Development of Timber Bridge Railing and Approach Guardrail Transition

Timber bridges with transverse and longitudinal bridge decks are increasingly utilized across the United States. However, limited research exists to develop bridge railing systems for high-service-level roadways that meet current vehicular impact safety standards. In response to this gap, the United States Department of Agriculture - Forest Service -Forest Products Laboratory (USDA-FS-FPL), in collaboration with the Midwest Roadside Safety Facility (MwRSF), initiated a research program to develop, crash test, and evaluate higher-service-level bridge railings and approach guardrail transitions (AGTs) compliant with the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH). This study continued a multi-phase project aimed at developing, crash testing, and assessing a MASH Test Level 4 (TL-4)-compliant glued-laminated (glulam) timber bridge railing system with a curb, as well as a MASH TL-3 AGT system. A comprehensive literature review identified critical deck configurations and parameters, including deck thickness, cantilevered overhang dimensions, and panel geometry. Additionally, soil-interaction dynamics for AGT posts were examined through prior bogie impact testing. Cost-effective design strategies were proposed to minimize the exposure of timber deck elements to prolonged water accumulation beneath the asphalt wearing surface. The AGT system design was based on an earlier crash-tested configuration. Dynamic impact testing of single, double, and triple AGT posts embedded in soil was conducted, yielding valuable insights into their impact behavior and performance. Data from these tests informed the calibration of BARRIER VII models, enabling the development of an optimized AGT system. Prototyping and iterative designing facilitated the creation of a reliable connection between the bridge railing and AGT systems. A critical impact point (CIP) investigation further identified locations most prone to failure under MASH TL-3 impact conditions for a final AGT system recommendation. The study culminated in the design of four dynamic component tests to evaluate critical timber deck configurations and connections. Test TRTD-1, involving a 7,186-pound bogie impacting a surrogate post on a 5⅛-inch-thick transverse Douglas Fir-Larch glulam deck, revealed premature tensile splitting of the deck due to insufficient restraint. This issue was addressed by introducing timber bolts through the scupper to enhance deck performance. Advisor: Ronald K. Faller