The AASHTO Roadside Design Guide recommends that guardrail be installed with the back edges of the guardrail post being 2 ft from a slope break. In many mountainous areas or in locations with tight environmental controls this width is difficult to provide. As a result, designers often have to make a trade-off between reduced shoulder width and a less than optimal guardrail placement. The WSDOT Design Manual provides for the placement of the guardrail post closer to or on slopes as steep as 1H:1V as illustrated in Figure 1.
Figure 1. Allowable Post on Slope Installation Cases from WSDOT Design Manual, Page 710-725
During the preceding phase of this research, TTI researchers conducted two full scale crash tests of a 31-inch high guardrail system placed on 2H:1V slope. The posts were placed 1-ft from the slope break such that the face of the guardrail was aligned with the slope break as shown in Figure 2.
Figure 2. Cross-Section of Guardrail on Slope System Tested by TTI.
Figure 3. Guardrail on Slope System as Tested by TTI.
Figure 4. Sequentials of the MASH Test 3-11 and 3-10 Crash Tests.
The crash tests conducted were MASH Test designation 3-11 and MASH test designation 3-10, which involves a 2270 kg pickup truck and a 1100 kg small car respectively. Both vehilces tests were setup so as they impact the CIP of the length of need section at a nominal speed of 100 km/h (62 mi/h) and a nominal angle of 25 degrees. Both vehicles redirected successfully as shown in the above sequential images and both tests metrics were within the acceptable criteria of MASH guidelines. Hence, both tests are considered pass and an eligibility letter is in the process of being issued by the FHWA.
Subsequently, TTI researchers conducted a research investigating the feasibility of guardrail placement on steeper slopes such as 1H:1V through nonlinear finite element analyses. Simulations for both MASH TL-3-10 and MASH TL-3-11 indicated a likelihood of successful outcome of testing for a regularly spaced posts 31-in tall W-beam guardrail system. The slope is 1H:1V. The posts were placed 1-ft from the slope break such that the face of the guardrail was aligned with the slope break. The posts are 9-ft long in the sloped section. The images below show the redirection of both test vehicles as predicted in the simulations.
Figure 5. Simulation of MASH Test 3-11 and 3-10 Crash Tests on Steeper Slope.
The objective of this study is to develop a MASH TL-3 compliant guardrail system meeting tests (3-11 and 3-10) for installing standard strong-post W-beam guardrail [Modified G4(1S)] with the face of the rail aligned with the break point of a 1H:1V slope.
This testing will provide a MASH compliant system to replace the non-tested and non-standard, inadequate systems, in place in hazardous locations.
A guardrail system in which the face of the rail is aligned with the slope break will provide significant increase in shoulder width in mountainous areas as well as other locations that have very restrictive space. This increased shoulder width will reduce nuisance hits while providing increased safety.
TTI will provide a report documenting the crash tests and their results per MASH recommendations. If both crash tests are deemed successful per MASH criteria, then TTI team will help in the preparation of an eligibility request as part of the project deliverables.
Implementation plans depends on the outcome of the crash tests and are to be defined for potential Phase V of this project.
The test installation drawing will be developed and TTI proving ground crew will construct the first test installation.
TTI will conduct the first MASH TL-3 test and upon the success of the first test, TTI proving ground shall rebuild the test installation and conduct the second MASH test. The sequence of which test shall be conducted first, 3-10 or 3-11 will be finalized during the early part of the project.
TTI will prepare documentation of both tests and help develop the content needed for the request of eligibility letter if both tests pass the MASH test evaluation criteria.
If the results of the full-scale crash tests will be deemed successful per MASH 2016, this study will generate documentation needed to support a request for eligibility.
|TTI Research Supervisor:
Akram Abu-Odeh, Ph.D.
Texas A&M Transportation Institute
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Joe Hall, P.E., P.S.
Technical Policy QA/QC Engineer
Value Engineering Coordinator
West Virgina Division of Highways
1334 Smith Street
Charleston, WV 25305-0430