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OCR for page 73
73 Figure 63. Simulation of 75% flattened guardrail leaning back by 10 degrees. Vehicle before impact on the left, and after the impact (t = 0.7s) on the right. speed was 73 kph. It was believed that by flattening the rails 25 percent to 100 percent. The following observations were before impact, the ability of the guardrail to absorb kinetic noted: energy was being reduced. To check if this was the case, the energy absorbed by the vehicle and guardrail was broken Vehicle roll and pitch increased with increasing degrees down by component. of flatness. The vehicle became unstable once the flatten- In the 100 percent flattening simulation, the guardrail ing reached 75 percent. At 100 percent flattening, the vehi- absorbed roughly 40 kJ less than, or 83 percent of, the energy cle rolled as it exited from the guardrail. Note that these that was absorbed by the undamaged simulation. When bro- simulations were conducted for perfectly upright posts. ken down even further, it was found that the rails actually Based on field inspections of damaged barriers, the research absorbed about 45 kJ less energy, but the other components team has observed that rail flattening almost always occurs of the guardrail absorbed 5 kJ more energy, resulting in the net in tandem with some degree off post and rail deflection. drop in energy absorption of 40 kJ. The components of the Any incline in the post would exacerbate the tendency guardrail that absorbed more energy were the posts and block- for vehicle rollover or instability. Therefore it is recom- outs, as the flattening of the guardrail allowed the vehicle to mended that all guardrails for which there is 50 percent engage these components more easily. or greater flattening be repaired as soon as possible due Since it was believed that the flattened rails created a ramp- to a greatly increased risk of vaulting and rollover. like surface, a supplementary simulation was performed to In any situations where there is a hazardous object directly examine how the angle of the guardrail would affect the per- behind the guardrail, the damage should be repaired imme- formance. The finite element model of 75 percent flattened diately because even a small amount of rail flattening rail was modified by bending the posts and rails in the area increased the maximum deflection of the guardrail by of contact backwards. This resulted in an 80-degree angle roughly 10 percent. between the post and ground line rather than the standard 90 degrees, as shown in Figure 63. The slight incline in the One observation from the field inspections of damaged bar- rails was sufficient to cause the vehicle to both vault and roll. riers was that it was difficult to quantify the amount of rail flat- From these results, it was evident that the angle of the guard- tening by direct measurement of the w-beam cross section. As rail, whether caused by damage or pre-existing because the an alternative method of determining the amount of rail flat- ground was sloped, could drastically alter the outcome of a tening, the research team is proposing a method where the crash. However, the same incline in an otherwise undamaged maintenance personnel can measure the maximum section guardrail had little effect on the outcome of the simulation. width of the flattened w-beam cross section, a much easier In the future, the full effect of combined incline and flatten- measurement to obtain. Based on finite element simulations ing should be examined in more detail. of flattened w-beam barriers, the research team has correlated the maximum deformed cross section height to the approx- imate portion of rail flattening. Fifty percent flattening cor- 13.4 Recommendation responds to a growth in section width from 12 inches for A series of finite element simulations of impacts into undamaged rail to 18 inches. In the guidelines, the 50 percent flattened strong-post w-beam guardrail were run and com- flattening limit is prescribed as a section width of 18 inches or pared to the performance of the undamaged guardrail sim- greater. See Exhibit 10.0 for recommendations for rail fatten- ulation. The flattening in these simulations varied from ing repair.

OCR for page 73
74 Exhibit 10.0. Recommendation for rail flattening repair. Damage Mode Repair Threshold Priority for damage above the threshold Rail Flattening Rail cross-section height more than 17 in. Medium (such as may occur if rail is flattened) Rail cross-section height less than 9 in. (such as a dent to top edge)