John D. Reid

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.

DEVELOPMENT OF THE MIDWEST GUARDRAIL SYSTEM

A revised guardrail system has been developed that should provide greatly improved performance for high-center-of-gravity light truck vehicles. The barrier incorporates W-beam guardrail and standard W6×9 steel posts. Primary changes to the design include raising the standard rail height to 635 mm, moving rail splices to midspan between posts, increasing blockout size, and increasing the size of post bolt slots. All of these changes were designed to improve the barrier’s performance with high-center-of-gravity vehicles. One full-scale crash test was conducted to verify that the guardrail would perform adequately with mini-sized automobiles when raised to 660 mm to the center of the rail. This test proved that the barrier can provide satisfactory performance when mounted at heights ranging from 550 mm (standard guardrail height) up to 660 mm. Hence, the new guardrail design provides approximately 110 mm (4.4 in.) of mounting height tolerance. When installed at the nominal mounting height of 635 mm, a 75-mm pavement overlay could be applied to the roadway without requiring adjustments to the barrier’s height.

DESIGN AND SIMULATION OF A SEQUENTIAL KINKING GUARDRAIL TERMINAL

The process of using non-linear, large deformation finite element analysis to design a new guardrail terminal system is summarized. The simulation program LS-DYNA was used to develop a sequential kinking process for energy dissipation. The sequential kinking process involves using a deflector plate to force a steel beam guardrail element to be bent around a rigid beam until it forms a plastic hinge. Deformation is then localized at the plastic hinge until the hinge contacts the deflector plate and a new kink develops. Critical steps in the design process include selection of the deflector plate geometry and design of structural components required to support the system during a full-scale vehicular impact. This paper describes the use of LS-DYNA for these critical portions of the design process. Predictions of the energy dissipation for the sequential kinking impact head were only 7% below values obtained from dynamic impact tests.

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.

LS-DYNA simulation influence on roadside hardware

Several new roadside safety appurtenances that can be directly linked to the use of nonlinear finite element analysis with LS-DYNA are being installed along the nations roadways. These advances can be attributed to FHWAs sponsored research program known as Centers of Excellence in DYNA3D Analysis. Some of the simulation efforts at the Midwest Roadside Safety Facility over the past 10 years are documented to help educate the roadside safety engineer and to promote the use of simulation. Noteworthy progress has been made in the use of LS-DYNA for the design and analysis of roadside safety hardware, and it is believed that a great deal more progress is possible.

DEVELOPMENT OF A SEQUENTIAL KINKING TERMINAL FOR W-BEAM GUARDRAILS

A new tangent energy-absorbing W-beam guardrail terminal that meets NCHRP Report 350 criteria has been developed. The terminal, designated the SKT-350, dissipates the energy of an encroaching vehicle by producing a series of plastic hinges in the W-beam as the terminal head is pushed down the guardrail. This energy-absorption concept allows for significantly lower dynamic forces on the encroaching vehicle, reducing the vehicle damage, the weight of the terminal head, the propensity for vehicle yaw and roll after impact, and the chances of buckling in the W-beam section. The energy required to move the head down the rail in this design is optimized for current criteria, but by modifying the bending geometry in the head, the average force to displace the head down the rail can be adjusted from values ranging from 11 to 60 kN (2,500 to 13,500 lb), meaning that the system can be easily modified to meet any future changes in safety performance standards. In addition to these important safety advantages, the terminal incorporates a unique cable anchor bracket that closely resembles a breakaway cable terminal anchor and a novel foundation tube design that facilitates the removal of broken posts during repair. Combining the features of reduced forces and head weight, a simple cable box, and more economical soil tubes allows the system to offer the advantages of both reduced cost and improved performance.

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.

Indenting, buckling and piercing of aluminum beverage cans

Analysis of indenting, buckling and piercing of aluminum beverage cans using both physical testing and computer simulation was performed in order to develop a better understanding of the sidewall structural strength of the cans. This understanding can be used to help design better cans and/or improve manufacturing processes.Simulation of the sidewall indentation was done with an impacting sphere. Parameters investigated included the sphere size and velocity, and the impact height along the sidewall. Buckling simulation of the dented can was then performed. Results from the deformed can buckling model compared well with physical testing based on buckled geometry, buckling load, and external work to buckle. Severe damage to a can that might occur during the manufacturing process was investigated by studying piercing of the can sidewall. Impacts of 5 and 10 m/s were performed with both blunt (flat) and sharp (45° tip) steel rods. It was found that separated elements with tied nodal constraints more accurately represent the failure behavior of the can subjected to a piercing load than merged element nodes. It was also found that the more crushing a can undergoes before piercing occurs, the more energy the can material absorbs. However, there is an upper limit to the crushing based on the speed and shape of the impactor.

Admissible modeling errors or modeling simplifications

Abstract Discovering modeling errors on large finite element models is complex and can often go unnoticed, particularly once good “correlation” is achieved. The signs of trouble and/or inaccuracies need to be recognized and, at the very least, the effects of system simplifications on a models accuracy need to be known. One danger in using simplified models occurs when the models, which have been correlated to test results, are used for predictive work. When predictive work begins, it is extremely difficult to determine if model “simplifications” have a small or large influence on the predictions. Fortunately, there are things that can be done to help recognize potential trouble. Similar to the examples shown in this paper, a library of small test models should be created in order to evaluate modeling techniques and code features. The evaluation should include determining limitations of the techniques and features that may cause modeling errors when applied to larger systems. This is analogous to the design process itself, where components must first be designed and thoroughly tested and understood before they can be reliably used in larger systems.