Investigating the use of Small-Diameter Softwood as Guardrail Posts (Dynamic Test Results)
Jason Hascall, Ronald Faller, John Reid, Dean Sicking
A modified version of the Midwest Guardrail System (MGS), utilizing small-diameter round wood posts, was developed, tested, and evaluated. Three systems were developed using different species of timber, Douglas Fir, Ponderosa Pine, and Southern Yellow Pine. A combination of Barrier VII computer simulation modeling and several series of cantilever bogie tests, conducted in both a rigid sleeve and soil, were used to determine the required diameter of post for each species in order to serve as an equivalent substitute for the standard steel post used in the barrier system. The final recommended nominal sizes were determined to be 184 mm (7 1/4 in.) for Douglas Fir, 203 mm (8 in.) for Ponderosa Pine, and 190 mm (7 1/2 in.) for Southern Pine. In addition, a grading criteria limiting knot size and ring density was established for each species. The final recommended knot sizes were limited to 51 mm (2 in.) and smaller for Douglas Fir, 102 mm (4 in.) or smaller for Ponderosa Pine, and 62 mm (2.5 in.) or smaller for Southern Yellow Pine. The minimum ring density for each post species was to exceed or equal 6 rings-per-inch for Douglas Fir, 6 rings-per-inch for Ponderosa Pine, and 4 rings-per-inch for Southern Yellow Pine. Two of the guardrail systems, one using Douglas Fir posts and another using Ponderosa Pine posts, were full-scale vehicle crash tested and reported in accordance to the Test Level 3 (TL-3) requirements specified in the National Cooperative Highway Research Program (NCHRP) Report No. 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features. Testing of the third system, using Southern Pine posts, was not conducted based on the adequacy of previous testing on 184-mm (7 1/4-in.) diameter Southern Yellow Pine posts on the standard W-beam guardrail system as well as a comparable post design strength to that of the other two species. The two full-scale crash tests showed that the modified Midwest Guardrail System functioned adequately for both wood species. Therefore, the three round wood post alternatives will serve as an acceptable substitute to the standard W152x13.4 (W6x9) steel post utilized in the MGS.
Highway Safety, Wood Posts, Round Posts, MGS, Longitudinal Barrier, W-Beam Guardrail, Douglas Fir, Ponderosa Pine, Southern Yellow Pine, Crash Testing
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