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15 Figure 11. Guardrail damage in TTI Test 404201-1. the 102-mm AASHTO type G curb was installed in combi- nation with a strong-post guardrail system. Test NEC-2 was conducted with impact conditions recommended in NCHRP Report 350 TL-3. The test vehicle was a 2000-kg pickup truck (1994 GMC 2500) and the impact speed and angle were 100.3 km/h and 28.6 degrees, respectively. The center of grav- ity of the test vehicle was 667 mm. The test installation was a modified G4(1S) guardrail with routed wood blockouts. In order to reduce the potential for rup- ture of the rail, two layers of 12-gauge W-beam were nested over a 26.67-m section of the guardrail. This modification was incorporated based on the results of test NEC-1, con- ducted in the first phase of the study, in which a splice rup- ture occurred during impact. The total length of the guardrail Figure 10. Guardrailcurb installation for TTI test was 53.34 m, including an inline breakaway cable terminal 404201-1. located at both ends of the system. The vehicle vaulted during impact and was airborne for the curb-and-barrier system at 101.8 km/h at an angle of much of the impact event. While the vehicle was airborne, it 25.2 degrees (i.e., NCHRP Report 350 Test 3-11). did get over the rail, as shown in Figure 14; however, the During the test, there was significant movement of the vehicle remained upright, came down on the front side of the anchor system as the foundation of the anchor posts moved in guardrail, and satisfied all safety requirements of NCHRP excess of 70 mm. The test was successful; however, there was Report 350. A summary of test NEC-2 is shown in Figure 15, considerable damage to the guardrail system, as shown in Fig- which was taken from Polivka et al. ure 11. The extent of damage to the system was much greater than that of previous crash tests on the G4(2W) guardrail sys- tem without a curb present (31). From reviewing the film from EFFECTS OF CURB TRIP ON VEHICLE STABILITY the crash test and the test report, it is believed that the exces- sive damage to the system is due, in part, to the use of poor DeLeys and Brinkman, 1987 (33 ) grade posts in the guardrail installation. Many of the posts split vertically during impact along preexisting splits passing Computer simulation was used in a study to determine the through the bolt hole location in the posts, as shown in Fig- dynamic response of small and large passenger cars travers- ure 12. A summary of Test 404201-1 is shown in Figure 13. ing various sideslope, fill-embankment, and ditch configura- tions. Both tracking and nontracking departures from the Polivka et al., 2001 (32 ) roadway were investigated. A modified version of HVOSM was used in this research that improved the program's appli- This study involved the second phase of the curb-and- cation to rollover situations. The modifications to the program barrier impact investigation conducted by MwRSF, in which were made by McHenry Consultants, Inc. These modifications

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16 included further development of the tire model and the addi- tion of a tire/deformable-soil interaction model to the program. A literature review and analysis of accident data recorded in the 197981 National Accident Sampling System (NASS) was performed; some of the principal findings from that review are quoted below: Embankments, ditches, and culverts are the roadside ter- rain features cited as being most frequently involved in overturn accidents. However, detailed information on the geometry of the terrain and whether the rollover was caused by vaulting, or by the wheels hitting a small obsta- cle, or by the wheels digging into soft soil and tripping the vehicle is generally lacking in accident data files. In most (50% to 80%) of the rollover accidents, the vehi- cles were skidding out of control at a large yaw angle prior to overturning. About half of all accidental departures from the road- way occurred at path angles greater than 15 degrees, and the majority of the vehicles were estimated to have been traveling at speeds less than 64 to 80 km/h. Full-scale tests were performed with an instrumented 1979 VW Rabbit automobile to provide data for evaluating the validity of the modified computer program. The tests included spinout of the car on level turf, dragging the car over a sod field, traversals of fill-embankments, and traversals of the front slope of a wide ditch. Motion-resistance force data were collected in these tests. They were used for obtaining tire/ ground coefficients of friction for typical roadside terrain surfaces, as well as for validating the computer simulation models. The drag tests were performed by attaching two steel cables to the center of the front and rear wheels on the right side of the vehicle. A load cell was installed on each cable to measure the forces as the vehicle was pulled sideways over the ground surface at speeds of 16 to 24 km/h. The data from the tests indicated that the average coefficient of friction between the tires of the VW Rabbit and the sodded ground surface was typically about 0.5. The modified version of HVOSM provided reasonable accuracy of the simulations of the tests on the various road- side terrains. The authors do point out, however, that "the study did not thoroughly establish the extent to which the model accounts for all of the various real-world conditions that contribute to vehicle rollover" (33). Over 200 HVOSM simulations of vehicles traversing var- Figure 12. Posts split vertically during TTI Test 404201-1 ious sideslopes, fill-embankments, and ditch configurations along preexisting splits in posts. were used to determine how much these roadside conditions affect the rollover tendencies of vehicles. In addition to the VW Rabbit model (1093-kg vehicle) that was developed and validated with the full-scale tests, two other vehicles were modeled: one was a relatively light vehicle and the other a much heavier vehicle. The lighter vehicle had a mass of

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Figure 13. Summary of results of TTI Test 404201-1 from Bullard and Menges. 17

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18 The conclusions that the authors made from the study, that pertain to the use of HVOSM for predicting the dynamic response of vehicles traversing various types and shapes of terrain, are presented below: The modified HVOSM has been demonstrated to be capable of predicting the response of vehicles operating on off-road terrains with reasonable accuracy. The development and incorporation of the deformable-soil model in HVOSM is considered an important improve- ment since it allows simulation for the effects of tire sinkage in soil which has been identified as one of the Figure 14. NCHRP Report 350 Test 3-11 impact with leading causes of rollover. However, evidence of the modified G4(1S) guardrail with nested 12-gauge W-beams validity of the deformable-soil model is clearly still very and a 102-mm curb under the rail (32). limited. The relatively few simulations that resulted in vehicle rollover in this study point to the dynamic nature of the 816 kg and was identical to the VW Rabbit model, except rollover phenomenon, which is sensitive to the complex that the mass and moments of inertia were different. The interactions of many factors whose effects are not inde- heavier vehicle model had a mass of 2,018 kg, representing pendent. Adequate vehicle parametric data for the severe the larger class of passenger cars, and its physical character- operating regime associated with the rollover response istics were defined in HVOSM using available data typical are generally lacking. Among the most important of for that vehicle type. these are definitive data for tire properties under the Figure 15. Summary of results of Test NEC-2 from Polivka et al. (32).

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19 high tire load and large slip and camber angle conditions Eight full-scale tests were conducted using four different that prevail in most rollover events. vehicle types to examine the rollover mechanics of vehicles Ultimately, the vehicle rollover potential associated with tripped by a curb, rolled off a dolly, and tripped by tire-soil roadside features is reflected by real-world accident expe- interaction. The test matrix and results from the study are rience. From the literature review performed as part of the presented in Table 3. study, it is apparent that the existing accident data base For the curb impact tests, a 152-mm-square section of steel lacks the comprehensive and detailed information neces- box tubing, rigidly affixed to the roadway, was used to repre- sary to define the conditions that lead to rollover for dif- sent a curb. The vehicles were towed sideways and released ferent vehicle types. For example, data contained in acci- just prior to contact with the curb. The friction between the dent data files, such as NASS and FARS, usually provide tires and the road surface was reduced by applying soap film little or no information regarding the geometrics of to the roadway. In order to more accurately represent the the accident site (e.g., steepness of slopes, embank- impact conditions of vehicles in real-world accidents, where ment height and roundings), whether the vehicles were an initial roll of the vehicle would be produced from the tire- tripped by a surface irregularity or as a result of tire ruts ground interaction, a roll angle of 2.5 degrees was built into in soft soil, where rollovers were initiated with respect to the test vehicles by extending the left suspension with wood the terrain feature (sideslope, backslope, toe of embank- blocks. ment, etc.), vehicle trajectory, and so forth Two of the five curb impact tests resulted in rollover. The three vehicles that did not rollover sustained excessive dam- Cooperrider et al., 1990 (34 ) age to their wheels or axles during impact. Failure or partial failure of these components may result in a reduction of load Researchers at Failure Analysis Associates, Inc. (FaAA) applied to the vehicle, which reduces the potential for roll- performed a study to investigate the mechanics of vehicle over. The tripping force must be applied for sufficient dura- rollovers. It was their perception that the experimental and tion to cause rollover. For the vehicles that did roll over, the analytical methods that were being used at that time (late average maximum decelerations at the center of gravity was 1980s) did not accurately represent real-world vehicle roll- 12.4 Gs, compared with maximum decelerations of 1.62 Gs overs. Their investigation involved full-scale tests in which and 1.3 Gs in the soil trip tests and dolly tests, respectively. vehicles were tripped by three different trip mechanisms: The curb trip tests resulted in peak angular velocities of sliding into a curb, sliding in soil, and being thrown from a 260 deg/sec and 300 deg/sec. The peak angular velocities in dolly. They also developed a simple analytical technique to the soil trip tests were similar with values of 230 deg/sec and characterize the mechanics of these different trip modes based 390 deg/sec. The peak angular velocity of the vehicle in the on a constant force method. dolly test was 460 deg/sec, which was much higher than the TABLE 3 Test matrix for Cooperrider et al. study (34 ) Test Vehicle model Trip method Test speed Results no. (km/h) 1 1981 Dodge Challenger Curb 48.1 no rollover 2 1981 Dodge Challenger Curb 47.6 rollover 3 1979 Datsun B210 Curb 47.2 rollover 4 1972 Chevrolet C20 Van Curb 47.6 no rollover 5 1981 Chevrolet Impala Curb 48.6 no rollover 6 1981 Dodge Challenger Dolly 48.6 rollover 7 1981 Dodge Challenger Soil 54.2 rollover 8 1979 Datsun B210 Soil 43.5 rollover