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61 CHAPTER 12 Evaluation of Post Separation from Rail This chapter evaluates the effect of post and rail separation Both roll and pitch decreased as more posts were detached in strong-post w-beam guardrail systems. This type of dam- from the guardrail indicating better vehicle stability. There age commonly occurs in combination with minor rail deflec- was also a small increase in the maximum amount of dynamic tion, but in this study it was considered in isolation. Figure 52 and static deflection in the guardrail system. However, the shows an example of post and rail separation in the field and increases associated with the rail and post separation damage a finite element model of this damage mode. mode were the smallest of all of the examined damage modes. In Table 26, the vehicle rotations (roll, pitch, and yaw) 12.1 Approach are shown. All of the simulations showed the same trends. As mentioned earlier though, the simulation of two posts Finite element simulations were completed in which a separated by 3 inches showed that the minor damage to the 2000P vehicle impacted a barrier (at TL-3 conditions) with guardrail improved the vehicle stability by lowering both the detached posts. Based on the field inspections of damaged roll and pitch. As a consequence, the vehicle also returned to barriers, the research team found that detached posts typically the neutral position at roughly 600 ms, which was faster than occur in tandem with some amount of post deflection away for the other two simulations which reached the neutral posi- from the rail. For this study two simulations were conducted-- tion at roughly 750 ms. The yaw did not vary between the three the first for a single detached post and the second for two adja- simulations. cent detached posts. To create a model of a rail with detached The vehicle velocities, shown in Figure 53, were also very posts, the connection to the rail was severed and both posts similar between all three simulations. There was a notice- were pushed out of line 3 inches with respect to their original able amount of lateral skidding during the impact (in the position perpendicular to the barrier face. The procedure by range of 50400 ms) that diminished as the vehicle began which the post and rail separation was induced in the finite to exit the guardrail. The exit speed for the vehicle was com- element model is described in the appendices. The resulting pletely unaffected by the minor damage and ranged between model is shown in Figure 52 for 3 inches of deflection of two adjacent posts. The vehicle was given the initial conditions 53 and 54 kph. specified by the NCHRP Report 350 test criteria. The impact The guardrail deflection did not vary much between the sim- velocity was 100 kph at an impact angle of 25 degrees. ulations of post and rail separation and the undamaged simu- lation. As shown in Figure 54, all of the simulations resulted in a maximum dynamic deflection of roughly 2.3 feet (0.7 meters) 12.2 Results and maximum static deflections around 1.8 feet (0.55 meters). The results of the rail and post separation simulations are summarized in Table 25. Many of the NCHRP Report 350 12.3 Discussion required criteria did not vary between the different simulations. The occupant impact velocity, occupant ridedown acceleration, Two simulations were run to evaluate the effect of post and and vehicle 50 ms moving average acceleration did not change rail separation. The first simulation modeled the effect of one between the undamaged and post-and-rail-separated simula- post separated by 3 inches and the second simulation modeled tions. The vehicle exit speed and angle were also unchanged. two posts separated by 3 inches. Intuitively, it was expected that The greatest changes between each of the simulations were the introduction of damage into the guardrail would worsen observed in the vehicle rotation, particularly the roll and pitch. the overall performance. However, this damage mode was

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62 Field Example FE Model Figure 52. Post separation from guardrail. found to have little effect on the safety of the vehicle and its The minimal effect of rail and post separation on the crash occupant. simulation results appeared reasonable. By design, the posts The maximum deflection of the guardrail itself did not and rails in strong-post systems are supposed to separate dur- change greatly as more posts were separated from the rail. ing impact. By allowing separation, the posts and rails can The most severe damage condition modeled, which was two deform by large amounts without the rails being pulled down posts separated by 3 inches, resulted in a 5.6 percent increase toward the ground. Because the posts were not connected in maximum dynamic deflection. The increases associated to the rails, the posts could deform more freely and reduce with rail and post separation were smaller than those of the the risk of the vehicle snagging on the posts. The posts were missing post, rail and post deflection, and rail flattening dam- still able to provide a significant amount of lateral resis- age conditions. It was interesting that the increase in maxi- tance to deflection even though they were not attached to the mum dynamic deflection for the simulation of one separated rail. Because of these factors, the ability of the guardrail to post increased by 0.1 meters (3.9 inches), which was roughly redirect the vehicle and absorb crash energy was not signifi- equal to the 3 inches which the damaged post was deflected. cantly reduced. Table 25. Results for rail and post separation simulations. Undamaged 1 Post 3 in. 2 Posts 3 in. Impact Conditions Speed (kph) 100 100 100 Angle (deg) 25 25 25 Exit Conditions Speed (kph) 53 53 54 Angle (deg) 14.5 14.8 14.8 Occupant Impact Velocity X (m/s) 7.5 7.7 7.7 Impact Velocity Y (m/s) 5.5 5.8 5.7 Ridedown X (G) -11.8 -13.1 -11.3 Ridedown Y (G) -12.3 -14.0 -12.0 50 ms Average X (G) -6.7 -7.3 -6.6 50 ms Average Y (G) -6.8 -7.2 -6.6 50 ms Average Z (G) -3.8 -2.6 -4.1 Guardrail Deflections Dynamic (m) 0.69 0.70 0.73 Static (m) 0.55 0.59 0.58 Vehicle Rotations Max Roll (deg) -14.4 -12.9 -10.0 Max Pitch (deg) -9.9 -8.8 4.6 Max Yaw (deg) 40.3 40.8 41.0

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63 Table 26. Roll, pitch, and yaw for post and rail separation simulations. 90 X - Roll 70 Y - Pitch Z - Yaw Angular Displacement (degrees) 50 30 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -10 -30 -50 Time (s) 90 90 X - Roll X - Roll Y - Pitch Y - Pitch 70 70 Z - Yaw Z - Yaw Angular Displacement (degrees) Angular Displacement (degrees) 50 50 30 30 10 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -10 -10 -30 -30 -50 -50 Time (s) Time (s)

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64 100 1200 Static Deflection Contour X Velocity Y Velocity Max Deflection Contour (t=0.165s) 80 1000 Z Velocity Total Velocity Distance from Guardrail (mm) 800 60 Velocity (kph) 600 40 400 20 200 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 -20 0 5000 10000 15000 20000 25000 Time (s) Guardrail Lengthwise Position (mm) 100 1200 X Velocity Static Deflection Contour Y Velocity Max Deflection Contour (t=0.16s) 80 Z Velocity 1000 Total Velocity Distance from Guardrail (mm) 60 800 Velocity (kph) 600 40 400 20 200 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 -20 0 5000 10000 15000 20000 25000 Time (s) Downstream Distance from Post 9 (mm) 100 1200 X Velocity Static Deflection Contour Y Velocity Max Deflection Contour (t=0.195s) 80 Z Velocity 1000 Total Velocity Distance from Guardrail (mm) 60 800 Velocity (kph) 40 600 400 20 200 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 -20 0 5000 10000 15000 20000 25000 Time (s) Downstream Distance from Post 9 (mm) Figure 53. Vehicle velocities for post and rail Figure 54. Guardrail damage contours for post and separation simulations. rail separation simulations.