Development and Implementation of the Simplified Midwest Guardrail System Stiffness Transition

The varied lateral stiffness between bridge rails and approach guardrail systems may cause vehicle pocketing or wheel snagging to occur near rigid bridge rail ends. To mitigate this potential hazard, an approach guardrail transition (AGT) is used to provide a gradual increase in the lateral stiffness of the barrier between the W-beam guardrail system and the bridge railing. However, these transitions can also cause a propensity for vehicle pocketing or wheel snagging if the change in lateral stiffness occurs too rapidly. Recently, a stiffness transition based on NCHRP Report 350 was developed for use with the Midwest Guardrail System (MGS) and a stiff Thrie beam AGT, and successful testing was performed close to the upstream end of the AGT. The transition was designed with three sizes of steel posts, one of which was nonstandard for state departments of transportation. Thus, a simplified version of the original MGS stiffness transition that used two common sizes of steel posts was developed and was subjected to full-scale crash testing according to Test Level 3 as set forth in the Manual for Assessing Safety Hardware. Subsequently, dynamic post properties obtained from bogie testing and numerical simulations were used to develop an equivalent wood post version of the simplified MGS stiffness transition. Recommendations are made regarding the attachment of the stiffness transitions to FHWA-accepted Thrie beam bridge rail AGTs.