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59 served even in crash tests of standard, unmodified guard- static testing. This has been attributed to the development of rails (Ray et al., 2001). These failures also occur at lower high localized stresses around the splices in full-scale crash tensions than the reported quasistatic tensile strength of tests. Localized tearing is possible in impacts where posts are 410 kN (92.2 kip). By removing posts from the guardrail, missing, but the finite element model was not configured to the forces of impact are concentrated on fewer posts. This look for element tearing resulting from localized stress con- increased the likelihood of a rail rupture. centrations because the model did not include any failure cri- To assess the possibility of rail rupture, measurements of teria for the steel components. However, based on the results rail tension in the simulations were made between different of the rail tension analysis, the likelihood of the rails ruptur- pairs of adjacent posts to identify the section carrying the ing during impact increased as more posts were removed. largest load. Tensions were tabulated for all rail sections located between post 9 and post 21, which included the entire area 11.5 Recommendation of contact between the rail and vehicle. There were clearly observed increases in the rail tension as the number of posts This study has examined the crash performance of strong- removed from the system increased. The rail tensions for all post w-beam guardrail with missing posts. The finite element simulations peaked at roughly 200 ms, although the tensions simulations conducted clearly showed that the removal of remained high during the full duration of vehicle redirection, posts from a guardrail had a strong adverse effect on the crash which occurred between 0 and 400 ms. performance of both the vehicle and guardrail, as summa- In Figure 51, the maximum observed tensions from the rized below: undamaged simulation are tabulated for each of the missing post simulations. For each additional post removed from the Vehicle collisions with guardrail systems missing posts have guardrail system, the maximum tension in the rail increased an increased risk of vehicle instability. While none of the by 2050 kN (4.511.2 kip). The maximum tension observed vehicles in the simulations overturned, several vehicles were was 352.8 kN (79.3 kip) for the guardrail missing three posts unstable after impact which could have led to rollover under with a mid-span impact. This was almost a 50% increase in some field conditions. Some of the vehicles exhibited signif- rail tension compared to the undamaged simulation, where icant skidding upon exiting the system, and these vehicles the maximum tension recorded was 237.4 kN (53.4 kip). would be easily tripped by irregularities in the ground. While the increase in rail tension as posts were removed was The removal of even one post can be expected to increase large, the maximum tension observed in the simulations was the system deflection by as much as 25 percent. Further still below the quasi-static tension limit of 410 kN. However, increases in deflection and stress were expected as more this did not necessarily mean that rail rupture could not occur, and more posts were removed from the system. The most as Ray et al. (2001) have shown that rail rupture typically oc- severe condition simulated (three posts missing) resulted curs at much lower rail tensions that can be reached in quasi- in a 50 percent increase in the maximum deflection of the 400 Beginning of Span 350 Middle of Span 300 Maximum Tension (kN) 250 200 150 100 50 0 Undamaged 1 Post Missing 2 Posts Missing 3 Posts Missing Figure 51. Maximum rail tension as a function of number of missing posts.

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