Bridge rail design procedure

The AASHTO Bridge Specifications recommend a yield line theory analysis to determine the structural capacity of concrete bridge railing based on static strength of concrete. However, this analysis technique has been shown to significantly underestimate the capacity of concrete bridge rails to withstand high speed truck impacts. Traditionally this shortcoming has been mitigated by artificial reductions in bridge rail design loads implemented into the design specifications. Fear of litigation associated with failure of a bridge rail to contain and redirect an errant vehicle has made continuing this policy unacceptable for most state highway agencies. On the other hand, existing barrier design guidelines contained in the Bridge Specifications are based upon National Cooperative Highway Research Program (NCHRP) Report 350. This document has been superseded by the Manual for Assessing Safety Hardware (MASH). The updated performance guidelines incorporate heavier vehicles, higher impact angles, and in one case, higher impact speeds. Full-scale crash testing has shown that the new testing criteria will require stronger and taller barriers. One study has attempted to generate new height and design load requirements for inclusion in the updated Bridge Design Specifications. The load recommended for implementation proved to be extremely high and was not well received by AASHTO's T7 committee on Guardrails and Bridge Rails. Thus, there is a national need for a more thorough evaluation of bridge rail design loads and minimum barrier heights required to meet the MASH guidelines. This thesis presents an improved method based on modified Yield Line Theory and dynamic strength of concrete to estimate the design impact loads to realistic levels without adjustment of the underlying analysis technique. The net effect of applying the new method to the design would be large decreases in the size and cost of bridge railing necessary to withstand the elevated loads. The objective of the research proposed herein includes: (1) Developing improved methods for estimating the structural capacity of bridge rails and cantilevered deck systems based on dynamic strength of concrete, and including the contribution of deflection of deck overhang, moment of inertia of the barrier and deck overhang sections, and mass of the vehicle and barrier and (2) Identifying appropriate design loads for use in the new methods that are representative of MASH recommended crash test conditions TL-2 through TL-5.