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35 The objective of this evaluation was to determine the effect of a missing blockout on barrier crash performance through pendulum testing (Figure 29). The performance of the barrier section with a missing blockout was compared to the perfor- mance of a similar barrier section with a blockout in place. The pendulum test setup is described in an earlier chapter on the research approach. For this damage mode, the blockout at the splice location was not installed. The post-rail bolt remained connected to simulate a wooden blockout that had split completely and was no longer present. Two pendulum tests were conductedâone at 19.0 mph and one at 18.2 mph. In both tests, there was approximately 178 mm (7 inches) of separation between the near flange of the post and the back of the w-beam rail. The splice location was thought to be the critical case as the splice is the weakest link in the rail element. 8.1 Results For the missing blockout damage, a high-speed test and low- speed test were performed. In the high-speed test (Test 03-7), the barrier was unable to contain the pendulum mass impact- ing at 30.6 km/hr (19.0 mph). Impact velocity was calculated based on the high-speed video footage. The barrier section failed at the splice due to the splice bolts pulling through holes in the rail with none of the individual splice bolts fracturing. This failure was similar to that observed in Test 02-1 with the horizontal tear damaged section. Based on an analysis of the overhead high-speed video data, the deflection of the rail was approximately 678 mm (26.7 inches) at 106 ms after the ini- tial impact, which was just prior to penetration of the w-beam. At 116 ms, the splice was completely separated. The deformation of the posts was less than in the undam- aged section test, as there was almost no torsion experienced by the post at the splice location. There was no visible crack- ing of either post. As with most previous tests, the post-rail bolt pulled through the w-beam rail at the non-splice location. The post-rail bolt at the non-splice location experienced large bending deformation while the bolt at the splice location frac- tured in the threaded region. Figure 30 shows the post dam- age and the splice failure. No tear developed and none of the splice bolts failed, but rather the bolt holes deformed enough to allow the two sections to separate. Figure 31 shows time sequential snapshots of the Test 03-7 obtained from the overhead high speed camera. In the lower speed test (overhead sequence not shown), the barrier was able to contain the pendulum mass impacting at 29.3 km/hr (18.2 mph) (Figure 32). Impact velocity was cal- culated based on the high-speed video footage. The deflection of the rail was approximately 467 mm (26.7 inches) at 121 ms after the initial impact. In Test 01-4, the barrier had a different impact performance. The barrier section contained the pendulum mass impacting at 30.9 km/hr (19.2 mph). Based on an analysis of the overhead high speed video data, the maximum dynamic deflection of the rail was 719 mm (28.3 inches) at 146 ms after the initial impact. The asymmetry caused by the missing blockout resulted in a significant twisting of the pendulum (approximately 6 degrees) in the horizontal plane. The vertical tear that developed at the splice location was 229 mm (9 inches) in length (approximately two-thirds of the total w-beam cross section) and along the line of the splice bolts. A close-up of the tear is shown in the center image in Figure 33. 8.2 Recommendation A pendulum test conducted at 20 mph of a strong-post w-beam barrier section with a missing blockout at the splice location resulted in successful containment of the pendulum mass (Test 01-4). There was strong evidence, however, that this damage mode could result in tearing of the w-beam. Dur- ing the test, the absence of the blockout allowed the rail to be C H A P T E R 8 Evaluation of Missing Blockout Damage
36 Field Example Pendulum Test Setup Figure 29. Missing blockout damage evaluated in pendulum tests. Figure 30. Test 03-7: splice failure (left) and post damage at splice (center) and non-splice location (right). driven into the steel post resulting in a 9-inch tear in the rail. It is possible that there may have been less of a propensity for tearing for wood posts. Another pendulum test was conducted at an impact speed of 17.5 mph to better represent the kinetic energy loading to a single section of barrier during a full-scale NCHRP Report 350 test. In this test, the barrier successfully contained the pendulum mass with no evidence of w-beam tearing. To be cautious, the research team has proposed a threshold of one or more missing blockouts. This damage has been assigned a medium priority based on the potential for rail tearing that has been observed in the pendulum test. With regards to potential vehicle instability resulting from impact, the research teamâs rationale was that collisions into barriers with a missing blockout would fall between a missing post case and the undamaged case. Finite element simulations of a 2000P pickup truck collision with the post missing the blockout were conducted to verify this. The results of finite element simulations of collisions into an undamaged barrier and into a barrier with a missing post are described later in this report. In the missing blockout simulations, the vehicle response was very similar to the missing post simulations in the early phase of the collision. Early in the collision, the vehicle interacts with a rail with no blockout or post support. However, once the vehicle has deflected the rail the width of a blockout, the vehicle begins to interact with the post. The simulation from this point on greatly resembles an ordinary collision into an undamaged rail section. The missing block- out case does however result in elevated vehicle instability, but not to the extent of the missing post case. The research team has set the repair priority of a missing post accordingly as âmediumâ which falls between the undamaged case and the high-priority repair of a missing post damage mode. Note that this repair priority is also consistent with the rail-post separa- tion case which allows small amounts of separation (less than 3 inches), but rates higher rail-post separation as a medium- priority repair (Exhibit 5.0).
37 Missing Blockout Damage, Test 03-7 (30.6 km/hr) 0.017 s 0.034 s 0.051 s 0.068 s 0.085 s 0.102 s 0.119 s 0.136 s Figure 31. Sequential overhead photographs for missing blockout damage. Figure 32. Test 07-5: overall damage (left), detail view of damage at splice (center), and post damage at non-splice location (right).
38 Figure 33. Test 01-4: overall damage (left), rail tear at splice (center), and post damage at splice location (right). Damage Mode Repair Threshold Relative Priority Missing Blockout Any blockouts that have the following issues: Missing, Cracked across the grain, Cracked from top or bottom of blockout through post-bolt hole, and Rotted. Medium Exhibit 5.0. Recommendation for missing blockout damage repair.
