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55 UNL OLS2 Crash Simulation of OLS2 Test Crash Test t = 0 ms t = 0 ms Figure 46. Simulated guardrail missing on post. t = 126 ms t = 125 ms guardrail section rather than a 12.5-foot guardrail section simulated in the research team's model. For this crash test, the finite element model was carefully adapted to match the exact length of nested rail and the substitution of weakened t = 206 ms t = 205 ms wooden controlled releasing terminal (CRT) posts near the unsupported span. Visually, good agreement was observed between the finite element model predictions and the reported outcome of the OLS2 crash test up to 760 ms. After 760 ms, the vehicle in the t = 254 ms t = 255 ms OLS2 crash test rolled whereas the simulated vehicle did not. A comparison between each crash test and simulation is shown in Figure 47. The results required by the NCHRP Report 350 test criteria for both the original crash test and the simula- tions reproducing the results are shown in Table 22. The t = 428 ms t = 430 ms post-impact exit speed of 55 km/hr (34.1 mph) was lower in the simulation than in the exit speed in the crash test of 66 km/hr (41.1 mph). The vehicle in the simulation did not overturn. These differences were attributed to the difficulty of modeling wooden posts. The maximum observed dynamic t = 760 ms t = 760 ms guardrail deflection was 0.3 meters (1 foot) lower in the simu- lation than in the crash test. The lower deflection of the simu- Figure 47. Comparison of missing post lation was related to the higher stiffness of the soil in the finite finite element model against OLS element model relative to the crash test. crash test. The initial point of impact had a strong effect on the sim- 11.3 Results ulation results. Simulations in which the vehicle struck the A series of finite element simulations was conducted to guardrail at the beginning of the unsupported span predicted determine the effect of missing posts on the guardrail crash less severe roll and pitch (all less than 10 degrees). Mid-span performance. In this model, unlike the long-span validation simulations, on the other hand, showed a much higher roll simulation, all of the posts around the impact area in these and much higher pitch values. Most severe was the system models were steel posts and none of the guardrails were nested. missing one post and impacted at the mid-span in which the Table 23 presents the results of simulations missing 1, 2, and vehicle pitched 45 degrees but maintained stability. 3 posts when the impact point was at the beginning of the The results for missing post simulations in which the impact span. Table 24 presents the results of simulations missing 1, point was mid-span are summarized in Table 23. For the vehi- 2, and 3 posts when the impact point was at the midpoint of cle, the exit speed decreased sharply for each additional miss- the unsupported span. Figure 48 presents a graphical com- ing post. The guardrail dynamic deflection also increased as parison of the simulations missing 1, 2, and 3 posts under more posts were removed as the lateral strength provided by both impact points. the posts was eliminated. When three posts were missing, the