Ronald K. Faller

Midwest Guardrail System for Standard and Special Applications

Development, testing, and evaluation of the Midwest Guardrail System were continued from the original research started in 2000. This new strong-post W-beam guardrail system provides increased safety for impacts with higher-center-of-mass vehicles. Additional design variations of the new system included stiffened versions using reduced (half and quarter) post spacings as well as a standard guardrail design configured with a concrete curb 152 mm (6 in.) high. All full-scale vehicle crash tests were successfully performed in accordance with the Test Level 3 requirements specified in NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features. The research study also included dynamic bogie testing on steel posts placed at various embedment depths and computer simulation modeling with BARRIER VII to analyze and predict dynamic guardrail performance. Recommendations for the placement of the original Midwest Guardrail System as well as its stiffened variations were also made.

DEVELOPMENT OF A NEW GUARDRAIL SYSTEM

The W-beam guardrail system has been the standard in the United States since the late 1950s and has proved to perform reasonably well under most impact conditions. However, in recent years the vehicle fleet has changed to include a relatively large percentage of light trucks, such as pickups, vans, and sport-utility vehicles. These vehicles have a higher center of mass and bumper mounting height than conventional automobiles and have been shown to have higher rollover and injury rates during guardrail accidents than conventional automobiles. Standard W-beam guardrails were not designed to capture the bumper of many of these vehicles. In recognition of the potential safety problems associated with light-truck accidents, safety performance standards were recently changed with the publication of NCHRP Report 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features. These performance standards require all new safety hardware to be tested with a full-size three-quarter-ton pickup to ensure acceptable performance for most vehicles in the light-truck category. In recognition of this, a guardrail system capable of capturing and redirecting a larger range of vehicle types and sizes was developed. A new guardrail system, called the Buffalo Rail, was designed with a new cross-sectional shape with an effective depth of 311 mm (compared to 194 mm for the W-beam), a rail thickness of 13 gauge, and a post spacing of 2500 mm. The safety performance of the Buffalo Rail was found to be acceptable according to the procedures and criteria recommended for the three-quarter-ton pickup truck at Test Level 3 in NCHRP Report 350.

Performance of Steel-Post, W-Beam Guardrail Systems

On the basis of the proposed changes to the NCHRP Report 350 guidelines, NCHRP Project 22-14(2) researchers deemed it appropriate to first evaluate two strong-post W-beam guardrail systems before finalizing the new crash testing procedures and guidelines. For this effort, the modified G4(1S) W-beam guardrail system and the Midwest Guardrail System (MGS) were selected for evaluation and comparison. Five full-scale vehicle crash tests were performed with the two longitudinal barrier systems, in accordance with the Test Level 3 (TL-3) requirements presented in the NCHRP Report 350 Update. For the modified G4(1S) testing program, two 2,270-kg pickup truck vehicles (2270P vehicles) were used: one 3/4-ton, two-door vehicle and one 1/2-ton, four-door vehicle. For the MGS testing program, two 2,270-kg pickup truck vehicles (2270P vehicles) and one 1,100-kg small-car vehicle (an 1100C vehicle) were used, with both pickup truck configurations being evaluated. On the basis of several findings, the NCHRP Project 22-14(2) researchers determined that the 1/2-ton, four-door pickup truck was better suited for use as a surrogate light truck test vehicle than the 3/4-ton, two-door pickup truck. The modified G4(1S) W-beam guardrail system, mounted at the top rail height of 706 mm, provided acceptable safety performance when it was crashed into by the 1/2-ton, four-door pickup truck vehicle, thus meeting the proposed TL-3 requirements presented in the NCHRP Report 350 Update. Testing of the modified G4(1S) W-beam guardrail system was not successful with a 3/4-ton, two-door pickup truck under the TL-3 impact conditions. The MGS was found to meet the TL-3 criteria presented in the NCHRP Report 350 Update for Test Designations 3-10 and 3-11. Satisfactory safety performance was observed with the MGS with both the 1/2-ton, four-door and 3/4-ton, two-door pickup truck vehicles.

Midwest Guardrail System W-Beam-to-Thrie-Beam Transition

For longitudinal barriers, it is common practice to use a standard W-beam guardrail along the required highway segments and to use a stiffened thrie-beam guardrail in a transition region near the end of a bridge. As a result of the differences in rail geometries, a W-beam-to-thrie-beam transition element is typically used to connect and provide continuity between the two rail sections. However, the W-beam-to-thrie-beam transition element has not been evaluated according to current impact safety standards. Therefore, an approach guardrail transition system, including a W-beam-to-thrie-beam transition element, was constructed and crash tested. The transition system was attached to Missouris thrie-beam and channel bridge railing system.

Midwest Guardrail System for Long-Span Culvert Applications

Long-span guardrail systems have been recognized as an effective means of shielding low-fill culverts. These designs are popular because, in comparison with other systems that attach posts to the top of the culvert, they are able to shield the culvert safely while creating little additional construction effort and limiting the damage to the culvert and the need for repair. However, previous long-span designs were limited by the need to use long sections of nested guardrail to prevent rail rupture and by the need for large lateral offsets between the barrier and the culvert. The Midwest Guardrail System (MGS) long-span design eliminates those two shortcomings by applying the benefits of the MGS to a long-span design. The MGS long-span design increased vehicle capture and stability because of the increased rail height, limited the potential for pocketing and wheel snag through the use of controlled-release terminal posts adjacent to the unsupported span, and greatly increased the tensile capacity of the rail through the movement of splices away from the posts and the use of shallower post embedment. These features allowed the system to be developed without the use of a nested guardrail and with a minimal barrier offset that placed the back of the guardrail posts even with the front face of the culvert. Two full-scale crash tests were conducted with the MGS long-span design according to the requirements in NCHRP Report 350 for Test Level 3 Test Designation 3-11. Both tests were conducted with the heavier 2,270-kg pickup truck to generate higher rail loads and dynamic deflections. The MGS long-span design met all safety requirements of NCHRP Report 350. The ability of the guardrail with the MGS long-span design to perform safely without a nested rail and a minimal barrier offset makes this new barrier a functional and safe option for the protection of low-fill culverts.

Midwest Guardrail System Adjacent to a 2:1 Slope

A W-beam guardrail is often used to protect motorists from steep roadside slopes adjacent to high-speed roadways. Although previously designed systems have demonstrated acceptable safety performance, the long posts and half-post spacing have proven to be costly and introduce maintenance challenges. Using longer posts is more economical to users than having a system with posts installed at half-post spacing. Furthermore, the improved redirective capacity of the Midwest Guardrail System (MGS) provides the opportunity to eliminate the need for half-post spacing and thereby greatly reduces the cost of placing a barrier at the slope break point. A stiffened version of the MGS was developed for use adjacent to steep roadside slopes. The new design incorporates 2,743-mm (9-ft) long posts with 1,905-mm (75-in.) spacing. With the top of the W-beam mounted at a height of 787 mm (31 in.), this guardrail was successfully crash tested according to the currently proposed NCHRP Report 350 Update safety performance evaluation criteria. Hence, the stiffened MGS guardrail design with full post spacing is acceptable for use on the National Highway System. This new guardrail design will provide a safe and economical alternative for use along highways with steep slopes very close to the travelway.

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.

TWO TEST LEVEL 4 BRIDGE RAILING AND TRANSITION SYSTEMS FOR TRANSVERSE TIMBER DECK BRIDGES

The Midwest Roadside Safety Facility, in cooperation with the Forest Products Laboratory, which is part of the U.S. Department of Agriculture’s Forest Service, and FHWA, designed two bridge railing and approach guardrail transition systems for use on bridges with transverse glue-laminated timber decks. The bridge railing and transition systems were developed and crash tested for use on higher-service-level roadways and evaluated according to the Test Level 4 safety performance criteria presented in NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features. The first railing system was constructed with glulam timber components, whereas the second railing system was configured with steel hardware. Eight full-scale crash tests were performed, and the bridge railing and transition systems were acceptable according to current safety standards.

APPROACH GUARDRAIL TRANSITION FOR CONCRETE SAFETY SHAPE BARRIERS

An approach guardrail transition for use with concrete safety shape barriers was developed and crash-tested. The transition was constructed with two nested thrie-beam rails, measuring 2.66 mm thick, and supported by nine W150 x 13.5 steel posts. Post spacings consisted of one at 292 mm, five at 476 mm, and three at 952 mm. Structural tube spacer blockouts were used in the transition system. The system successfully met the Test Level 3 requirements specified in NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features.

Long-Span Guardrail System for Culvert Applications

A long-span guardrail for use over low-fill culverts was developed and successfully crash tested. The guardrail system was configured with 30.48 m of nested, 12-gauge W-beam rail and centered around a 7.62-m-long unsupported span. The nested W-beam rail was supported by 16 W152×13.4 steel posts and 6 standard CRT posts, each with two 150-mm×200×360 mm wood block-outs. Each post was 1830 mm long. Post spacings were 1905 mm on center, except for the 7.62-m spacing between the two CRT posts surrounding the long span. The research study included computer simulation modeling with Barrier VII and full-scale vehicle crash testing, using 3/4-ton (680-kg) pickup trucks in accordance with the Test Level 3 (TL-3) requirements specified in NCHRP Report 350. Three full-scale vehicle crash tests were performed. The first test was unsuccessful because of severe vehicle penetration into the guardrail system. This penetration resulted from a loss of rail tensile capacity during vehicle redirection when the swagged fitting on the cable anchor assembly failed. A second test was performed on the same design, which contained a new cable anchor assembly. During vehicle redirection, the pickup truck rolled over and the test was considered a failure. The long-span system was subsequently redesigned to incorporate double block-outs on the CRT posts and crash tested again. Following the successful third test, the long-span guardrail system was determined to meet TL-3 criteria.