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Criteria for Restoration of Longitudinal Barriers (2010)

Chapter: Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection

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Page 42
Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 43
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Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 44
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Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 45
Page 46
Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 46
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Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 47
Page 48
Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 48
Page 49
Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 49
Page 50
Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 50
Page 51
Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 51
Page 52
Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 52
Page 53
Suggested Citation:"Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection." 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 53

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42 Rail and post deflection is one of the most prevalent types of damage in guardrail, most often caused by a lower severity crash. An example of this damage mode is shown in Figure 37. Impacts where the vehicle speed is lower may result in local- ized minor damage. Depending on the impact angle, the dam- age may be incurred only to the rail element, with minimal or no damage to the supporting posts and soil. Impacts with a higher speed but shallower angle can also result in more distributed rail and post deflection. The amount of deflection that can be sustained by a guard- rail before its safety is compromised is a major concern. Main- tenance crews and highway agencies are often forced to balance the expense of continual repairs against the potential liability if the damaged guardrail is struck again. The survey of U.S. states and Canadian provinces presented earlier in this report revealed that very few agencies have quantitative criteria underlying the decision of when to replace a deflected guardrail. Among those agencies that have quantitative guidelines, the threshold deflec- tion was most commonly set at 6 inches (152 mm) of deflection. This is also the recommended limit for minor damage specified by the Federal Highway Administration (FHWA, 2008). Indi- vidual agencies had limits as low as 3 inches (76 mm) or as high as 12 inches (305 mm). This study is intended to test the per- formance of guardrail with rail and post deflection to support a unified deflection limit based on quantitative data. 10.1 Objective The objective of this chapter is to present the results of an evaluation of crash induced guardrail rail and post deflection. The evaluation was conducted using a combination of full- scale crash tests and simulations. 10.2 Evaluation Through Crash Tests In August 2009, a test series was conducted by the MGA Re- search Corporation to evaluate the performance of damaged strong-post w-beam guardrails. This test was comprised of two parts: (1) an initial, low-speed impact to create a realistic repre- sentation of minor rail deflection and (2) a subsequent full-scale impact into the damaged section of guardrail. The goal of this test was to observe the effect of the guardrail damage on the ve- hicle performance and to test the outcome as well as to provide additional data for the validation of the finite element models. The strong-post w-beam guardrail installed for the pur- pose of this two-part crash test was 162 feet (49.5 meters) in total length from end-to-end. All posts were steel strong posts and the w-beam rails were spaced out from the posts via the use of routed wood blockouts. Tensioned end terminals were installed at both ends of the guardrails. The guardrail was ori- ented such that the vehicles would approach at a 25-degree angle of impact with the initial point of impact located 1.94 feet (591 mm) before post 11. This impact point was computed using the critical impact point procedures described in NCHRP Report 350. No modifications were made to the guardrail in between the first and second impacts. Further details on the guardrail design can be found in the crash test reports (Fleck and Winkelbauer, 2008a, 2008b). 10.2.1 Low-Speed Impact Test In the first impact, the impacting vehicle, a 1997 Chevrolet C2500 pickup weighing 4,632 lb (2101 kg), struck the guardrail at a speed of 30 mph (48.3 km/hr) at 26.0 degrees. This impact resulted in damage to 36 feet (11 meters) of barrier length of the total 162-foot (49.4 meters) length. The maximum perma- nent post and rail deflection was approximately 14.5 inches (368 mm). The barrier successfully contained the vehicle. The vehicle came to rest alongside the barrier due to the low initial speed of the vehicle. Figure 38 shows an overhead time series of the impact. 10.2.2 High-Speed Impact Test The day following the first, low-severity impact, a high-speed test was run. In this test, another 1997 Chevrolet C2500 pickup C H A P T E R 1 0 Evaluation of Crash-Induced Rail and Post Deflection

43 truck impacted the guardrail at the same initial impact point and area damaged by the previous vehicle. The impact con- ditions were 62.1 mph (99.9 km/hr) at 25.5 degrees (NCHRP Report 350 Test 3-11 conditions). Due to the damage that had already incurred to the guardrail, the vehicle failed to redirect and overrode the guardrail. The vehicle returned to the ground on the opposite side of the guardrail and continued to travel at 43.2 mph (69.5 km/hr) and at an angle of 18.7 degrees from the guardrail. Post 13 failed to separate from the guardrail despite the significant amount of post and rail deflection dur- ing the test. A series of photographs showing the vehicle as it vaulted over the guardrail is shown in Figure 39. As shown in these photographs, the pickup truck vaulted over the barrier and came to rest upright behind the test installation. Figure 37. Guardrail with rail and post deflection. 0 ms 212 ms 71 ms 294 ms 142 ms 376 ms 177 ms 458 ms Figure 38. Time series for low-speed impact.

10.2.3 Results of the MGA Tests With the exception of the vehicle roll, pitch, and yaw for the initial low-speed test, all of the NCHRP Report 350 criteria were computed. These values are shown in Table 16. The results for Test TTI 405421-1, a crash test of undamaged strong-post w-beam guardrail with a Chevrolet C2500 pickup truck, are provided for comparison purposes. For the low-speed crash test, all values were below what would be expected from a standard crash test. This was expected as the initial speed of the vehicle was much lower than for a TL-3 crash test. For the high-speed crash test, all of the accelerations and OIV scores were lower than observed in the test of undamaged guardrail. This was because the vehi- cle overrode the guardrail rather than being redirected, result- ing in less crash energy being dissipated. This was reflected in the higher exit speed. The guardrail deflection was far larger for the full-scale MGA test than for the typical TL-3 crash test into an undamaged guardrail. This difference was attributed to the difference in 44 0 ms 500 ms 100 ms 600 ms 200 ms 700 ms 300 ms 800 ms 400 ms 900 ms Figure 39. Time series for second, high-speed impact into damaged guardrails.

45 outcome, i.e., redirection vs. vaulting, as well as the increased deflection of the supporting posts. 10.2.4 Conclusions of the Crash Tests The outcome of the crash tests demonstrated that there are limits to the amount of damage that can be sustained by guardrails while still maintaining its functional capacity. The test series showed that a guardrail with 14.5 inches (368.3 mm) of rail and post deflection in a guardrail represents an unaccept- able condition that warrants high-priority repair. The perfor- mance of guardrail with lower amounts of deflection was evaluated with finite element models, as described in the following sections, to determine the limit of deflection that can be allowed. 10.3 Evaluation Through Finite Element Modeling The crash tests show that 14 inches of lateral post and rail deflection is a damage level which requires repair. The level of damage below 14 inches of post/rail deflection which may be acceptable was investigated by finite element simulation. A series of simulations was conducted to determine how much deflection could be permitted in a strong-post w-beam guardrail without compromising the safety of the system. All simulations were conducted with the LS-DYNA software as described earlier in this report. Simulations with com- bined rail and post deflection were conducted for 3, 6, 9, and 11 inches of deflection. A small number of simulations with rail deflection only were also conducted for 3 and 6 inches of deflection. Higher extents of rail only deflection were not con- sidered since it was unlikely that the posts would be unaffected as well. Rail and post deflection is typically produced by a low- severity impact. Therefore, the best way to reproduce this damage would be to simulate such an impact. Low-speed impacts in the range of 30–60 kph (18.6–37.3 mph) with an impact angle of 25 degrees were used to cause 3, 6, 9, and 11 inches of deflection in the rails, often with concurrent post deflection as well. In some models, artificial constraints were introduced to prevent post motion so that the effects of the rail deflection could be studied in isolation. An example of a completed full-scale model with 6 inches of rail and post deflection is shown in Figure 40. TTI Test 405421-1 MGA C08C3-027.1 MGA C08C3-027.2 Impact Conditions Speed (mph) 63.1 30.0 62.1 Angle (deg) 25.5 26.0 25.5 Exit Conditions Speed (mph) 34.2 12.9 43.2 Angle (deg) 16.0 11.4 18.7 Occupant Kinematics Longitudinal OIV (m/s) 7.1 3.8 6.1 Lateral OIV (m/s) 4.4 3.3 3.7 Longitudinal Ridedown (G) -7.9 -3.3 -6.1 Lateral Ridedown (G) 8.4 -1.9 -5.6 Vehicle Kinematics 50 ms X Acceleration (G) -5.3 -2.9 -5.5 50 ms Y Acceleration (G) 4.3 -2.6 -3.1 50 ms Z Acceleration (G) -4.8 1.8 -4.1 Guardrail Deflections Dynamic deflection (ft) 3.3 1.4 7.2 Static Deflection (ft) 2.3 1.2 3.3 Vehicle Rotation Maximum Roll (deg) -10 Not reported 29.7 Maximum Pitch (deg) -4 Not reported 12.1 Maximum Yaw (deg) 42 Not reported -10.2 Table 16. NCHRP Report 350 criteria for MGA test. Figure 40. Simulated guardrail with rail and post deflection.

46 10.3.1 Validation of the Finite Element Model The model was validated using the MGA test series in which two Chevrolet 2500 pickup trucks impacted a guardrail (MGA, 2008a; 2008b). By using multiple crash tests, the acceptability of using the finite element approach to model a wide range of crash conditions could be assured. The MGA crash tests were conducted to evaluate the per- formance of a guardrail with rail and post deflection. In the first test, a 30 mph (47 kph) impact was used to create 14.5 inches (368 mm) of deflection. The second test was performed accord- ing to NCHRP Report 350 standards with the impact occur- ring at the same point as the previous test. Because of the damage incurred by the first impact, the second test resulted in a failure due to the vehicle vaulting over the guardrail. These MGA crash tests were invaluable as a source of validation data for the finite element models. A series of photos from the second crash test and associ- ated simulation is shown in Figure 41. For this test, there was visually good agreement between the real crash test and the finite element model. The models were also compared by the NCHRP Report 350 criteria, as shown in Table 17. The first MGA impact, a low-speed collision intended to cause a minor amount of deflection, was successfully repro- duced. A simulation speed of 32 mph (52 km/hr) was required to reproduce the 14.5 inches (368 mm) of deflection observed in the 30 mph (48.3 km/hr) crash test. For the second MGA crash test, initial attempts at reproducing the results were unsuccessful. After an investigation, a critical factor in the outcome of the crash test was found to be the failure of a sin- gle post, located roughly 12.8 feet (3.9 meters) downstream of the impact point, to separate from the rail during both the first and second impacts. The addition of a constraint on the same post in the finite element model resulted in a drastic change in the predicted outcome of the impact, changing a successful crash into a failure with the vehicle vaulting over the guardrail. Occupant impact velocities and ridedown accelera- tions were below the NCHRP Report 350 limits in all the crash tests and simulations. 10.3.2 Results of Finite Element Simulations The MGA tests demonstrated that the separation of posts from the rails can radically change the crash performance of strong-post w-beam guardrail. Finite element modeling may not be able to accurately predict which behavior will occur in a real crash when relevant factors such as soil strength or bolt position are not known. The approach was to bracket the crash performance by conducting two series of simulations. In the first series, the rails and posts were allowed to separate. In the second series, a single post was prevented from sepa- rating. The post to which this constraint was applied was 12.5 feet (3.8 meters) downstream, which maximized the ef- fect on vehicle performance. In the first set of simulations, a guardrail with combined rail and post deflection of 3, 6, 9, and 11 inches (76, 152, 229, and 279 mm) was tested. The NCHRP Report 350 test values recorded for each simulation are shown in Table 18. Despite the huge difference in performance between the MGA test simulation of 14.5 inches of deflection and the undamaged simulation, there was very little variation in performance between the simulations of lesser damage. Even the simulation with 11 inches of deflection yielded virtually the same crash results and test values as the un- damaged simulation. MGA Crash Test MGA C08C3-027.2 Simulation of MGA Crash Test t = 0 ms t = 0 ms t = 120 ms t = 125 ms t = 242 ms t = 250 ms t = 360 ms t = 350 ms t = 490 ms t = 500 ms t = 690 ms t = 700 ms Figure 41. Comparison of finite element simulations against second MGA crash test.

tion of the vehicles in each of these crash test simulations at 700 ms after impact. The vehicle began to move upward and roll with increasing amounts of prior deflection damage. The vehicle eventually rolled onto its side when the deflection dam- age reached 11 inches (279 mm). However, even at 6 inches (152 mm) of deflection, the roll was very high and reached over 35 degrees before the vehicle began to recover. Figure 43 shows the local vehicle velocity at the center of gravity as a function of time for both the normal and fixed post simulations. There was almost no difference in the velocity between the undamaged simulation and the unmodified rail and post deflection simulations. All of the exit speeds were in the range of 31–35 mph (50–56 km/hr). The velocities for the simulations with a fixed post were a little more varied. The vehicle in the 11-inch simulation retained the most speed due to rolling on its side, which limited the amount of inter- action with the guardrail. The 3-inch simulation vehicle showed the lowest amount of roll and lost more speed because of more opportunities to interact with the posts. The maximum deflection of the guardrail increased as the extent of rail and post deflection increased for both sets of simulations as shown in Figure 44. The increases were much larger for the simulations without separation due to the de- flecting posts pulling the rails out. However, for both sets, each additional 3 inches (75 mm) in pre-existing deflection yielded only 0.8–1.6 inches (20–40 mm) of extra dynamic de- flection. The limited effect of the pre-existing deflection was attributed to the narrow range over which the damage was in- curred on the rail. 47 MGA Crash Test C08C3- 027.2 MGA Crash Test Simulation Impact Conditions Speed (kph) 99.9 100.0 Angle (deg) 26.4 26.4 Exit Conditions Speed (kph) 69.5 57.0 Angle (deg) 18.7 5.7 Occupant Impact Velocity X (m/s) 6.1 9.3 Impact Velocity Y (m/s) 3.7 5.4 Ridedown X (G) -6.1 -10.4 Ridedown Y (G) -5.6 -5.4 50 ms Average X (G) -5.5 -10.0 50 ms Average Y (G) -3.1 -6.3 50 ms Average Z (G) -4.1 -6.5 Guardrail Deflections Dynamic (m) 2.2 1.00 Static (m) 1.0 0.80 Vehicle Rotations Max Roll (deg) 30 7.1 Max Pitch (deg) 12 11.5 Max Yaw (deg) -10.2 -21.3 Table 17. Validation of finite element simulations against second MGA crash test. Undamaged Model 3 in. Rail and Post Deflection 6 in. Rail and Post Deflection 9 in. Rail and Post Deflection 11 in. Rail and Post Deflection Impact Conditions Speed (kph) 100 100 100 100 100 Angle (deg) 25 25 25 25 25 Exit Conditions Speed (kph) 53 53 52 56 50 Angle (deg) 14.5 13.2 13.8 15.6 15.0 Occupant Impact Velocity X (m/s) 7.5 8.0 8.0 8.6 8.3 Impact Velocity Y (m/s) 5.5 5.6 5.5 5.5 5.9 Ridedown X (G) -11.8 -12.0 -12.2 -10.7 -12.8 Ridedown Y (G) -12.3 -13.0 -10.1 -12.0 -10.4 50 ms Average X (G) -6.7 -6.7 -6.8 -7.9 -7.1 50 ms Average Y (G) -6.8 -6.7 -6.5 -6.5 -6.8 50 ms Average Z (G) -3.8 -3.9 -3.0 -4.2 -4.6 Guardrail Deflection Max Dynamic (m) 0.69 0.72 0.74 0.76 0.78 Static Deflection (m) 0.55 0.62 0.55 0.60 0.64 Pre-existing deflection (m) 0.00 0.07 0.15 0.22 0.28 Vehicle Rotation Max Roll (deg) -14.4 -12.9 -13 -16.6 -13.2 Max Pitch (deg) -9.9 -10 -6.6 -5.6 10 Max Yaw (deg) 40.3 40 40 41 40.5 Table 18. Simulation results for rail and post deflection with no rail and post separation constraints. In the second set of simulations, the models were set up in an identical manner, except that a constraint was added to a post located 12.5 feet (3.8 meters) downstream of the impact point to prevent the post and rail from separating. The NCHRP Report 350 test values recorded for each simu- lation are shown in Table 19. Figure 42 shows the orienta-

Rail Deflection Only Simulations. To determine the relative contributions of the rails versus those of the posts, two simulations were conducted in which rail deflection was introduced between two adjacent posts. No post deflection was permitted in the first impact. The posts were free to move however in the second impacts of these simulations. These rail deflection only simulations were limited to 3 and 6 inches (76 and 152 mm) of deflection since larger rail deflections generally do not occur without also deflecting the posts. The NCHRP Report 350 test criteria were almost entirely unchanged from the values recorded for the undamaged simulation. Between the undamaged and 6 inch rail only de- flection simulation, the roll and pitch decreased by less than 4 degrees and the maximum dynamic deflection increased by less than 3 percent. The longitudinal occupant impact veloc- ity showed the greatest increase, rising to 27 ft/s (8.2 m/s) from 24.6 ft/s (7.5 m/s), but was still within the recom- mended limit. The lack of change in crash test outcome for 48 Undamaged Model 3 in. Rail and Post Deflection 6 in. Rail and Post Deflection 9 in. Rail and Post Deflection 11 in. Rail and Post Deflection Impact Conditions Speed (kph) 100 100 100 100 100 Angle (deg) 25 25 25 25 25 Exit Conditions Speed (kph) 53 46 55 55 64 Angle (deg) 14.5 19.1 12.3 15.6 3.4 Occupant Impact Velocity X (m/s) 7.5 8.4 7.9 8.1 7.9 Impact Velocity Y (m/s) 5.5 5.5 5.1 5.2 5.4 Ridedown X (G) -11.8 -11.7 -13.2 -14.5 -8.8 Ridedown Y (G) -12.3 -10.4 -11.9 -11.7 -8.7 50 ms Average X (G) -6.7 -8.8 -8.2 -8.3 -7.0 50 ms Average Y (G) -6.8 -6.5 -6.2 -6.2 -6.0 50 ms Average Z (G) -3.8 -3.9 -3.7 4.6 5.2 Guardrail Deflection Max Dynamic (m) 0.69 0.82 0.86 0.86 0.90 Static Deflection (m) 0.55 0.64 0.66 0.67 0.77 Pre-existing deflection (m) 0.00 0.07 0.15 0.22 0.28 Vehicle Rotation Max Roll (deg) -14.4 32.1 35.5 39.7 Roll Max Pitch (deg) -9.9 -14.6 -19.8 22.7 28.2 Max Yaw (deg) 40.3 46.2 35.8 39.0 23.3 Table 19. Simulation results for rail and post deflection with one rail and post separation constraint. (a) Undamaged rail (b) Rail with 3” prior deflection (c) Rail with 6” prior deflection (d) Rail with 9” prior deflection (e) Rail with 11” prior deflection Figure 42. Damaged guardrail simulations after impact (t = 700 ms).

rail deflection only supports the earlier theory that the con- tributions of the posts may be more important in predicting the outcome of a crash. 10.4 Discussion 10.4.1 Importance of Rail and Post Separation A critical contribution to the vaulting of the vehicle in the MGA crash test was believed to be the failure of some of the posts to detach from the guardrail. In the second MGA crash test, a post failed to separate from the rail during impact. In a preliminary simulation of this crash, the post did separate from the rail, and the vehicle was successfully redirected. When a constraint was added to prevent the rail from sepa- rating from the post, the vehicle vaulted over the guardrail. The deflection of this post during impact was believed to have pulled the rail downward which permitted the vehicle to vault over the guardrail. 10.4.2 Simulations of Rail and Post Deflection In the simulations of the 3, 6, 9, and 11 inches (76, 152, 229, and 279 mm) of rail and post deflection with no separation con- straints, minor rail and post deflection had very little effect on the simulation results. The OIV, ridedown, and 50 ms average accelerations were satisfactory and the increases in maximum deflection were less than the increase in prior deflection. When the simulations were altered to prevent a post from separating from the rail, different outcomes were observed. The vehicle roll increased with increasing preexisting deflection. The vehicle overturned during impact with a guardrail having 11 inches (279 mm) of pre-existing rail deflection. Even for as little as 6 inches of rail deflection, substantial rolling was observed. When the rail and posts fail to separate, two different haz- ardous conditions can be created. If the post remains mostly upright the vehicle may be at greater risk of snagging. Another possible outcome was reflected in the results of the MGA crash test. If an unseparated post was deflected backwards and 49 0 10 20 30 40 50 60 70 80 90 100 0 0.2 0.4 0.6 0.8 1 Time (s) 0 0.2 0.4 0.6 0.8 1 Time (s) Ve lo ci ty (k ph ) Ve lo ci ty (k ph ) Undamaged 3 Inch 6 Inch 9 Inch 11 Inch 0 10 20 30 40 50 60 70 80 90 100 Undamaged 3 Inch 6 Inch 9 Inch 11 Inch Figure 43. Vehicle velocities for rail and post deflection simulations (left) and the same simulations with one post prevented from separating (right). 0 100 200 300 400 500 600 700 800 900 1000 0 5000 10000 15000 20000 25000 Downstream Distance from Post 9 (mm) 0 5000 10000 15000 20000 25000 Downstream Distance from Post 9 (mm) D ef le ct io n (m m) D ef le ct io n (m m) Undamaged 3 Inch 6 Inch 9 Inch 11 Inch 0 100 200 300 400 500 600 700 800 900 1000 Undamaged 3 Inch 6 Inch 9 Inch 11 Inch Figure 44. Maximum dynamic deflection for rail and post simulations (left) and the same simulations with one post prevented from separating (right).

downwards, as in the simulations with greater than 6 inches (152 mm) of deflection, the rail is pulled downward as well and the risk of vaulting is increased. The vehicle behavior for both 3 and 6 inches (76 and 152 mm) of prior rail deflection without post deflection was no different from that of the undamaged rail simula- tion. The static and dynamic guardrail deflections were also unchanged. These results provide further support for the theory that the behavior of the posts in strong-post guardrail systems can strongly influence the outcome of a crash test. 10.4.3 Effects of Prior Damage on Rail Height Existing literature has suggested that rail height can be a major contributor to vaulting (Marzougui et al., 2007). The rails in the finite element simulations were examined to de- termine whether the minor rail deflection incurred in the first impact resulted in changes in the rail height that could be cor- related to the outcome of the simulated second impact. The hypothesis was that the pre-existing damage would lower the rail height and lead to the vehicle vaulting. Figure 45 and Table 20 present the minimum height of the rail bottom, maximum height of the rail top, and the length of pre-existing deflection after the first impact but before the second impact. All of the measurements were made from the simulations with a separation constraint added. This situa- tion represented the worst case scenario for vaulting because the deflection of the post would pull the rail downward as it deflected. Figure 45 shows that one consequence of an impact is that the rail flattens. The bottom of the rail moved downward from 15.3 inches (388.6 mm) to 12.6 inches (320 mm) above the ground surface. The top of the rail moved upward from 27.9 inches (709 mm) to 31.8 inches (808 mm). The maxi- mum height of the guardrail increased with increasing deflec- tion, indicating that the guardrail was becoming increasingly flattened. The length of deflection also increased with increas- ing magnitude of deflection. These results indicate that, in addition to rail height, the flattening of the rail and the dam- age length may also exert a significant influence on the crash outcome in these simulations. However, further testing will be needed to draw any conclusions about the relative impor- tance of each of these factors on vaulting or rollover risk. 10.4.4 Evaluation of Rail Rupture Potential Ray et al. conducted a study on rail rupture in crash tests which showed that rails can carry up to 92.2 kip (410 kN) 50 0 5 10 15 20 25 30 35 0 3 6 9 12 15 Prior Rail Deflection (in) R ai l H ei gh t ( in) Bottom of Rail Top of Rail 0 5 10 15 20 25 0 2 4 6 8 10 12 14 16 Prior Deflection (in) Le ng th o f D ef le ct io n (ft ) Figure 45. The height of the rails (left) and the length of damage (right) vs. the extent of prior deflection. Minimum Height of Rail Bottom (mm) Maximum Height of Rail Top (mm) Difference in Max and Min (mm) Length of Damage (m) Undamaged 388 709 321 NA 3 in. 320 742 422 2.3 6 in. 317 755 438 3.5 9 in. 317 769 452 4.7 11 in. 314 781 467 5.8 14.5 in. 334 807 473 6.6 Table 20. The height of guardrails and length of damage in simulations with pre-existing damage.

under quasistatic loading. However, rupture may also occur at lower rail tensions. Localized tearing is possible in impacts of this type, but the research team’s model was not configured to accurately compute element tearing resulting from local- ized stress concentrations and did not include failure criteria for the steel components. The model was meshed using large element sizes 0.4–0.6 in (10–40 mm), which were appropri- ate for determining vehicle dynamics but were too coarse to realistically model the initiation and propagation of tears. As an alternative, the tension carried by the rails was used to determine the relative risk of rail rupture. The tensions for the rail and post simulations are tabu- lated in Table 21 under the column for separation allowed simulations. The tension for the rail deflection only did not vary significantly from the undamaged simulation. However, all of the post and rail deflection simulations showed an increase in rail tension compared to the undamaged simu- lation, with the tension steadily increasing to a maximum of 292.6 kN at 9 inches of deflection. Although this tension was below the 410 kN limit of w-beam rail, rupture can occur at a lower tension (Ray et al., 2001). The higher tension carried by the damaged rails implied that there was a modest increase in the chance that a rail rupture would occur. The tension was also tabulated for the simulations in which post separation was not permitted. These tensions are listed under the “No separation allowed” column. The recorded maximum tensions were not much different than that of the undamaged simulation. Because the connection between the post and rail was maintained, more of the crash energy was transmitted to the posts. 10.5 Conclusions This study has examined the crash performance of strong- post w-beam guardrail with rail and post deflection from a previous impact. The MGA crash tests and finite element simulations of second impacts into damaged guardrails have shown that the combination of rail and post deflection can negatively affect the crash performance. The research team’s conclusions are the following: • Crash tests demonstrated that 14.5 inches (368 mm) of post and rail deflection with a damage length of 36 feet (11 meters) was a damage level requiring high-priority re- pair. Two full-scale crash tests were conducted to evaluate the limits of acceptable rail and post deflection in crash- damaged strong-post w-beam guardrail. The damaged bar- rier failed to contain a Chevrolet 2500 pickup truck that impacted the damaged section at 62 mph (100 km/hr) and 26.4 degrees. The vehicle vaulted over the guardrail and came to rest upright behind the barrier. A critical factor in the outcome of the test was the failure of a post near the area of impact to separate from the rails during impact. • Finite element simulations were employed to investigate the acceptability of damage levels below 14.5 inches (368 mm) of rail and post deflection. Simulations were conducted for post and rail deflection varying from 3 to 11 inches (76 to 279 mm). A series of simulations were run in which a sin- gle post was prevented from separating from the rail. In this simulation series, the vehicle experienced a significant roll beginning at 6 inches (152 mm) of deflection and eventually rolled over when the deflection reached 11 inches (279 mm). The crash performance of rail with 3 inches of deflection was not markedly different than undamaged rail. • Finite element simulations were conducted of impacts into guardrail with rail deflection between two adjacent posts. No post deflection was permitted in the first impact. The posts were free to move however in the second impacts of these simulations. Rail deflection of 3 and 6 inches between the posts was investigated. The vehicle and guardrail per- formance in these simulations were almost unchanged from the undamaged simulation. These results support the con- clusion that the contributions of the post during an impact were important. • Rail tension was examined in all simulations as an indi- cator of rail rupture potential. The tension carried by the 51 Maximum Rail Tension (kN) % Increase Over Undamaged Separation allowed No separation allowed Separation allowed No separation allowed Undamaged 237.4 0.0% 3 in. Rail and Post 247.1 258.1 4.1% 8.7% 6 in. Rail and Post 282.9 229.2 19.2% -3.5% 9 in. Rail and Post 292.6 237.6 23.3% 0.1% 11 in. Rail and Post 261.1 235.5 10.0% -0.8% Table 21. Maximum rail tensions in rail and post deflection simulations.

guardrail changed very little for any simulation where a post was prevented from separating. However, when the posts could freely separate from the posts, rail tension increased with increasing pre-existing rail deflection. For 9 inches (229 mm) of pre-existing deflection, peak rail tension was 23 percent higher than the rail tension in the undamaged rail simulation. Peak rail tension in the simulation of 6 inches (152 mm) of deflection was 19 percent higher than in the undamaged rail simulation. These results indicated that the risk of rupture increased modestly as the magnitude of prior rail/post deflection increased. • The maximum rail height and length of deflection both increased with increasing amounts of pre-existing deflec- tion. The minimum rail height was roughly constant for any amount of prior deflection. Rail height, length of dam- age, and flattening extent were all factors which appeared to contribute to crash income, but the relative influence of each could not be isolated. Further study will be needed to better understand these factors. 10.6 Recommendation This guideline was based upon two quantitative metrics: (1) lateral post and rail deflection and (2) post height. 10.6.1 Lateral Post and Rail Deflection Two full scale crash tests were conducted to evaluate this guideline. In the first test, a 2000P vehicle (Chevrolet C-2500 pickup truck) impacted a strong-post guardrail system at 30 mph (48.3 km/hr) and 26 degrees to induce damage to the barrier. The barrier successfully contained the vehicle in this lower speed test. The vehicle came to rest alongside the bar- rier. The impact resulted in damage to 36 feet (11 meters) of barrier length and a maximum post and rail deflection of ap- proximately 14.5 inches (368 mm). In the second test, another Chevrolet C-2500 pickup truck collided with the area dam- aged by the first test at a speed of 62 mph (100 km/hr) and 25 degrees. The damaged barrier failed to contain the impact- ing 2000P vehicle. The 2000P vehicle vaulted over the barrier and came to rest upright behind the test installation. These tests show that 14.5 inches of lateral post and rail de- flection is a damage level damage mode which requires repair. The levels of damage below 14.5 inches of post and rail deflec- tion which may be acceptable were investigated by finite ele- ment simulation. Simulations were conducted for post and rail deflection varying from 3 to 11 inches (76 to 279 mm). The crash performance of rail with 3 inches of deflection was not markedly different than an impact into undamaged rail. However, the vehicle experienced significant roll beginning at 6 inches (152 mm) of deflection and eventually rolled over when the deflection reached 11 inches (279 mm). For strong soils, the crash performance of barriers with post deflection up to 9 inches was satisfactory, whereas higher amounts of deflection were not. Impacts into rail with 11 inches of prior deflection resulted in a rollover in the simulation. The crash test into a rail with 14.5 inches of deflection resulted in the vehicle vaulting over the rail. Adjusting for an extra margin of safety, e.g., to account for softer soils or overlapping damage modes, the limit of acceptable post and rail deflection was set to 9 inches. Impacts into rail with 6–9 inches of prior deflection were satisfactory in the simulations, but were associated with significant amounts of vehicle instability. The presence of any amount of deflection in the guardrail was found to in- crease the amount of maximum dynamic deflection, so re- pairs to guardrail with hazardous objects directly behind the guardrail should also be repaired as quickly as practical. Based on these initial simulations, a damage threshold of 6 inches of post and rail deflection has been set as the threshold for strong-post w-beam barrier repair. Deflection from 6–9 inches was associated with significant amounts of vehicle insta- bility, and should be repaired with medium priority. Barriers with rail and post deflection above 9 inches should be repaired with a high priority as vehicle stability and rollover appears to be a significant threat with barrier damage of this magnitude. The guideline further requires that this damage be fairly local- ized and form a pocket in the rail in order to require repair. The deflection must occur over a 25-foot or shorter length of rail. The rationale is that 6 inches of deflection spread over 300 feet of rail would have an insignificant effect on performance whereas the pocket formed by 6 inches of deflection measured over 25 feet of rail would be a potential safety concern. 10.6.2 Post Height Depending on the extent of post and rail deflection, the height of the w-beam in the damaged section may be lower than the original height of the strong-post w-beam. Several crash studies conducted in New York (Zweden and Bryden, 1977; Carlson et al., 1977; Bryden, 1984) have shown lower rail heights to be associated with more vehicle penetrations. Con- cern over the effects of lower rail height have led to the recent development of new barrier systems such as the Midwest Guardrail System (Faller et al., 2007), the Gregory Mini-Spacer (Baxter, 2006), and the Trinity T-31 Barrier (Baxter, 2005). In these new systems, the top of the rail is 31 inches from the ground line compared to 27 inches for the modified G4(1S) strong post barrier system. More recently, full-scale crash test- ing at FOIL has shown that a strong-post w-beam rail height that is 2 inches lower than the standard installation height re- sulted in the 2000P test vehicle vaulting over the barrier (Mar- zougui et al., 2007). Based on this information, the research team recommends barrier repair for any crash damaged strong- post barrier where the top of the w-beam rail is 2 or more inches below the original top of the rail height (Exhibit 7.0). 52

53 Damage Mode Repair Threshold Relative Priority One or more of the following thresholds: More than 9 inches of lateral deflection anywhere over a 25-ft length of rail. Top of rail height 2 or more inches lower than original top of rail height. HighPost and Rail Deflection 6-9 inches of lateral deflection anywhere over a 25-ft length of rail. Medium Rail Deflection Only 6-9 inches of lateral deflection between any two adjacent posts. Note: For deflection over 9 inches, use post and rail deflection guidelines. Medium Exhibit 7.0. Recommendations for crash-induced rail and post deflection.

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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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