Median barriers are needed to prevent cross-over accidents. Roadways with ample median space, a flexible median barrier such as a cable barrier can be used. However, when median width is limited, a rigid concrete barrier is typically used. Concrete median barriers are typically cast-in-place continuous concrete barriers. These barriers can also be slipped formed. In some instances, underground utilities and other structures are located in the alignment of the rigid median barrier. There is a need to span across these structures/underground utilities with something other than the rigid median barrier.
The purpose of this research is to design a transition railing attachment that can span across and open space in the median barrier. This median barrier attachment will be crash tested to Manual for Assessing Safety Hardware (MASH) Test Level-3 (TL-3) (1).
The design tested for this project will enable the State Departments of Transportation (DOT)’s the ability to use a MASH TL-3 crashworthy gap transition across an open space in a rigid concrete median barrier. A 36-inch tall single slope median barrier is planned for this project. If the results of the full-scale crash test are successful per MASH 2016, this barrier design will be submitted to FHWA for eligibility for federal funds reimbursement.
There is a need for a crashworthy barrier structure to span across open gaps in rigid concrete median barriers. Sometime underground utilities and other structures are located in the alignment of rigid concrete barriers located in the median. The TTI research team has performed MASH crash testing on rigid concrete median barriers. The TTI research team tested a median barrier embedded 10 inches into soil base (TTI Project No. 405160-13). This research presents a design to restrict lateral deflection of a concrete barrier when placed adjacent to steep slopes or on top of Mechanically Stabilized Earth (MSE) walls, without using a concrete moment slab. This design was developed through the use of full-scale finite element vehicle impact analysis and crash testing. The design incorporates precast 20-ft long single slope barrier segments with grouted rebar grid connection. The barrier segments are embedded 10 inches in soil and are placed in front of a 1.5H:1V slope. The offset of the barrier from the slope break point of the soil embankment is restricted to a minimum of 2 ft. MASH Test 3-11 was performed to evaluate the performance of the embedded barrier. The barrier performed acceptably. The permanent lateral deflection of the barrier was 5.5 inches. Figure 1 illustrates a photo of the design and Figure 2 illustrates a brief cross-section detail of the design.
Oftentimes, obstructions or other features are located along the alignment for barrier in the median. There is a need to span across these obstructions or features. This results in open gaps in rigid concrete barriers located in the median. Manholes and drainage inlets are often obstructing the continuity and the median barrier placement. Due to access reasons, these manholes and other features do not permit the use of rigid barrier located directly on top of the structure. Therefore, a barrier structure of some type is needed to span across gaps in the median barrier.
The objective of this research is to design a tubular barrier gap rail system for use on a 36-inch high single slope barrier. Thrie-beam, W-beam, and tubular rail elements will be considered for the barrier rail design. The maximum open gap used for the design will be 8 ft. The new design will be tested to MASH TL-3. TTI has received preliminary details from Minnesota Department of Transportation (MinnDOT) on the barrier details that will be considered for the design. The TTI research team will incorporate much of this information into the initial design concept(s). Figure 3 illustrates these details. Figure 4 illustrates a preliminary concept developed for the barrier rail system.
Figure 3. Proposed Median Barrier Details from MinnDOT for Type 36A Barrier.
Figure 4. Preliminary Details for Median Barrier Gap Rail.
This project will provide a new barrier gap rail design that can be used on 36-inch tall median barriers with a maximum gap opening of 8-ft for MASH TL-3 Applications.
The TTI research team will generate a report providing the research results on the performance of a new median barrier gap rail design for use median barriers meeting the requirements of MASH TL-3. The report will provide detailed engineering drawings of the design for implementation of median barrier projects requiring MASH TL-3 requirements.
Detailed engineering drawings of the approved MASH TL-3 crash tested gap barrier design will be available immediately upon completion of the project.
The TTI researcher will work closely with the project Technical Representative to develop engineering details for the new gap barrier system. Engineering analyses will be performed to determine the strength of the barrier system to meet the strength requirements of MASH TL-3. These drawings and details will be developed and submitted for approval prior to the construction and final drafting effort planned for Task 2.
Upon completion of Task 1, TTI will develop and finalize full-scale test installation drawings that accurately reflect the details developed from Task 1. After receiving final approval of the test installation drawings from the technical representative, TTI will construct a full-scale test installation. Sufficient length of the barrier system will be constructed for performing MASH Test Designations 3-10 and 3-11.
Upon completion of Task 3, TTI will perform full-scale crash tests on the new gap barrier system with respect to MASH TL-3 specifications. Two crash tests are planned for this project.
After completion of the crash testing, TTI will prepare a final report that summarizes the details of the test installation, final drawings, and the findings and conclusions. If the results of the full-scale crash test will be deemed successful per MASH 2016, this study will generate documentation needed to support a request for FHWA eligibility.
|TTI Research Supervisor:
William F. Williams, P. E.
Associate Research Engineer
Texas A&M Transportation Institute
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Michael Elle, P.E.
Design Standards Engineer
Minnesota Department of Highways
395 John Ireland Blvd, MS696
St. Paul, MN 55155-1899