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38 Time = 0.000 seconds Time = 0.061 seconds 18 17 16 15 14 Time = 0.119 seconds Time = 0.180 seconds Time = 0.241 seconds Time = 0.361 seconds Time = 0.480 seconds Time = 0.600 seconds Figure 22. Sequential photographs for TTI Test 471470-26 (top) and G4(2W) finite element simulation (bottom), overhead view. PARAMETRIC ANALYSES element simulations. Another variable that could be inves- USING COMPUTER SIMULATIONS tigated is vehicle speed. It is of interest to highway engi- neers to know the maximum impact speed that a system can withstand. Such information could be used for determining As demonstrated in the simulations, the potential for either which system would be the most effective along a given stretch barrier failure or vehicle vaulting can be assessed in much the of roadway where site and operating conditions are known. same way that physical crash tests are evaluated. The advan- Due to the fact that the project had limited funds, the proj- tage of finite element simulations is that once a model is ect team and panel had to balance the number of simula- developed and validated, the impact conditions, as well as the tions with the number of possible scenarios that could be basic geometry of the installation, can be varied easily. Per- investigated. forming ten finite element simulations with the curb located at different distances from the face of the post, for example, would be straightforward and inexpensive and would allow Analysis of CurbBarrier Combinations the analyst to determine the effect of the curb offset on the performance of the barrier. Likewise, curbs with heights vary- Analyses involving curbbarrier combinations were per- ing from 0 to 300 mm could be evaluated easily using finite formed using the LS-DYNA finite element software. A matrix

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TABLE 9 Summary of major impact events of test 471470-26 and G4(2W) finite element simulation (46 ) G4(2W) Summary of impact events Full-scale test Finite element simulation Time (sec) Speed Time (sec) Speed (km/h) (km/h) Initial Contact 0.000 100.8 0.000 100.8 Vehicle starts to yaw 0.056 100.8 0.044 100.6 Wheel impacts post 15 0.104 90.2 0.101 91.3 Wheel impacts post 16 0.193 74.8 0.190 75.7 Rear of vehicle contacts 0.203 73.2 0.207 73.0 guardrail Wheel Detaches 0.215 69.4 0.215 71.3 Vehicle parallel with 0.283 68.0 0.264 69.0 guardrail Vehicle exits guardrail = 13.5E 64.0 = 14.3E 63.0 Front View Overhead View Figure 23. Finite element simulation of a 2000P vehicle striking a G4(2W) with a 150-mm-high AASHTO Type B mountable curb.

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40 G4(2W) with Curb G4(2W) without Curb Figure 24. Sequential photographs of finite element simulations comparing the impact performance of the G4(2W) with and without the AASHTO Type B curb. of simulations was developed to provide information regard- beam), G2 (weak-post W-beam), or G1 (weak-post cable), ing the impact performance of the G4(1S) guardrail system in but it was decided to investigate combinations of curbs with combination with various types of curbs at impact conditions the more widely used systems. The G4(2W) and the modi- specified by NCHRP Report 350 Test 2-11 and Test 3-11. fied G4(1S) (i.e., steel posts with wood blockouts) are widely Both these tests involve the 2000-kg pickup truck impacting used systems and were good candidates for the research. at 25 degrees. The impact speed for Test 2-11 is 70 km/h, Since both systems have successfully passed NCHRP Report which is in the intermediate speed range (i.e., 60 to 80 km/h), 350 TL-3 impact conditions, poor performance of these sys- and the impact speed for Test 3-11 is 100 km/h, which rep- tems combined with a curb can be directly attributed to the resents the higher speed range (i.e., > 80 km/h). The perfor- presence of the curb and not necessarily to structural inade- mance of certain curbbarrier systems was also investigated quacy of the barrier systems. Since there were a limited num- at 85 km/h, which represented the upper limit of intermedi- ber of analyses that could feasibly be conducted, only the ate speed roadways (i.e., 60-80 km/h). modified G4(1S) guardrail was used in the study so that the There are many barrier systems that could have been maximum number of curb types and impact conditions could investigated in the study, such as the G4(2W), G9 (thrie- be investigated. The G4(1S) is the most widely used strong-

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41 post guardrail in the United States, thus information regard- bility of the vehicle and (2) to provide information regarding ing its performance with curbs should be the most beneficial the trajectory and path of the vehicle after impact with curbs. to the states. Most of the curb impact studies that were identified in the NCHRP Report 350 Test 2-11 and Test 3-11 impact condi- literature involved vehicles encroaching the curb in a track- tions were chosen for the matrix of simulations because they ing manner. Another aspect of collisions with curbs involves involve the 2000-kg pickup, which is much more unstable than an out-of-control vehicle impacting the curb in a nontracking the 820-kg small car and also produces a more severe impact position. In these situations, vehicle tripping may be highly due to the larger mass of the pickup. The simulations were probable during impact. Nontracking impacts with curbs may used to determine the most effective curbbarrier combinations result in vehicle instability and rollover, especially impacts for those impact conditions. involving vehicles with high centers of gravity. The side friction between the tires and ground for an out- of-control vehicle will cause the vehicle to roll, such that the Analysis of Vehicle Impacts with Curbs vehicle has an initial roll-rate at the onset of impact with the curb. This factor is much more significant for vehicles with Analyses involving the simple impact of a vehicle and curb a high center of gravity, such as pick-up trucks and SUVs were also investigated using LS-DYNA. There are a number which make up a large percentage of the vehicle population of variables that would have been interesting to investigate in currently on the road. this study, such as vehicle type (e.g., small car, pickup, SUV), As documented in NHTSA's Rollover Status Report in curb type, impact speed, and angle of impact. Due to limita- Traffic Safety Facts 1996 (51), rollover crashes, particularly tions in time and computational constraints, only a limited single-vehicle (SV) accidents in light pickup trucks and number of impact conditions were investigated. A matrix of SUVs, continue to take the lives of thousands of Americans simulations was developed to provide information regarding each year. In 1996, almost 9,500 passenger vehicles (e.g., the vehicle's response when crossing a number of different passenger cars, pickup trucks, vans, and SUVs were involved curb types at various impact conditions. The information col- in fatal rollover crashes. Rollovers accounted for 36% of all lected in this phase of the study served two purposes: (1) to fatal crashes involving SUVs and 24.5% of all fatal crashes quantify the effects of vehicle impact with curbs on the sta- involving pickup trucks, as illustrated in Figure 25. It is also Figure 25. Rollover occurrence as a percent of all crashes, by vehicle type and crash severity (51).