National Academies Press: OpenBook

Criteria for Restoration of Longitudinal Barriers (2010)

Chapter: Chapter 11 - Evaluation of Missing or Broken Posts

« Previous: Chapter 10 - Evaluation of Crash-Induced Rail and Post Deflection
Page 54
Suggested Citation:"Chapter 11 - Evaluation of Missing or Broken Posts." 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 54
Page 55
Suggested Citation:"Chapter 11 - Evaluation of Missing or Broken Posts." 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 55
Page 56
Suggested Citation:"Chapter 11 - Evaluation of Missing or Broken Posts." 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 56
Page 57
Suggested Citation:"Chapter 11 - Evaluation of Missing or Broken Posts." 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 57
Page 58
Suggested Citation:"Chapter 11 - Evaluation of Missing or Broken Posts." 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 58
Page 59
Suggested Citation:"Chapter 11 - Evaluation of Missing or Broken Posts." 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 59
Page 60
Suggested Citation:"Chapter 11 - Evaluation of Missing or Broken Posts." 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 60

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

54 This section aims to quantitatively assess the effects of miss- ing posts in an otherwise undamaged section of strong-post w-beam guardrail. The ultimate goal was to develop recom- mendations to be used by maintenance personnel for the repair priority of missing posts. Posts can be missing from a guardrail for a variety of reasons. The posts in steel guardrail may be missing, severely twisted, or completely flattened from a prior crash. The posts from a wooden post guardrail might be missing due to rot, insect damage, or shattering due to a crash. Note that for this study, the research team also catego- rizes posts as missing if they are present but so weakened due to damage or deterioration that they present little to no effec- tive support of the rail. 11.1 Approach A series of finite element simulations were run to deter- mine the number of posts which could be removed from the strong-post w-beam guardrail while still maintaining accept- able crash performance. Simulations were conducted using the LS-DYNA software (LSTC, 2003) for strong-post w-beam guardrail with 1, 2, and 3 missing posts. For each missing post simulation, two different impact points were used to examine the effect that the impact point had on the crash performance. These impact points were (1) at the post begin- ning at the unsupported span and (2) the mid-point of the unsupported span. The missing post damage mode was a straightforward damage condition to simulate. To reproduce the damage, the entire post, along with all the supporting elements, was deleted from the model. The supporting elements consisted of the soil, post bolt, post nut, and blockout. No compen- satory options such as nesting were added to the model to im- prove the strength of the resulting section of unsupported rail. An example of a completed full-scale model with a miss- ing post is shown in Figure 46. 11.2 Validation of Finite Element Model Ideally, the validation of strong-post w-beam systems with missing posts could be determined from crash tests, but there were no crash tests to the research team’s knowledge with miss- ing posts in unmodified strong-post w-beam guardrail. As an alternative, the finite element model was validated against a crash test of a specialized variation of guardrail called a long- span system (Polivka et al., 1999a; Polivka et al., 1999b). Long-span systems have posts that are missing by design. They are used wherever posts cannot be driven into the ground, most commonly due to the presence of medium to large cul- verts under the roadway. Long-span systems are typically modified in order to compensate for the loss of one or more posts. Typically, the rails are nested (doubled up) over the unsupported portion of the guardrail and the adjacent sec- tions of the rail that would be involved in the impact. Other long-span systems may also incorporate changes to the post spacing, the number of blockouts, or the substitution of wooden posts near the unsupported area to reduce the chance of snagging. The crash test selected for validation was the crash test of a long-span guardrail system missing three posts, performed by the University of Nebraska-Lincoln (UNL) as part of a study of long-span systems (Polivka et al., 1999a). In this report, this test will be referred to as OLS2. This guardrail system included a 25-foot (7.62 meters) unsupported span with nested guard- rail used as compensation for the reduced strength in the un- supported region. A TL 3-11 impact of a Chevrolet C-2500 pickup truck into the long-span section test resulted in the pickup truck overturning as it exited the guardrail. This crash test differed from the missing/broken model in several respects: (1) OLS2 used wood rather than steel posts, (2) OLS2 used nested guardrail rather than the standard single guardrail simulated in this study, and (3) the test used a 25-foot C H A P T E R 1 1 Evaluation of Missing or Broken Posts

55 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 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 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 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 guardrail deflection was 0.3 meters (1 foot) lower in the simu- lation than in the crash test. The lower deflection of the simu- lation was related to the higher stiffness of the soil in the finite element model relative to the crash test. 11.3 Results A series of finite element simulations was conducted to determine the effect of missing posts on the guardrail crash performance. In this model, unlike the long-span validation simulation, all of the posts around the impact area in these models were steel posts and none of the guardrails were nested. Table 23 presents the results of simulations missing 1, 2, and 3 posts when the impact point was at the beginning of the span. Table 24 presents the results of simulations missing 1, 2, and 3 posts when the impact point was at the midpoint of the unsupported span. Figure 48 presents a graphical com- parison of the simulations missing 1, 2, and 3 posts under both impact points. The initial point of impact had a strong effect on the sim- ulation results. Simulations in which the vehicle struck the guardrail at the beginning of the unsupported span predicted less severe roll and pitch (all less than 10 degrees). Mid-span simulations, on the other hand, showed a much higher roll and much higher pitch values. Most severe was the system missing one post and impacted at the mid-span in which the vehicle pitched 45 degrees but maintained stability. The results for missing post simulations in which the impact point was mid-span are summarized in Table 23. For the vehi- cle, the exit speed decreased sharply for each additional miss- ing post. The guardrail dynamic deflection also increased as more posts were removed as the lateral strength provided by the posts was eliminated. When three posts were missing, the Figure 46. Simulated guardrail missing on post. UNL OLS2 Crash Test Simulation of OLS2 Crash Test t = 0 ms t = 0 ms t = 126 ms t = 125 ms t = 206 ms t = 205 ms t = 254 ms t = 255 ms t = 428 ms t = 430 ms t = 760 ms t = 760 ms Figure 47. Comparison of missing post finite element model against OLS crash test.

56 dynamic deflection increased by a little over 50 percent. All occupant injury metrics, i.e., occupant ridedown acceleration and occupant impact velocities, were well below the NCHRP Report 350 limits. In Table 24, the results for the missing post simulations for which the point of impact was the beginning of the unsup- ported span are presented. Maximum rail deflection, maximum rail tension, and vehicle exit speed increased as the number of missing posts increased. All occupant injury metrics, i.e., occu- pant ridedown acceleration and occupant impact velocities, were well below the NCHRP Report 350 limits. 11.4 Discussion For all simulations, there was a large increase in dynamic deflection for each post that was removed from the system. The maximum dynamic deflection contours are shown in Figure 49. For most of the simulations, the maximum deflec- tion typically occurs around 0.2 seconds after impact. At this time, the vehicle was just beginning to redirect due to contact with the rails. As would be expected, the dynamic deflection increased as more posts were removed from the system. For the simulation with three missing posts, the guardrail deflec- tion exceeded that of the OLS validation simulation, which was also missing three posts. However, the OLS test made use of nested guardrail to reduce the deflection, so this was not unexpected. The static deflection varied greatly between simulations. This was partly due to twisting in the rails, but also because of the manner in which the vehicle exited the guardrail. For the simulations with one and two posts miss- ing, the snagging of the vehicle tires on the posts caused the vehicle to slide away from the rails. In the simulations with undamaged and 3 posts missing, the vehicle remained in con- tact with the rails longer which caused the damage contour to smooth out more. OLS Crash Test OLS2 Simulation Impact Conditions Speed (kph) 102.7 102.7 Angle (deg) 24.5 24.5 Exit Conditions Speed (kph) 66.2 55.0 Angle (deg) 16.7 16.3 Occupant Impact Velocity X (m/s) 6.7 8.6 Impact Velocity Y (m/s) 5.0 -6.3 Ridedown X (G) 6.4 -14.0 Ridedown Y (G) 8.3 14.6 50 ms Average X (G) NR -9.1 50 ms Average Y (G) NR 8.6 50 ms Average Z (G) NR -4.9 Guardrail Deflections Dynamic (m) 1.3 1.0 Static (m) 1.0 0.7 Vehicle Rotations Max Roll (deg) Rolled 14.3 Max Pitch (deg) NR -15.5 Max Yaw (deg) NR -43.5 Table 22. Comparison of missing post model results with UNL long-span crash test OLS2. Undamaged 1 Post Missing 2 Posts Missing 3 Posts Missing Impact Conditions Speed (kph) 100 100 100 100 Angle (deg) 25 25 25 25 Exit Conditions Speed (kph) 53 60 47 32 Angle (deg) 14.5 20.9 29.4 13.3 Occupant Impact Velocity X (m/s) 7.51 7.55 6.81 7.63 Impact Velocity Y (m/s) 5.54 5.56 3.65 3.56 Ridedown X (G) -11.77 -9.50 -13.98 -12.88 Ridedown Y (G) -12.27 -9.02 -9.87 -9.71 50 ms Average X (G) -6.68 -6.67 -8.11 -8.52 50 ms Average Y (G) -6.82 -6.10 -7.00 -6.44 50 ms Average Z (G) -3.85 4.29 -3.18 -6.73 Guardrail Deflections Dynamic (m) 0.69 0.86 0.97 1.05 S tatic (m) 0.55 0.71 0.78 0.60 Vehicle Rotations Max Roll (deg) -14.4 -15.4 -13.2 -19.4 Max Pitch (deg) -9.9 -44.6 -17.8 -23.4 Max Yaw (deg) 40.3 44 78.8 40 Max Rail Tension 237.4 267.5 299.8 352.8 Table 23. Results for missing post simulations with mid-span impacts.

Because of the coarse sampling, the damage contours shown in Figure 49 do not always show the same maximums that were recorded in Tables 23 and 24. However, the contours are use- ful for observing the shape of the guardrail during the time of maximum deflection. The contours shown all begin at that same point, starting at post 9. The deflection was sampled roughly every 953 mm (3.1 feet) until post 21 was reached. The total length sampled was just under 23 meters (75.5 feet) which covers the full area of contact. In all of the curves, the peak was formed around the corner of the vehicle, with rela- tively smooth leading and trailing edges created by the vehicle’s front and side, respectively. The difference in the locations of the peak deflections was due to changes in the impact point relative to the reference post. Figure 50 shows the vehicle velocity for each simulation. All velocities were reported in the vehicle local coordinate system. Undamaged 1 Post Missing 2 Posts Missing 3 Posts Missing Impact Conditions Speed (kph) 100 100 100 100 Angle (deg) 25 25 25 25 Exit Conditions Speed (kph) 53 39 57 63 Angle (deg) 14.5 -11.5 11.4 15.3 Occupant Impact Velocity X (m/s) 7.51 8.88 7.53 6.51 Impact Velocity Y (m/s) 5.54 5.81 5.75 5.48 Ridedown X (G) -11.77 -9.10 -12.13 -10.22 Ridedown Y (G) -12.27 -10.08 -8.89 -10.30 50 ms Average X (G) -6.68 -7.93 -6.37 -6.32 50 ms Average Y (G) -6.82 -6.76 -6.75 -7.27 50 ms Average Z (G) -3.85 -4.82 3.21 1.71 Guardrail Deflections Dynamic (m) 0.69 0.78 0.89 1.00 Static (m) 0.55 0.51 0.68 0.70 Vehicle Rotations Max Roll (deg) -14.4 -7.6 -7.8 -6.5 Max Pitch (deg) -9.9 -7.6 -6.9 2 Max Yaw (deg) 40.3 23.8 37 40 Max Rail Tension (kN) 237.4 268.4 286.2 336.7 Table 24. Results for missing post simulations with beginning of span impacts. 1 post missing, beginning of span impact (t = 0.7s) 1 post missing, mid-span impact (t = 0.7s) 2 posts missing, beginning of span impact (t = 0.7s) 2 posts missing, mid-span impact (t = 0.7s) 3 posts missing, beginning of span impact (t = 0.7s) 3 posts missing, mid-span impact (t = 0.7s) Figure 48. Post-impact behavior of the vehicle for missing post simulations. 57

58 Many of the vehicles showed decreases in velocity due to fric- tion after exiting the guardrail. All exit velocities were recorded at 700 ms as a common reference velocity. Although this did not eliminate any loss in speed due to friction, this approach ensured that the measurements were consistent across all the simulations. The magnitude of the Y and Z velocity compo- nents tended to be the highest for the simulations where there was a short distance between the point of impact and the first downstream post. This was attributed to the front left tire snag- ging on the downstream posts. For simulations where the impact point was at the beginning of the unsupported span, the exit speed of the vehicle increased as the number of posts removed from the system was increased. The most likely explanation for this was that the increased dis- tance to the next post in the guardrail prevented severe wheel snagging from occurring. By contrast, for the three simulations were the impact point was at the middle of the unsupported span the exit speed decreased as more posts were removed. To explore the possible causes of this difference, the distance between the vehicle’s point of impact and the first downstream post was examined. For mid-span impacts, the distances to the next post were 1.9, 2.86, and 3.8 meters (6.2, 9.4, and 12.5 feet) for 1, 2, and 3 posts missing, respectively. For the beginning of span impacts, the same distances were 3.8, 5.7, and 7.6 meters (12.5, 18.7, and 24.9 feet). The two simulations where the vehi- cle was 3.8 meters (6.2 feet) from the next post resulted in the two lowest exit velocities, whereas the exit speed for the vehi- cle increased as the distance either increased or decreased. This behavior was attributed to the existence of a critical impact point for which the chance of the vehicle snagging on the posts was maximized. 11.4.1 Evaluation of Rail Rupture Potential Rail rupture is a great concern for guardrails with long stretches of unsupported rail. Ruptures are occasionally ob- 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 -5000 0 5000 10000 15000 20000 Position Relative to Impact Point (mm) -5000 0 5000 10000 15000 20000 Position Relative to Impact Point (mm) D ef le ct io n (m m) Undamaged 1 Post Missing 2 Posts Missing 3 Posts Missing 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 D ef le ct io n (m m) Undamaged 1 Post Missing 2 Posts Missing 3 Posts Missing Figure 49. Maximum dynamic deflection contours; impacts at the beginning of the unsupported span (left) and the middle of the unsupported span (right). 0 10 20 30 40 50 60 70 80 90 100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (s) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (s) To ta l V el oc ity (k ph ) Undamaged 1 Post Missing 2 Posts Missing 3 Posts Missing 0 10 20 30 40 50 60 70 80 90 100 To ta l V el oc ity (k ph ) Undamaged 1 Post Missing 2 Posts Missing 3 Posts Missing Figure 50. Vehicle velocity at center of gravity. Velocity for impacts at the beginning of the unsupported span (left) and the middle (right).

served even in crash tests of standard, unmodified guard- rails (Ray et al., 2001). These failures also occur at lower tensions than the reported quasistatic tensile strength of 410 kN (92.2 kip). By removing posts from the guardrail, the forces of impact are concentrated on fewer posts. This increased the likelihood of a rail rupture. To assess the possibility of rail rupture, measurements of rail tension in the simulations were made between different pairs of adjacent posts to identify the section carrying the largest load. Tensions were tabulated for all rail sections located between post 9 and post 21, which included the entire area of contact between the rail and vehicle. There were clearly observed increases in the rail tension as the number of posts removed from the system increased. The rail tensions for all simulations peaked at roughly 200 ms, although the tensions remained high during the full duration of vehicle redirection, which occurred between 0 and 400 ms. In Figure 51, the maximum observed tensions from the undamaged simulation are tabulated for each of the missing post simulations. For each additional post removed from the guardrail system, the maximum tension in the rail increased by 20–50 kN (4.5–11.2 kip). The maximum tension observed was 352.8 kN (79.3 kip) for the guardrail missing three posts with a mid-span impact. This was almost a 50% increase in rail tension compared to the undamaged simulation, where the maximum tension recorded was 237.4 kN (53.4 kip). While the increase in rail tension as posts were removed was large, the maximum tension observed in the simulations was still below the quasi-static tension limit of 410 kN. However, this did not necessarily mean that rail rupture could not occur, as Ray et al. (2001) have shown that rail rupture typically oc- curs at much lower rail tensions that can be reached in quasi- static testing. This has been attributed to the development of high localized stresses around the splices in full-scale crash tests. Localized tearing is possible in impacts where posts are missing, but the finite element model was not configured to look for element tearing resulting from localized stress con- centrations because the model did not include any failure cri- teria for the steel components. However, based on the results of the rail tension analysis, the likelihood of the rails ruptur- ing during impact increased as more posts were removed. 11.5 Recommendation This study has examined the crash performance of strong- post w-beam guardrail with missing posts. The finite element simulations conducted clearly showed that the removal of posts from a guardrail had a strong adverse effect on the crash performance of both the vehicle and guardrail, as summa- rized below: • Vehicle collisions with guardrail systems missing posts have an increased risk of vehicle instability. While none of the vehicles in the simulations overturned, several vehicles were unstable after impact which could have led to rollover under some field conditions. Some of the vehicles exhibited signif- icant skidding upon exiting the system, and these vehicles would be easily tripped by irregularities in the ground. • The removal of even one post can be expected to increase the system deflection by as much as 25 percent. Further increases in deflection and stress were expected as more and more posts were removed from the system. The most severe condition simulated (three posts missing) resulted in a 50 percent increase in the maximum deflection of the 0 50 100 150 200 250 300 350 400 Undamaged 1 Post Missing 2 Posts Missing 3 Posts Missing M ax im um T en si on (k N) Beginning of Span Middle of Span Figure 51. Maximum rail tension as a function of number of missing posts. 59

60 guardrail. Therefore, it would be especially important to repair missing posts whenever there is a substantial crash risk immediately behind the barrier. • Rail tension, a possible predictor of rail rupture, increased as posts were removed from the system regardless of where the impact point was located. With three posts missing from the system, the tension was increased by nearly 50 percent but was still below the quasi-static fail- ure limit of 410 kN. However, rail ruptures have been observed in crash tests at much lower tensions than can be reached at quasi-static loading due to localized stresses around the splices. Thus, the increased rail tension, com- bined with the higher stresses, indicates an increased risk of the rail rupturing during impact. The research team’s recommendation is that maintenance crews should repair any strong-post w-beam systems that are missing any number of posts (Exhibit 8.0). Even a single missing post in a strong-post w-beam guardrail can seriously degrade the performance. Impacts into guardrail systems with missing posts were found to have a higher risk of vehicle insta- bility, greater maximum guardrail deflection, and an increased risk of rail rupture. Damage Mode Repair Threshold Relative Priority Missing/Broken Posts 1 or more posts missing Cracked across the grain Broken Rotted With metal tears High Exhibit 8.0. Recommendations for repair of missing or broken posts.

Next: Chapter 12 - Evaluation of Post Separation from Rail »
Criteria for Restoration of Longitudinal Barriers Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!