Presence of an intersecting secondary road close to a bridge on the main road requires special guardrail design due to limitation of space needed to transition the guardrail for attaching it to the bridge parapet. Furthermore, the curved region of the guardrail at the intersection of the two roads results in a more stringent design conditions due to higher impact angle that an errant vehicle can achieve prior to impact. A special guardrail system, commonly known as short radius guardrail (SRG) system is used for this purpose. Due to the higher impact severity, complexity of the design, and a relatively large number of design impact conditions, there are very limited number of SRG systems available. National Cooperative Research Program (NCHRP) Project 15-53 developed a Manual for Assessing Safety Hardware (MASH) Test Level 3 (TL-3) compliant SRG system (1,2). This SRG system was crash tested with an 8-ft radius. In the same research, impact simulations were conducted with 16-ft and 24-ft radii of the SRG system. The researchers concluded that the design was expected to perform successfully for these radii as well. Under this project, SRG system design was developed for 90-degree intersections. The guardrail system extended approximately 45 ft along the secondary road. In-field applications of the SRG system often require variations from the crash tested design conditions. Due to the complexity and sensitivity of the SRG system to design variations, there is a need to evaluate MASH compliance of common in-field design variations. Among these are effects of using larger radii to allow large truck turning, use of shorter side-road length for restricted spaces, and use on intersections where the secondary and main roads intersect with a larger or smaller angle than the crash-tested 90-degree intersection.

 

The research objective is to evaluate MASH performance of SRG systems for various design variations using finite element (FE) modeling and simulation. The research team will evaluate design variations that will allow use of the SRG system developed under NCHRP Project 15-53 for additional site conditions. These variations will include evaluating larger SRG turn radii, shorter secondary-road guardrail length, and variations in intersection angles between the main and secondary roads.

Benefits

Successful performance assessment of the SRG system with common field variations will expand the use of the system to more site conditions.

Products

The TTI research team will provide a final report that will include the results of the simulation analyses performed under the project and design details of SRG system variations that were determined to be MASH compliant.

Work Plan

The work plan for this research includes the following tasks.

Task 1: Develop Simulation Matrix
In this task, TTI research team will develop a simulation matrix with input from the Technical Representative and the members of the Roadside Safety Pooled Fund states. The research team will develop and conduct a poll of the member states to determine and prioritize SRG design variations for evaluation under this project.
In the current understanding of the research team, following variations are expected to be considered under this project.

– Radius: 32-ft, 40-ft, and 96-ft radii of the SRG system
– Intersection Angle: Up to ±45-degree variation from the standard 90-degree intersection
– Side-Road Length: 12.5-ft reduction in guardrail length

The research team will determine the SRG system design variations that are included in the final simulation matrix based on the results of the polling and subject to the funds needed to perform modeling and simulation of the variations with the allocated project funds. It should be noted that not all design variations require the same amount of modeling and simulation effort. Change in SRG system radius, for example is a significant change that requires remodeling of significant portions of the SRG system. On the other hand, reduction in guardrail length of the side-road is expected to be a relatively minor modeling change. Depending on the nature of the modeling changes prioritized in the simulation matrix, the time and effort to implement these changes in the FE models are expected to be vary. Both significant and minor changes, however, still require performance of impact simulations for the various MASH test impact conditions – which has similar impact in terms of time and effort. Since the exact nature of the SRG system changes is not fully determined at this time, the research team will prioritize design variations in consultation with Technical Representative to stay within the project funds allocation.

Task 2: Conduct Simulation Analysis
In this task, the TTI research team will perform modeling and simulation of the SRG system design variations prioritized in Task 1. The research team will build upon the SRG system FE model developed and used by researchers of NCHRP Project 15-53. This model is available to the TTI research team. The research team will make modifications to this existing model to develop FE models of the selected design variations. For each design variation, the research team will perform impact simulations using impact conditions of MASH. It should be noted that MASH does not provide a specific test matrix for evaluating SRG systems, which have functional similarities with longitudinal barriers and crash cushions. While an appropriate test matrix is currently being debated and researched in the roadside safety community, NCHRP Project 15-53 used MASH Tests 3-31, 3-32, 3-33, and 3-35 for evaluating the SRG design that has formed the basis of this new research. The research team will therefore use impact conditions and evaluation criteria of the same test designations for the simulation analysis. To optimize the number of simulations, the research team will only analyze test conditions that are considered necessary for the evaluation of a particular design variation. For each design variation, the research team will assess MASH compliance based on simulation results. It should be noted that this research project was not scoped to include crash test evaluation of the SRG system design variations. SRG systems are inherently sensitive to design changes and FE models have limitations with regard to material rupture, bolts and weld failures, etc. For these reasons, the research team will take a conservative approach in assessing performance of the SRG system design variations when considering their MASH compliance.

Task 3: Develop Design Guidance and Final Report
In this task, TTI research team will develop guidance for variations in SRG system design with respect to the as-tested design. The guideline is expected to be in the tabular format, providing ranges of acceptable design parameters that can be considered MASH compliant based on simulation results. The research team will also prepare a final report documenting the work performed in this research, including the simulation matrix selected, FE modeling and simulation results, and the design guidance for the SRG system.

Time Schedule

Started: April 2025
Time frame: 24 months

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April 9, 2025