adjacent to 1.5H:1V slopes. Dynamic bogie testing of various anchor alternatives determined the capacity of each anchor. Subsequent, computer simulation modeling with BARRIER VII analyzed and predicted dynamic barrier performance with each anchor alternative. One full-scale crash test, 2,034-kg (4,484-lb) pickup truck impacting at a speed of 98.1 km/h (61.0 mph) and at an angle of 26.2 degrees, was conducted and reported in accordance with the requirements specified in the National Cooperative Highway Research Program (NCHRP) Report No. 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features. The full-scale crash test was conducted to verify that the cable guardrail adjacent to a steep slope performs adequately with the weakest anchor alternative, the driven steel post as shown through computer modeling, but was determined to be unacceptable according to the Test Level 3 (TL-3) evaluation criteria specified in NCHRP Report No. 350 due to vehicle vaulting and rollover. The poor performance observed in the full-scale crash test warranted design modifications. Consequently, additional computer simulation with BARRIER VII predicted that increasing the cable guardrail stiffness with reduced post spacing and increasing the offset from the slope breakpoint significantly improved the barrier's performance. Therefore, it is recommended that a full-scale crash test be conducted on the modified cable barrier to verify its safety performance. Furthermore, the steel post anchor design performed as intended and provided adequate cable tension for the barrier system. However, since the impacting vehicle was not contained nor smoothly redirected, it is recommended that the new anchor alternatives continue to be evaluated in the future.