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OCR for page 52
52 guardrail changed very little for any simulation where a post For strong soils, the crash performance of barriers with post was prevented from separating. However, when the posts deflection up to 9 inches was satisfactory, whereas higher could freely separate from the posts, rail tension increased amounts of deflection were not. Impacts into rail with 11 inches with increasing pre-existing rail deflection. For 9 inches of prior deflection resulted in a rollover in the simulation. The (229 mm) of pre-existing deflection, peak rail tension was crash test into a rail with 14.5 inches of deflection resulted in 23 percent higher than the rail tension in the undamaged rail the vehicle vaulting over the rail. Adjusting for an extra margin simulation. Peak rail tension in the simulation of 6 inches of safety, e.g., to account for softer soils or overlapping damage (152 mm) of deflection was 19 percent higher than in the modes, the limit of acceptable post and rail deflection was set undamaged rail simulation. These results indicated that to 9 inches. Impacts into rail with 69 inches of prior deflection the risk of rupture increased modestly as the magnitude of were satisfactory in the simulations, but were associated with prior rail/post deflection increased. significant amounts of vehicle instability. The presence of The maximum rail height and length of deflection both any amount of deflection in the guardrail was found to in- increased with increasing amounts of pre-existing deflec- crease the amount of maximum dynamic deflection, so re- tion. The minimum rail height was roughly constant for pairs to guardrail with hazardous objects directly behind the any amount of prior deflection. Rail height, length of dam- guardrail should also be repaired as quickly as practical. age, and flattening extent were all factors which appeared Based on these initial simulations, a damage threshold of to contribute to crash income, but the relative influence of 6 inches of post and rail deflection has been set as the threshold each could not be isolated. Further study will be needed to for strong-post w-beam barrier repair. Deflection from 69 better understand these factors. inches was associated with significant amounts of vehicle insta- bility, and should be repaired with medium priority. Barriers with rail and post deflection above 9 inches should be repaired 10.6 Recommendation with a high priority as vehicle stability and rollover appears to This guideline was based upon two quantitative metrics: be a significant threat with barrier damage of this magnitude. (1) lateral post and rail deflection and (2) post height. The guideline further requires that this damage be fairly local- ized and form a pocket in the rail in order to require repair. The deflection must occur over a 25-foot or shorter length of rail. 10.6.1 Lateral Post and Rail Deflection The rationale is that 6 inches of deflection spread over 300 feet of rail would have an insignificant effect on performance Two full scale crash tests were conducted to evaluate this whereas the pocket formed by 6 inches of deflection measured guideline. In the first test, a 2000P vehicle (Chevrolet C-2500 over 25 feet of rail would be a potential safety concern. pickup truck) impacted a strong-post guardrail system at 30 mph (48.3 km/hr) and 26 degrees to induce damage to the barrier. The barrier successfully contained the vehicle in this 10.6.2 Post Height lower speed test. The vehicle came to rest alongside the bar- Depending on the extent of post and rail deflection, the rier. The impact resulted in damage to 36 feet (11 meters) of height of the w-beam in the damaged section may be lower than barrier length and a maximum post and rail deflection of ap- the original height of the strong-post w-beam. Several crash proximately 14.5 inches (368 mm). In the second test, another studies conducted in New York (Zweden and Bryden, 1977; Chevrolet C-2500 pickup truck collided with the area dam- Carlson et al., 1977; Bryden, 1984) have shown lower rail aged by the first test at a speed of 62 mph (100 km/hr) and heights to be associated with more vehicle penetrations. Con- 25 degrees. The damaged barrier failed to contain the impact- cern over the effects of lower rail height have led to the recent ing 2000P vehicle. The 2000P vehicle vaulted over the barrier development of new barrier systems such as the Midwest and came to rest upright behind the test installation. Guardrail System (Faller et al., 2007), the Gregory Mini-Spacer These tests show that 14.5 inches of lateral post and rail de- (Baxter, 2006), and the Trinity T-31 Barrier (Baxter, 2005). In flection is a damage level damage mode which requires repair. these new systems, the top of the rail is 31 inches from the The levels of damage below 14.5 inches of post and rail deflec- ground line compared to 27 inches for the modified G4(1S) tion which may be acceptable were investigated by finite ele- strong post barrier system. More recently, full-scale crash test- ment simulation. Simulations were conducted for post and ing at FOIL has shown that a strong-post w-beam rail height rail deflection varying from 3 to 11 inches (76 to 279 mm). that is 2 inches lower than the standard installation height re- The crash performance of rail with 3 inches of deflection was sulted in the 2000P test vehicle vaulting over the barrier (Mar- not markedly different than an impact into undamaged rail. zougui et al., 2007). Based on this information, the research However, the vehicle experienced significant roll beginning team recommends barrier repair for any crash damaged strong- at 6 inches (152 mm) of deflection and eventually rolled over post barrier where the top of the w-beam rail is 2 or more inches when the deflection reached 11 inches (279 mm). below the original top of the rail height (Exhibit 7.0).

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53 Exhibit 7.0. Recommendations for crash-induced rail and post deflection. Damage Mode Repair Threshold Relative Priority Post and Rail Deflection One or more of the following High thresholds: More than 9 inches of lateral deflection anywhere over a 25-ft length of rail. Top of rail height 2 or more inches lower than original top of rail height. 6-9 inches of lateral deflection Medium anywhere over a 25-ft length of rail. Rail Deflection Only 6-9 inches of lateral deflection Medium between any two adjacent posts. Note: For deflection over 9 inches, use post and rail deflection guidelines.