Safety investigation and guidance for retrofitting existing approach guardrail transitions

Approach guardrail transition systems have been designed, tested, und evaluated according to various impact safety standards. Unfortunately, approach guardrail transitions that arc installed in the field may deviate from the as-tested configuration (e.g., by missing posts); this deviation reduces the desired lateral stiffness and strength of the transition system. Validated BARRIER VII computer models of 18-ft 9-in. (5.7-m)-long and 31-ft 3-in. (9.5-m) long systems of NCHRP Report 350 crashworthy approach guardrail transitions were used to gain an understanding of how they would perform, with and without deficiencies, when subjected to Test Level 3 impacts at various locations throughout each system. Each simulation was evaluated on the basis of three criteria: (a) maximum wheel-rim snag on the upstream edge of the bridge rail, (b) maximum dynamic deflection within the nested Thrie section, and (c) maximum vehicle pocketing angle within the nested Thrie section. According to simulation results, when transition posts were missing, installed adjacent to sloped terrain, or exposed more than 3 in. (76 mm), excessive dynamic deflection or vehicle snag on the upstream end of the bridge rail could occur. Recommendations and retrofits are provided to upgrade approach guardrail transitions for the safest performance.