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42 CHAPTER 10 Evaluation of Crash-Induced Rail and Post Deflection Rail and post deflection is one of the most prevalent types parts: (1) an initial, low-speed impact to create a realistic repre- of damage in guardrail, most often caused by a lower severity sentation of minor rail deflection and (2) a subsequent full-scale crash. An example of this damage mode is shown in Figure 37. impact into the damaged section of guardrail. The goal of this Impacts where the vehicle speed is lower may result in local- test was to observe the effect of the guardrail damage on the ve- ized minor damage. Depending on the impact angle, the dam- hicle performance and to test the outcome as well as to provide age may be incurred only to the rail element, with minimal additional data for the validation of the finite element models. or no damage to the supporting posts and soil. Impacts with The strong-post w-beam guardrail installed for the pur- a higher speed but shallower angle can also result in more pose of this two-part crash test was 162 feet (49.5 meters) in distributed rail and post deflection. total length from end-to-end. All posts were steel strong posts The amount of deflection that can be sustained by a guard- and the w-beam rails were spaced out from the posts via the rail before its safety is compromised is a major concern. Main- use of routed wood blockouts. Tensioned end terminals were tenance crews and highway agencies are often forced to balance installed at both ends of the guardrails. The guardrail was ori- the expense of continual repairs against the potential liability if ented such that the vehicles would approach at a 25-degree the damaged guardrail is struck again. The survey of U.S. states angle of impact with the initial point of impact located 1.94 feet and Canadian provinces presented earlier in this report revealed (591 mm) before post 11. This impact point was computed that very few agencies have quantitative criteria underlying the using the critical impact point procedures described in NCHRP decision of when to replace a deflected guardrail. Among those Report 350. No modifications were made to the guardrail in agencies that have quantitative guidelines, the threshold deflec- between the first and second impacts. Further details on the tion was most commonly set at 6 inches (152 mm) of deflection. guardrail design can be found in the crash test reports (Fleck This is also the recommended limit for minor damage specified and Winkelbauer, 2008a, 2008b). by the Federal Highway Administration (FHWA, 2008). Indi- vidual agencies had limits as low as 3 inches (76 mm) or as high 10.2.1 Low-Speed Impact Test as 12 inches (305 mm). This study is intended to test the per- formance of guardrail with rail and post deflection to support a In the first impact, the impacting vehicle, a 1997 Chevrolet unified deflection limit based on quantitative data. 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 10.1 Objective the total 162-foot (49.4 meters) length. The maximum perma- The objective of this chapter is to present the results of an nent post and rail deflection was approximately 14.5 inches evaluation of crash induced guardrail rail and post deflection. (368 mm). The barrier successfully contained the vehicle. The The evaluation was conducted using a combination of full- vehicle came to rest alongside the barrier due to the low initial scale crash tests and simulations. speed of the vehicle. Figure 38 shows an overhead time series of the impact. 10.2 Evaluation Through Crash Tests 10.2.2 High-Speed Impact Test In August 2009, a test series was conducted by the MGA Re- search Corporation to evaluate the performance of damaged The day following the first, low-severity impact, a high-speed strong-post w-beam guardrails. This test was comprised of two test was run. In this test, another 1997 Chevrolet C2500 pickup
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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 Figure 37. Guardrail with rail and post deflection. 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. 0 ms 212 ms 71 ms 294 ms 142 ms 376 ms 177 ms 458 ms Figure 38. Time series for low-speed impact.
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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. 10.2.3 Results of the MGA Tests 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 With the exception of the vehicle roll, pitch, and yaw for the of the accelerations and OIV scores were lower than observed initial low-speed test, all of the NCHRP Report 350 criteria were in the test of undamaged guardrail. This was because the vehi- computed. These values are shown in Table 16. The results cle overrode the guardrail rather than being redirected, result- for Test TTI 405421-1, a crash test of undamaged strong-post ing in less crash energy being dissipated. This was reflected in w-beam guardrail with a Chevrolet C2500 pickup truck, are the higher exit speed. provided for comparison purposes. The guardrail deflection was far larger for the full-scale MGA For the low-speed crash test, all values were below what test than for the typical TL-3 crash test into an undamaged would be expected from a standard crash test. This was guardrail. This difference was attributed to the difference in