Development of a Standardized Buttress for Approach Guardrail Transitions

Approach guardrail transitions (AGTs) incorporate increased post and rail sizes, reduced post spacings, and specialized buttress end geometries to smoothly transition from deformable W-beam guardrail to rigid barriers. This transition in barrier stiffness makes AGTs sensitive systems that require specific combinations of these components to function properly. Changing components, or even the removal of a curb below the rail, can negatively affect the safety performance of an otherwise crashworthy system. However, recent full-scale crash testing has indicated that a properly designed buttress at the downstream end of an AGT may be utilized with multiple AGT systems. Thus, the objective of this project was to develop a standardized buttress to reduce vehicle snag and be compatible with a wide variety of previously developed Thrie beam AGT systems, either with or without a curb. The standardized buttress was designed with a dual taper on its front upstream edge. A longer lower taper was designed to mitigate tire snag below the rail, while a shorter upper taper was designed to prevent vehicle snag and limit the unsupported span length of the rail. This buttress design was evaluated in combination with a critically weak AGT without a curb, which represented the worst-case scenario. The standardized buttress was successfully crash tested to MASH TL-3. Guidance was provided for both the attachment of the buttress to various Thrie beam AGTs as well as how to transition the shape of the buttress to adjacent bridge rails or rigid parapets downstream of the AGT.