Jason A. Hascall

Midwest Guardrail System with Round Timber Posts

A modified Midwest Guardrail System (MGS) was developed by using small-diameter round wood posts. The barrier system was configured with three timber species: Douglas fir (DF), ponderosa pine (PP), and southern yellow pine (SYP). Barrier VII computer simulation, combined with cantilever post testing in a rigid sleeve and soil, was used to determine the required post diameter for each species. The recommended nominal sizes were 184 mm (7.25 in.) for DF, 203 mm (8 in.) for PP, and 190 mm (7.5 in.) for SYP. A grading criterion limiting knot size and ring density was established for each species. The recommended knot sizes were limited to 38 mm (1.5 in.) or smaller for DF, 89 mm (3.5 in.) or smaller for PP, and 64 mm (2.5 in.) or smaller for SYP. The minimum ring densities equaled or exceeded 6 rings per inch (rpi) for DF, 6 rpi for PP, and 4 rpi for SYP. Two guardrail systems–-one using DF posts and another using PP posts–-were crash tested according to the Test Level 3 requirements specified in NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features. Crash testing was not conducted on the SYP system because of the adequacy of previous testing on 184-mm (7.25-in.) diameter SYP posts in a standard W-beam guardrail system and post design strength comparable to that in the other two species. Both crash tests showed that the modified MGS functioned adequately for both wood species. Three round wood post alternatives were recommended as an acceptable substitute for the standard W152×13.4 (W6×9) steel post used in the MGS.

Inertial effects during impact testing

Shortcomings were identified in historic dynamic guardrail post testing methods that could overestimate the strength of guardrail posts by a factor of two. This inaccuracy resulted from the influence of inertia that was not previously accounted for during dynamic component testing. The effects of inertia were verified and quantified with several analysis methods including nonlinear finite element analysis, which was used to develop a computer simulation of the dynamic tests. Several alternative testing procedures that could significantly reduce the effects of inertia were identified and investigated. One of these testing alternatives–-use of a crushable impact head–-was shown with computer simulation to reduce the effects of inertia and maintain the benefits of strain rate effects. The use of such an impact head is recommended for all future dynamic post strength testing. The results of the study imply that changes should be made not only in the standard design procedures used for guardrail and other vehicular barriers, which are clearly influenced by inertial effects, but in all designs that are based on dynamic strength testing. Dynamic testing is very common in the field of transportation engineering and care should be taken to ensure that inertial effects are considered when performing such tests.

Test Level 4 Noise Wall for Attachment to Concrete Traffic Barriers

A new transparent noise barrier system was developed for use on rigid bridge railings and other rigid structures. The crashworthy noise barrier system incorporated specially designed, strategically located structural hardware into a noncrashworthy noise barrier system. The noise barrier system was anchored to the back face of a reinforced concrete parapet with vertical steel posts. Transparent sound panels surrounded by a metallic frame were dropped between the spaced posts to create a noise barrier. Three horizontal rails were used to prevent errant vehicles from penetrating into sound panels, snagging on the support posts, or both. The test installation consisted of a 34.56-m-long Paraglas Soundstop TL-4 noise barrier system supported by 18 steel posts. Two full-scale crash tests—one with a single-unit truck and one with a pickup truck—were conducted and reported in accordance with the requirements specified in NCHRP Report No. 350: Recommended Procedures for the Safety Performance Evaluation of Highway...