Full-scale 4, 500-pound vehicle impact tests at 60 mph and 25 degrees were conducted on four new guardrail-bridgerail "transition" designs for use in nebraska in which the first wood post from the bridge end was left out. The post was left out to represent a common field problem in which a concrete footing prevents installing the post.To compensate for the missing post, a stronger beam member and heavier wood posts were used in addition to a 4:1 tapered end on the concrete bridgerail. The tapered end was used to (1) reduce the unsupported span length and (2) provide a smooth guardrail deflection curve during vehicle redirection. All of the transition designs were identical except for the transition beam member. the designs consisted of two heavy 10 inch x 10 inch posts followed by four heavy 8 inch x 8 inch posts. The remaining posts were standard 6 inch x 8 inch posts. Over a guardrail length of 18 feet 9 inches, the posts were spaced 3 feet 1.5 Inches on centres, whereas, over the remaining length, a standard post spacing of 6 feet 3 inches was used. The posts were installed in a "native" silty clay (type cl) soil. In terms of the evaluation guidelines in national co-operative highway research program (NCHRP) report 230, the overall performance of the transition designs was as follows: single thrie beam transition--unsatisfactory, double thrie beam transition--satisfactory, tubular thrie beam transition--satisfactory, double w-beam transition-unsatisfactory.
Abstract