Development of a Standardized Buttress for Approach Guardrail Transitions
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2020-11-10
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Edition:Final Report: 2014 – 2020
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Abstract:Approach guardrail transitions (AGTs) incorporate increased post and rail sizes, reduced post spacings, specialized buttress end geometries, and other roadway features to smoothly transition from deformable W-beam guardrail to rigid barriers. This transition in barrier lateral stiffness requires specific combinations of these components to function properly. Changing components, or even the addition or 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 thrie-beam 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, 31-in. tall, 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 shallower lower taper was designed to mitigate tire snag below the rail, while a steeper upper taper was designed to prevent vehicle snag and limit the unsupported span length of the guardrail. This buttress design was evaluated in combination with a critical AGT (i.e., an AGT with lower stiffness than other crashworthy AGTs) without a curb, representing a worst-case scenario. The standardized buttress was successfully crash tested to the 2016 Manual for Assessing Safety Hardware (MASH) Test Level 3 (TL-3) safety criteria. Guidance was provided for both the attachment of the buttress to various crashworthy thrie-beam AGTs as well as how to transition the shape of the buttress to adjacent bridge rails or rigid parapets downstream from the AGT.
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