Split Single Slope Median Wall For Grade Separations (2011-TN/28)
PROBLEM STATEMENT
When widening of an existing divided highway with depressed median is proposed, adding inside lanes by using the depressed median opening is the preferred method since no additional right of way (ROW) is required. Extending the pavement at super elevations, however, requires the use of a retaining wall along with the concrete median barrier due to the grade separation that occurs.
BACKGROUND
Median barriers are typically installed to prevent errant vehicles from crossing the a divided area between travel ways so as to prevent a collision with oncoming traffic (AASHTO, 2004). The application of median barriers depend on a multitude of factors, including median width. traffic volume, adverse geometries (split elevations), and severity of consequences due to vehicular penetration into opposing traffic lanes (AASHTO, 2002; AASHTO, 2004). Special considerations are taken when the travel ways are at different elevations.
The correct median barrier must be selected such that the maximum dynamic deflection that occurs is less than one half of the median width (AASHTO, 2002). The barrier should prevent the errant vehicle from penetrating into oncoming traffic lanes and redirect the vehicle to the correct direction of travel (AASHTO, 2004).
Median barrier can be categorized as flexible, semi-rigid, and rigid (AASHTO, 2002). Examples of typical median barriers include, weak post W-beam, three strand cable, box beam median barrier, blocked out W-beam strong post, blocked out thrie beam strong post, modified thrie beam median barrier, and concrete barrier (AASHTO, 2002).
The use of W-beam, box beam, and cable systems are limited to flat medians and are typically not used when a split elevation between traffic ways greater than 3:1 occur (AASHTO, 2004). In the case of split elevation highways with little to no median width, such as when inside lanes are added by using the depressed median, dynamic deflection is restricted. In this situation, use of rigid barriers are most appropriate.
Concrete median barriers are the most common types of rigid barriers and include the New Jersey, F-shape, single slope barrier and vertical walls (AASHTO, 2002). These systems present low life cycle cost due to their effective performance and maintenance free life. For the research conducted herein, the single slope barrier will be considered exclusively (AASHTO, 2002).
A single slope barrier can have either a 9.1 or 10.8 degree slope and may be used as either a temporary or permanent longitudinal barrier (Beason et al., 1989). Each has been successfully tested according to criteria presented in NCHRP Report 350 (Ross et al., 1993, AASHTO, 2002). The primary advantage of the single slope barrier is that the pavement adjacent to the sloped face may be overlaid multiple times without degrading the performance of the barrier (Beason et al., 1989). These barriers are typically 42 inches tall, but may be found as short as 30 inches (AASHTO, 2002).
OBJECTIVE
The purpose of the study is to explore the use of two independent single slope barriers to provide a median barrier on split level highways. The scope of the study will include analysis of the structure and stability of the wall, as well as finite element modeling to determine its crashworthiness.
BENEFITS
The use of two independent half size single slope barrier walls, backing up to each other, provides design and construction flexibility as shoulder elevations vary along the road. This type of design and construction provides an economical way to construct a median wall on split elevation highways.
PRODUCTS
TTI will provide a detailed design of two independent single slope barriers for use as a median wall. TTI will generate standard sheets including design details and drawings based on the study results of the proposed device.
IMPLEMENTATION
If acceptable test results are achieved, methods for the use of two independent single slope barriers as a median wall will be provided. Details and drawings will be used to develop standard sheets. These standards will be provided in the Task Force 13 format for submittal to AASHTO to be included in the AASHTO/ARTBA/AGC Barrier Hardware Guide.
WORK PLAN
Task 1 –Slope stability and geotechnical analysis
The researchers will investigate the strength of the system based on a geotechnical prospective. The loads imposed on the wall due to soil pressures and superimposed lane loading will be determined. In addition, the soil slope stability will be analyzed.
Task 2 – Wall structural stability
The researchers will determine the wall’s structural strength to resist soil and any superimposed loads from traffic on the higher elevation. In addition, the adequacy of the wall’s foundation will be analyzed.
In the case the wall cannot adequately retain the soil, a new design will be investigated and guidelines proposed for the implementation of the installation.
Task 3 – Perform finite element analysis for median crashworthiness
The researchers will investigate and determine the crashworthiness of the split single slope median wall by performing finite element analysis for a proposed installation. The crashworthiness of the median will be verified by simulating the impact conditions according to the Manual for Assessing Safety Hardware (MASH) test 3-11 (AASHTO, 2009). Test 3-11 involves a 5000-lb pickup (designation 2270P) impacting the test article at 25 degrees and 62 mph. The research team feels that performing test 3-10 with the 2425-lb passenger car (designation 1100C) is unnecessary.
Task 4 – Evaluation and Reporting
The researchers will generate a final report documenting the findings from the analysis and design details and drawings. In addition, the report will include drawings and details that may be used in the development of standard sheets for AASHTO Task Force 13.
| TTI Research Supervisor: Chiara Silvestri, Ph.D. Texas Transportation Institute Texas A&M University System 3135 TAMU College Station, Texas 77843-3135 (979) 845-8971 c-silvestri@ttimail.tamu.edu |
Pooled Fund Technical Representative: Jeff Jones, Director Design Division Tennessee Department of Transportation Suite 1300 James K. Polk State Office Building Nashville, TN 37243-0348 (615) 741-2221 Jeff.C.Jones@state.tn.us |
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AASHTO (2002). Roadside Design Guide. American Association of State Highway and Transportation Officials. Washington, D.C.
AASHTO (2004). A Policy on Geometric Design of Highways and Streets. American Association of State Highway and Transportation Officials. Washington, D.C.
AASHTO (2009). Manual for Assessing Safety Hardware. American Association of State Highway and Transportation Officials. Washington, D.C.
Beason, W. L., J. Ross, H. E., et al. (1989). “Development of a Single-Slope Concrete Median Barrier.” Research Report 9429CDK. Texas Transportation Institute, College Station, TX.
Ross, H. E., D. L. Sicking, et al. (1993). “Recommended Procedures for the Safety Performance Evaluation of Highway Features.” NCHRP Report 350. Washington D.C.
