National Academies Press: OpenBook

Criteria for Restoration of Longitudinal Barriers (2010)

Chapter: Chapter 4 - Evaluation of Vertical Tear Damage

« Previous: Chapter 3 - Research Approach
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Suggested Citation:"Chapter 4 - Evaluation of Vertical Tear Damage." 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 4 - Evaluation of Vertical Tear Damage." 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 4 - Evaluation of Vertical Tear Damage." 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 4 - Evaluation of Vertical Tear Damage." 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 4 - Evaluation of Vertical Tear Damage." 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 27

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23 The objective of this evaluation was to determine the effect of a vertical tear on barrier crash performance (Figure 14). The approach was to subject a barrier test section with an artificially introduced vertical tear to a pendulum test. The performance of the barrier section with the flaw was com- pared to the performance of a similar barrier section with no flaw. This chapter presents the pendulum test results of both the baseline undamaged section and the section with the ver- tical tear. The pendulum test setup was described in an ear- lier chapter on the research approach. 4.1 Baseline Tests Pendulum tests were conducted at two impact speeds: 32.2 km/hr (20 mph) and 28.2 km/hr (17.5 mph). In Test 03-2, the undamaged barrier contained the pendulum mass impact- ing at 30.9 km/hr (19.2 mph). The impact speed was calculated using the data from the pendulum-mounted accelerometers. The maximum dynamic deflection of the test section was 739 mm (29.1 inches) at 140 ms after the initial impact, com- puted from the pendulum acceleration data. The maxi- mum static crush at the center of the w-beam was 356 mm (14 inches). The overall damage and individual post damage is shown in Figures 15 and 16. The post at the splice location (center in Figure 15) experienced more torsion than the non- splice post and had some minor cracking at the flange. The post at the splice location also remained connected to the rail while the post bolt at the non-splice location pulled through the slot in the rail. There were no failures in the anchor cables in this test, and there was no visible separation of the cable from the swaged portion of the anchor cable assembly. At the splice location, there was approximately 19 mm (0.75 inches) of relative movement between the two w-beam rail sections. No tears were evident in the guardrail and no bolt failure was observed. The left portion of Figure 17 shows time sequence snapshots of Test 03-2 obtained from the high speed camera positioned overhead. In general, the remaining two undamaged barrier tests (not shown) had similar results to Test 03-2. Tests 01-2 and 07-1 both had successful containment of the pendulum mass. In both tests, the post-rail connection at the splice remained intact and post-rail bolt pullout was evident at the non-splice location. The separation at the splice was also approximately 19 mm (0.75 inches) in both tests. For Test 01-2, both the impact speed and maximum deflection were slightly higher compared to the analogous 2-cable test (Test 03-2, Table 15). In all three baseline tests, the undamaged barrier section demonstrated satisfactory impact performance by containing the pendulum mass. The cable end fixture designs provided adequate connection of the w-beam section to the existing rigid posts on either side of the pendulum. With the excep- tion of the shorter swaged cables, the end fixtures were con- structed with standard barrier hardware. The cable end fixtures provide a very rigid connection of the w-beam to the essen- tially rigid posts on either side of the pendulum. Based on an analysis of data from the rigid post mounted accelerometers, the maximum motion of each rigid post was approximately 1 inch (data not shown) toward the pendulum mass, which would have a tendency to slightly reduce the tension in the w-beam rail. This rigid connection coupled with the 32.2 km/hr (20 mph) pendulum impact speed, though, provides a very severe impact to the barrier section, which approaches the limit of the strong post barrier section. The ability of the w-beam barrier to withstand an impact of this severity is a tes- tament to its structural robustness. The pendulum tests appear to be an appropriate surrogate for determining the structural adequacy of w-beam barriers. A limitation of this test methodology is an inability to evalu- ate vehicle trajectory/stability as well as occupant risk. Most importantly, these tests provide insight into the crash per- formance of modified G4 (1S) strong-post w-beam barriers C H A P T E R 4 Evaluation of Vertical Tear Damage

24 Field Example Pendulum Test Setup – 4-in. Tear Pendulum Test Setup – ½-in. Tear Figure 14. Vertical tear evaluated in pendulum tests. Figure 16. Test 03-5: overall damage. that have sustained minor damage. Several of these damage modes have been tested with repeated tests, albeit with slightly different impact speeds. 4.2 Method of Introducing the Vertical Tear A vertical tear was simulated by cutting a “V” shaped notch in the w-beam using a reciprocating saw. The point at the end of the notch was intended to provide a stress concentrator similar to those observed in the field in a crash-induced ver- tical tear. In all vertical tear tests, the location of the tear cor- responded to the pendulum mass impact location, as this was believed to have the largest risk for rail rupture. Two dif- ferent length tears were tested: a 4-inch (102 mm) tear and a 0.5-inch (13 mm) tear. All tears started from the top of the Figure 15. Test 03-2: overall damage (right) and post damage at splice (center) and non-splice location (right).

w-beam section. For the 4-inch tear, the width of the tear was 0.5 inches at the w-beam edge and tapered to a point. For the 0.5-in (13 mm) tear, the width of the tear was approximately 4 mm (0.15 inches). 4.3 Results In Test 03-5, the barrier section with a 4-inch (102 mm) ver- tical tear was unable to contain the pendulum mass impact- ing at 20.4 mph (32.8 km/hr) (see Table 15). Impact speed was computed by analysis of the high-speed overhead video footage. A vertical tear developed from the bottom tip of the induced vertical tear and continued (approximately straight downward) through the entire w-beam cross section result- ing in a complete transection of the w-beam at the impact location. Based on an analysis of the overhead high-speed video data, the deflection of the rail was 533 mm (21 inches) at 90 ms after initial impact, which was just prior to penetra- tion of the w-beam section. At 118 ms after initial impact, the w-beam was completely transected. The overall damage and the post damage due to impact are shown in Figure 16. Other than at the damage location, there were no other tears evident in the w-beam. The right portion of Figure 17 shows time sequence snap- shots of Test 03-5 obtained from the high speed camera posi- tioned overhead. The performance of the analogous 3-cable test (not shown), Test 01-3, was very similar with the pen- dulum penetrating the barrier section due to a full cross- section tear at the impact location. Post damage and relative movement of the w-beam sections at the splice was similar. Again, the post-rail connection at the splice remained intact while the post-rail bolt pulled through the rail at the non- splice location. For the 0.5-inches (13 mm) vertical tear damage in Test 08-2, the barrier was able to contain the pendulum mass with a maximum deflection of 711 mm (28 inches). A vertical tear developed from the bottom tip of the induced vertical tear, at 13 mm from the w-beam edge, and continued (roughly straight downward) through approximately half of the w-beam cross section. The overall damage and close-up views of the tear propagation are shown in Figure 18. 4.4 Recommendation Pendulum testing of a strong-post w-beam barrier with a vertical (transverse) tear resulted in complete rail rupture. This was due both to the loss in an available cross section to carry the tensile load as well as the introduction of a stress concentrator. Although the vertical tear in the pendulum test was relatively severe (roughly one-fourth of the w-beam cross section), the research team believes that any vertical tear rep- resents a stress concentrator sufficient to cause further tear- ing should the barrier be subjected to a secondary impact. A second pendulum test was conducted for a small vertical tear of 13 mm (0.5 inches) in length. A pendulum impact at 20 mph caused this small tear to grow ominously to a length of 4 inches The research team concluded that no vertical tear is safe. To be cautious, the research team has classified vertical rail tears of any length as a damage mode that should be repaired with high priority (Exhibit 1.0). 25 Damage Mode Test # End Fixture Impact Speed (km/hr) Maximum Deflection* (mm) Time of Max Deflection (ms) Crash Performance 01-2 3-Cable 32.7 767 136 Containment 03-2 2-Cable 30.9 739 140 Containment Undamaged 07-1 2-Cable 28.3 610 140 Containment 01-3 3-Cable 32.0 592* 84 Penetration, Tear at center Vertical Tear (4 in.) 03-5 2-Cable 32.8 533* 90 Penetration, Tear at center Vertical Tear (0.5 inch) 08-2 2-Cable 32.9 711 144 Containment, Tear at center Table 15. Pendulum testing summary for vertical tears.

26 Undamaged Rail, Test 03-2 (30.9 km/hr) 4-inch Vertical Tear Damage, Test 03-5 (32.8 km/hr) 0.02 s 0.06 s 0.03 s 0.06 s 0.10 s 0.14 s 0.09 s 0.12 s 0.18 S 0.22 s 0.15 s 0.18 s 0.26 S 0.30 s 0.21 s 0.24 s Figure 17. Sequential overhead photographs for undamaged section (left) and 4-inch vertical tear damage (right). Figure 18. Test 08-2: overall damage (left) and detail views of the additional tearing caused by the pendulum impact (center), and post damage at splice location (right).

27 Damage Mode Repair Threshold Relative Priority Horizontal tears Horizontal (longitudinal) tears greater than 12 in. long or greater than 0.5 in. wide should be repaired with a medium priority. Note: for horizontal tears less than 12 in. in length or less than 0.5 in. in height, use the non-manufactured holes guidelines. Medium Exhibit 1.0. Recommendations for vertical tear damage repair.

Next: Chapter 5 - Evaluation of Horizontal Tear Damage »
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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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