Guardrails on Slopes – Phase II (405160-20)

To See Phase 1

Problem Statement

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 (1) 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 710-25.

Background

Earliest known research about guardrail placement on slopes was conducted by ENSCO, Inc. (2) which included a battery of pendulum tests on a single post and three full scale crash tests. Two tests of a large sedan impacting a G4(1S) guardrail system installed on a break point of a 2H:1V slope were considered to be successful to redirect the sedan per NCHRP Report 230. One of the tests had a 6 ft post length while the other had a 7 ft post length. The 7 ft post length installation had a better performance (rail deflection and vehicle impact speed change) than the 6 ft post length installation.

Polivka, et al (3) performed another battery of bogie tests and a crash test of steel post guardrail system with a 2000P test vehicle per NCHRP Report 350 Test level 3. A region that encompassed the impact point had 7 ft long W6x8.5 steel posts placed 3 ft-1.5 inches on center. These posts were placed on the break of a 2H:1V slope with 4 ft-7 inch embedment depth. The crash test was considered successful per NCHRP Report 350 test evaluation criteria.

During the first phase of this research (4), TTI researchers conducted a full scale crash test of a 27-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 and Figure 3.

Figure 2 -- Cross section of the guardrail on slope system tested by TTI.

Figure 3 -- Guardrail on slope system as tested by TTI.

The system was 175 ft in total length and was comprised of 12 gauge W-beam mounted on W6x8.5 steel posts. The guardrail length of need was 100 ft, and a 37.5 ft long ET Plus terminal anchored the guardrail on each end. A 2H:1V sloped ditch was excavated behind the rail to represent the sloped terrain. The ditch was centered along the installation length and was 68 ft-9 inches long and 8 ft wide. Six-ft long posts were placed at 6 ft-3 inch spacing on the flat terrain portion of the guardrail. Along the sloped section, the 8-ft long posts were placed at 3 ft-1.5 inch spacing. Standard size 6 inch x 8 inch x 14 inch routed wood blockouts were used in the length of need section.

The crash test conducted was NCHRP Report 350 test designation 3-11, which involves a 2000 kg (4409 lb) pickup truck impacting 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. The vehicle was redirected by the guardrail system; however it rolled on its side upon exiting the guardrail system. Figure 4 shows sequential photos of the impact event that lead to the rollover of the 2000P test vehicle.

Figure 4 -- Impact sequential of TTI test 405160-4-1.

Objective

The objective of this study is to identify an acceptable method for installing standard strong-post W-beam guardrail [Modified G4(1S)] with the face of the rail aligned with the break point of a 2H:1V slope.

Benefits

A guardrail system in which the face of the rail is aligned with the slope break will provide significant savings in shoulder width in mountainous areas as well as other locations that have very restrictive space.

Products

TTI will provide composite video and photographic documentation of the crash test and a final report suitable for submittal to Federal Highway Administration (FHWA) documenting the research and testing performed.

Upon successful completion of full-scale crash testing in Phase II, TTI will provide all the supporting information and any needed written discussion for submitting a request to FHWA for acceptance of the guardrail placed on slope design for use on the National Highway System.

If the impact performance of the guardrail system is found to be acceptable, TTI will also provide drawings of the system in a format suitable for incorporation into the AASHTO-ATRBA-AGC Task Force 13 Barrier Hardware Guide.

Implementation

As stated above, TTI will provide all the supporting information and any needed written discussion for submitting a request to FHWA for acceptance of the guardrail on slope design to be developed under this study for use on the National Highway System.

Engineering drawings of the system will be provided to facilitate development of standard detail sheets and specifications for the new guardrail system. Drawings provided for Task Force 13 guides will further support implementation of the research.

Work Plan

Task 1 – Perform Engineering Review and Computer Simulation

The researchers will review the design details of guardrails on slope previously developed to evaluate the behavior of the guardrail when subjected to NCHRP Report 350 tests. Design features that have been found to be important in terms of mitigating the rollover event of the guardrail are rail height, block out depth, and post placement. It is apparent that the front left tire of the vehicle snagged on one or more of the guardrail posts as shown in Figure 5 below.

Figure 5 -- Post twisted and pushed back from an apparent front left tire snagging.

The previous crash test will be simulated to calibrate the vehicle and guardrail system models. It should be noted that current vehicle models lack sufficient fidelity in the suspension model to capture common tire/suspension failure modes. Once the model is sufficiently calibrated against data from the previous crash test, a series of impact simulations will be performed to investigate design modifications and establish the sensitivity of the guardrail’s impact performance to selected design variables.

The results of these simulations will be summarized and submitted to the state pool fund technical representative along with a recommended design for further evaluation through full-scale crash testing.

Task 2 – Perform Full-Scale Vehicle Crash Test

The researchers will perform test 3-11 of NCHRP Report 350 (2000P vehicle, 100 km/hr, 25 deg) on the selected guardrail design. It is believed that this is the critical test for this design and that test 3-10 (820C vehicle, 100 km/hr, 20 deg) is not required. TTI will provide the test facility, test vehicle, instrumentation of the vehicle, high-speed film, video, still photographs, and a final report suitable for submittal to Federal Highway Administration (FHWA).

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(1.) Design Manual M 22-01, Chapter 710 “Traffic Barriers”, Washington State DOT, January 2005.
(2.) Stout, D., et al. “Force-Deflection Characteristics of Guardrail Posts”, Report FHWA-RD-88-193, Performed by ENSCO, Inc., August 31, 1988.
(3.) Polivka, K. A., et al. “Development of a W-Beam Guardrail System for Use on a 2:1 Slope”, Midwest Roadside Safety Facility, October 16, 2000.
(4.) 4. Abu-Odeh, A. Y. et al. “Crash Testing and Evaluation of the Modified G4(1S) W-Beam Guardrail on 2:1 Slope.”, Texas Transportation Institute, November 2008.