One of the most common forms of transitions is an approach guardrail transition to a bridge rail. Such stiffness transitions are used to transition the stiffness from the more flexible approach guardrail to the stiffer barrier bridge rail. The stiffness is transitioned through the use of stiffer rail elements (e.g., nested rails) and changes to post size, embedment depth, and/or spacing. Proper transitioning of the stiffness mitigates barrier pocketing and vehicle snagging on the end of the stiffer barrier system. There are different configurations of approach guardrail transition systems. One of the more common configurations uses nested thrie beam rail and reduced post spacing adjacent to the bridge rail end. Such transitions are typically 18 ft-9 in to 25 ft in length. Site conditions, such as intersecting roads or driveways in proximity to the bridge end, can restrict the distance available to accommodate barrier length of need and make a shortened transition system desirable (see Figure 1). Under a project sponsored by the Roadside Safety Pooled Fund, a shortened approach transition, shown in Figure 2, was successfully developed and crash tested in accordance with the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH) Test Level 3 (TL-3) criteria (1, 2). The transition system uses a 6 ft-3 in section of nested thrie beam and a 6 ft-3 in asymmetric thrie beam-to-W-beam transition section with reduced post spacing. The transition further incorporates a lower rubrail element to help mitigate potential for vehicle snagging on the end of the rigid concrete bridge parapet. The system provides state departments of transportation (DOTs) with a desirable solution when limited space exists at the end of bridges.



When solid bridge rails or metal bridge rails mounted on concrete parapets are used, drainage occurs at the bridge ends. To avoid erosion around the bridge wing walls, the steep side slopes that are often adjacent to a bridge end, and the transition posts, state DOTs often control the location of the bridge end drainage using curb elements that abut the end of the bridge rail, such as shown in Figure 3. Such curbs are typically 4 to 6 inches in height and may have different shapes.



When curbs are used at bridge ends, the transition must be designed to incorporate the curb element. In certain scenarios, the presence of a curb element may help reduce bridge end snagging potential and eliminate the need for a rubrail below the primary transition rail element. The goal of this research is to expand the use and functionality of the previously developed shortened thrie beam transition (2) by incorporating a curb element to accommodate bridge end drainage. A MASH TL-3 transition of this type would be useful in areas with limited space and with a curb to accommodate bridge end drainage

Objective

The project objective is to develop a MASH TL-3 shortened thrie beam transition with curb. If successful, it will expand the use and functionality of the shortened thrie beam transition with rubrail that was developed under pooled fund project 618981. The design will incorporate a 4 to 6-inch curb to facilitate use of the transition at locations that have bridge end drainage.

Benefits

This project will assess MASH compliance and overall impact performance of a shortened thrie beam transition with curb. If this is verified through this research, the transition will provide state DOTs with a cost effective transition design for application at approaches to bridges that have limited space and utilize a curb to control drainage around the wingwalls and slopes.

Products

The TTI research team will provide a final report that will include the results of the simulation, testing, and evaluation performed under the project. The report will contain recommended design details for a shortened thrie beam transition with curb that can be attached to a rigid concrete parapet.

Work Plan

The work plan for this research includes four tasks. Details of these tasks are described below.

Task 1 – Determination of Recommended Transition Design
The research team will review previous and ongoing research projects to evaluate the characteristics of thrie beam transitions to rigid bridge rail parapets with curb. The previous short thrie beam transition design with rubrail successfully developed under pooled fund project 618981 will also be critically reviewed (2). Results from the review will be used to assess possible design changes for shortened thrie beam transition with a 4-inch to 6-inch-tall curb. Drawings of the recommended design configuration will be prepared and submitted to the technical representative for review and approval.

Task 2 – Simulation Analysis
Once the design details are approved by the technical representative, the research team will develop a finite element model of the short thrie beam transition with rubrail and validate the model against the previous MASH crash test. The validated model will then be revised to reflect the details of the recommended transition configuration with curb. Predictive simulations will be performed following MASH Test 3-20 and 3-21 impact conditions. The research team will use the simulation analyses results to recommend a transition design configuration for further evaluation through full-scale crash testing. The simulation results will also be used to determine the critical impact location for the full-scale crash test that will be conducted under Task 3.

The research team will evaluate the selected design configuration of the shortened thrie beam transition with curb according to MASH TL-3 criteria for transitions. A prototype of the recommended transition design will be constructed. The test installation will be comprised of the short thrie beam transition with curb attached to a vertical concrete bridge parapet on the downstream end and an approach W-beam guardrail on the upstream end. MASH Test 3-21 has been budgeted to evaluate the impact performance of the transition. Test 3-20 with a small passenger car is an optional test and is not budgeted under the current project. If there is a reasonable uncertainty regarding the impact performance of the transition system for impacts with small passenger cars based on the simulation results, Test 3-20 will be recommended. The test results will be used to assess MASH compliance of the shortened thrie beam transition with curb.

The research effort will be documented in a final report. The report will document all aspects of the research including simulation and full-scale crash testing of a shortened thrie beam transition to rigid parapet with curb. The report will include detailed drawings for the recommended transition design. A draft final report will be submitted to the technical representative for review and comment. Any comments received will be addressed in a revised final report.

Time Schedule

Started: May 2026 Time frame: 20 Months

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May 28, 2026