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OCR for page 93
93 Each component of the guardrail model was validated both CONCLUSIONS quantitatively and qualitatively with laboratory tests, with the exception of the anchor system for which no test data The result of the foregoing analyses and testing was the were available. The modified NCAC C2500R (reduced ele- development of recommended guidelines for the use of curbs. ment) pickup truck model (i.e., model with modifications The results of the study of tracking vehicles traversing curbs made to the suspension system by WPI) was used to simulate where guardrails are not present indicated that the front the impact of a 2000-kg pickup truck. The NCAC C2500R bumper trajectory is only slightly affected by the impact model had been widely used in previous studies to analyze speed, impact angle, and slope of the curb face. The most sig- vehicle impact into roadside barriers and therefore the model nificant factor influencing the trajectory and vehicle stability had been generally debugged. in these tracking impacts is the height of the curb. The accuracy of the model's results was quantified prior to Vehicles also often interact with curbs in a nontracking con- being used in this study. The model was first used to simu- figuration. A tripping risk index (TRI) was developed to quan- late a 2000-kg pickup impacting the modified G4(1S) guard- tify the performance of curbs in nontracking situations. The rail at 100 km/h at an angle of 25 degrees. The results were index was developed using full-scale live-driver curb traversal validated by comparing them to a full-scale crash test docu- tests and finite-element simulations of a 2000-kg pickup truck mented in the literature, and it was concluded that the model traversing a variety of curbs in nontracking impacts. TRI val- provided realistic behavior of both the guardrail and vehicle ues above 45 were considered to indicate that vehicles were in such an impact event. at high risk of tripping whereas TRI values less than 20 pre- sented a very low risk. TRI values between 20 and 45 were The validated model was then used in a parametric analy- considered moderate. The best curb evaluated in this study sis to investigate the effects of various curb types in tracking was the New York curb which resulted in a TRI of just over impacts with a 2000-kg pickup truck on the stability and tra- 12. The AASHTO Types C and G curbs presented moderate jectory of the vehicle during simple curb traversals. The para- risk on high-speed roads and the AASHTO Types B and D metric analysis involved six curb types (AASHTO Types A, presented high risk for high-speed roads. When curbs must B, C, D and G and the 100-mm New York curb), two impact be used on high-speed roads, the shortest possible curb height speeds (70 and 100 km/h) and three impact angles (5, 15, and and flattest slope should be used to minimize the risk of trip- 25 degrees). ping the vehicle in a nontracking collision. The model was also used in a parametric study to investi- Guidelines for use of curbs in conjunction with guardrails gate the crashworthiness of curbbarrier combinations in were also developed. tracking impacts with the 2000-kg pickup truck. The para- Any combination of a sloping-faced curb that is 150-mm metric analysis involved the modified NCAC C2500R pickup or shorter and a strong-post guardrail can be used where the truck model impacting the modified G4(1S) guardrail model curb is flush with the face of the guardrail up to an operating (1) at impact speeds of 70, 85, and 100 km/h; (2) at an impact speed of 85 km/h. angle of 25 degrees; (3) and at offset distances from curb to Guardrails installed behind curbs should not be located barrier of 0, 2.5, and 4 m. The results of the curb traversal closer than 2.5 m for any operating speed in excess of 60 km/h. study indicated that the stability of the pickup truck was not The vehicle bumper may rise above the critical height of the compromised in tracking impacts, but the trajectory of the guardrail for many road departure angles and speeds in this front bumper was sufficient to imply a risk of barrier over- region, making vaulting the barrier likely. A lateral distance ride when a standard strong-post guardrail is placed anywhere of at least 2.5 m is needed to allow the vehicle suspension from 0.5 m to 7.0 m behind 150-mm high curbs or 0.6 m to to return to its predeparture state. Once the suspension and 7.0 m behind 100-mm high curbs. bumper have returned to their normal position, impacts with The results of the pickup truck model impacting various the barrier should proceed successfully. curbguardrail combinations confirmed that the presence of For roadways with operating speeds of 70 km/h or less, curbs was potentially hazardous. The results of the paramet- guardrails may be used with 150-mm high or shorter sloping- ric study were used to identify certain combinations that were face curbs as long as the face of the guardrail is located at more likely to result in acceptable barrier performance and least 2.5 m behind the curb. Vehicles traveling at speeds those more likely to result in unacceptable barrier perfor- greater than 70 km/h may vault over the guardrail for some mance, and guidelines defining proper curb type and barrier departure angles. placement were presented. It should be noted that even cases In cases where guardrails are installed behind curbs on identified as being successful resulted in poorer performance roads with operating speeds between 71 and 85 km/h, a lateral of the guardrail and a higher risk of injury for the occupants distance of at least 4 m is needed to allow the vehicle suspen- of the vehicle than was the case when the curb was not pres- sion to return to its predeparture position. Once the suspension ent. These guidelines were validated by full-scale crash tests and bumper have returned to their normal position, impacts of curbguardrail combinations. with the barrier should proceed successfully. Guardrails may
OCR for page 94
94 be used with 100-mm high or shorter sloping-face curbs as sloping face of the curb must be 13 or flatter and must be long as the face of the guardrail is located at least 4 m behind 100-mm high or shorter. the curb. Vehicles traveling at speeds greater than 85 km/h These recommended guidelines should help practitioners may vault over the guardrail for some departure angles. select appropriate curb and guardrail combinations at sites Above operating speeds of 85 km/h, guardrails should where both curbs and guardrails are necessary. Curbs should only be used with 100-mm high or shorter sloping-faced only be used on higher speed roadways when concerns about curbs, and they should be placed with the curb flush with the drainage make them essential to the proper maintenance of the face of the guardrail. Above operating speeds of 90 km/h, the highway.