National Academies Press: OpenBook

Criteria for Restoration of Longitudinal Barriers (2010)

Chapter: Chapter 12 - Evaluation of Post Separation from Rail

« Previous: Chapter 11 - Evaluation of Missing or Broken Posts
Page 61
Suggested Citation:"Chapter 12 - Evaluation of Post Separation from Rail." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Suggested Citation:"Chapter 12 - Evaluation of Post Separation from Rail." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
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Page 62
Page 63
Suggested Citation:"Chapter 12 - Evaluation of Post Separation from Rail." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
×
Page 63
Page 64
Suggested Citation:"Chapter 12 - Evaluation of Post Separation from Rail." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
×
Page 64
Page 65
Suggested Citation:"Chapter 12 - Evaluation of Post Separation from Rail." National Academies of Sciences, Engineering, and Medicine. 2010. Criteria for Restoration of Longitudinal Barriers. Washington, DC: The National Academies Press. doi: 10.17226/14374.
×
Page 65

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61 This chapter evaluates the effect of post and rail separation in strong-post w-beam guardrail systems. This type of dam- age commonly occurs in combination with minor rail deflec- tion, but in this study it was considered in isolation. Figure 52 shows an example of post and rail separation in the field and a finite element model of this damage mode. 12.1 Approach Finite element simulations were completed in which a 2000P vehicle impacted a barrier (at TL-3 conditions) with detached posts. Based on the field inspections of damaged barriers, the research team found that detached posts typically occur in tandem with some amount of post deflection away from the rail. For this study two simulations were conducted— the first for a single detached post and the second for two adja- cent detached posts. To create a model of a rail with detached posts, the connection to the rail was severed and both posts were pushed out of line 3 inches with respect to their original position perpendicular to the barrier face. The procedure by which the post and rail separation was induced in the finite element model is described in the appendices. The resulting model is shown in Figure 52 for 3 inches of deflection of two adjacent posts. The vehicle was given the initial conditions specified by the NCHRP Report 350 test criteria. The impact velocity was 100 kph at an impact angle of 25 degrees. 12.2 Results The results of the rail and post separation simulations are summarized in Table 25. Many of the NCHRP Report 350 required criteria did not vary between the different simulations. The occupant impact velocity, occupant ridedown acceleration, and vehicle 50 ms moving average acceleration did not change between the undamaged and post-and-rail-separated simula- tions. The vehicle exit speed and angle were also unchanged. The greatest changes between each of the simulations were observed in the vehicle rotation, particularly the roll and pitch. Both roll and pitch decreased as more posts were detached from the guardrail indicating better vehicle stability. There was also a small increase in the maximum amount of dynamic and static deflection in the guardrail system. However, the increases associated with the rail and post separation damage mode were the smallest of all of the examined damage modes. In Table 26, the vehicle rotations (roll, pitch, and yaw) are shown. All of the simulations showed the same trends. As mentioned earlier though, the simulation of two posts separated by 3 inches showed that the minor damage to the guardrail improved the vehicle stability by lowering both the roll and pitch. As a consequence, the vehicle also returned to the neutral position at roughly 600 ms, which was faster than for the other two simulations which reached the neutral posi- tion at roughly 750 ms. The yaw did not vary between the three simulations. The vehicle velocities, shown in Figure 53, were also very similar between all three simulations. There was a notice- able amount of lateral skidding during the impact (in the range of 50–400 ms) that diminished as the vehicle began to exit the guardrail. The exit speed for the vehicle was com- pletely unaffected by the minor damage and ranged between 53 and 54 kph. The guardrail deflection did not vary much between the sim- 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) and maximum static deflections around 1.8 feet (0.55 meters). 12.3 Discussion Two simulations were run to evaluate the effect of post and rail separation. The first simulation modeled the effect of one post separated by 3 inches and the second simulation modeled two posts separated by 3 inches. Intuitively, it was expected that the introduction of damage into the guardrail would worsen the overall performance. However, this damage mode was C H A P T E R 1 2 Evaluation of Post Separation from Rail

found to have little effect on the safety of the vehicle and its occupant. The maximum deflection of the guardrail itself did not change greatly as more posts were separated from the rail. The most severe damage condition modeled, which was two posts separated by 3 inches, resulted in a 5.6 percent increase in maximum dynamic deflection. The increases associated with rail and post separation were smaller than those of the missing post, rail and post deflection, and rail flattening dam- age conditions. It was interesting that the increase in maxi- mum dynamic deflection for the simulation of one separated post increased by 0.1 meters (3.9 inches), which was roughly equal to the 3 inches which the damaged post was deflected. The minimal effect of rail and post separation on the crash simulation results appeared reasonable. By design, the posts and rails in strong-post systems are supposed to separate dur- ing impact. By allowing separation, the posts and rails can deform by large amounts without the rails being pulled down toward the ground. Because the posts were not connected to the rails, the posts could deform more freely and reduce the risk of the vehicle snagging on the posts. The posts were still able to provide a significant amount of lateral resis- tance to deflection even though they were not attached to the rail. Because of these factors, the ability of the guardrail to redirect the vehicle and absorb crash energy was not signifi- cantly reduced. 62 Field Example FE Model Figure 52. Post separation from guardrail. 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 Table 25. Results for rail and post separation simulations.

63 -50 -30 -10 10 30 50 70 90 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (s) An gu la r Di s pl ac em en t ( de gr e e s ) X - Roll Y - Pitch Z - Yaw -50 -30 -10 10 30 50 70 90 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (s) An gu la r Di s pl ac em en t ( de gr e e s ) X - Roll Y - Pitch Z - Yaw -50 -30 -10 10 30 50 70 90 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (s) An gu la r Di s pl ac em en t ( de gr e e s ) X - Roll Y - Pitch Z - Yaw Table 26. Roll, pitch, and yaw for post and rail separation simulations.

64 0 200 400 600 800 1000 1200 0 5000 10000 15000 20000 25000 Guardrail Lengthwise Position (mm) D is ta nc e fro m G ua rd ra il (m m) Static Deflection Contour Max Deflection Contour (t=0.165s) 0 5000 10000 15000 20000 25000 0 200 400 600 800 1000 1200 Downstream Distance from Post 9 (mm) D is ta nc e fro m G ua rd ra il (m m) Static Deflection Contour Max Deflection Contour (t=0.16s) 0 5000 10000 15000 20000 25000 0 200 400 600 800 1000 1200 Downstream Distance from Post 9 (mm) D is ta nc e fro m G ua rd ra il (m m) Static Deflection Contour Max Deflection Contour (t=0.195s) Figure 54. Guardrail damage contours for post and rail separation simulations. -20 0 20 40 60 80 100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (s) Ve lo ci ty (k ph ) X Velocity Y Velocity Z Velocity Total Velocity -20 0 20 40 60 80 100 Time (s) Ve lo ci ty (k ph ) X Velocity Y Velocity Z Velocity Total Velocity 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -20 0 20 40 60 80 100 Time (s) Ve lo ci ty (k ph ) X Velocity Y Velocity Z Velocity Total Velocity 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Figure 53. Vehicle velocities for post and rail separation simulations.

65 12.4 Recommendation The simulations conducted for this study indicate that the separation of up to two adjacent posts from the rails of the guardrail did not pose a risk to the vehicle or occupant. Indeed, it was found that the crash performance from this damage mode was almost indiscernible from that of the undamaged barrier performance. The recommended repair threshold is 2 or more adjacent posts that are detached from the rail; each deflected no more than 3 inches away from the rail. The pri- ority assigned to this damage was medium for consistency with the post and rail deflection guidelines. Post and rail sep- aration which exceeds 3 inches begins to take on the charac- teristics of the missing blockout damage mode. Repair for any single post with post and rail separation over 3 inches is rec- ommended. The priority assigned to this damage was medium to be consistent with the missing blockout guideline. Post and rail separation rarely occurs without post and rail deflection or damage to other components. In the recommen- dation in Exhibit 9.0, the research team recommends that the damage should also be evaluated using the deflected post and rail guidelines. Similarly, if the blockout is damaged or miss- ing, maintenance personnel should use the damaged block- out repair guidelines. Damage Mode Repair Threshold Relative Priority 2 or more posts with blockout attached with post-rail separation less than 3 in. Post-rail separation which exceeds 3 in. Note: If the blockout is not firmly attached to the post, use the missing blockout guidelines. Note: Damage should also be evaluated against post/rail deflection guidelines. MediumPosts Separated from Rail 1 post with blockout attached with post-rail separation less than 3 in.. Note: If the blockout is not firmly attached to the post, use the missing blockout guidelines. Note: Damage should also be evaluated against post/rail deflection guidelines. Low Exhibit 9.0. Recommendations for posts separated from rail repair.

Next: Chapter 13 - Evaluation of Rail Flattening »
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 656: Criteria for Restoration of Longitudinal Barriers explores the identification of levels of damage and deterioration to longitudinal barriers that require repairs to restore operational performance.

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