Final Report: Single Slope Median Wall for Grade Separations
|TTI Research Supervisor:
Chiara Silvestri Dobrovolny, Ph.D.
Associate Research Scientist
Texas A&M Transportation Institute
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Ali Hangul, P.E.
Civil Engineering Manager
Tennessee Department of Transportation
James K. Polk State Office Building
Nashville, TN 37243-0348
This research explored design options of median barriers for use as grade separation on split level highways. Such type of barrier needs to provide design and construction flexibility as shoulder elevations vary along the roadway and also perform as a retaining wall. Strength and stability of the barrier were investigated to evaluate the structural ability to provide adequate stability with respect to sliding, overturning, and bearing capacity. Crashworthiness of the barrier design(s) chosen were investigated through finite element modeling analyses at American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH) Test Levels 3 and 4.
An initial barrier design was proposed by Tennessee Department of Transportation (TDOT) considered the use of two independent half-size single slope barrier walls backing up to each other. The researchers proposed a slightly different design of median barrier for grade separation consisting in removing the small barrier (51-inch barrier) and maintaining only the tall barrier (112.5-inch barrier). Because of the possibility for the median vertical wall proposed design to be considered a hazard for head contact during a vehicle-barrier collision, the researchers and TDOT worked together to propose an alternative median barrier design option which should resolve the head slap concern. The median vertical wall was modified into a single slope median barrier of a total maximum height of 112.5 inches, a soil backfill height of maximum 60 inches acting on one side, and 4H:21V slope on both sides of the barrier. Stability and yield line analyses for the sloped barrier design were evaluated for the proposed designs, as well as their crashworthiness and stability through finite element analyses for MASH test level 3 impact conditions. These analyses resulted in acceptable barrier performance according to the criteria set forth in MASH for longitudinal barriers, and soil retention according to AASHTO 2007.
In a second phase of the project, researchers optimized the minimum barrier segment length needed to resist soil forces and MASH TL-3 and TL-4 impact conditions. The barrier segments stability and crashworthiness were evaluated according to the LRFD method and computer simulations.