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OCR for page 85
85 CHAPTER 6 DESIGN GUIDELINES FOR THE USE OF CURBS WITH GUARDRAILS DEVELOPMENT AND VALIDATION be found in the literature review (Chapter 2). FEA simulations, OF DESIGN GUIDELINES indicated with a triangle in Figure 44, indicated that guardrail performance was not generally degraded by the presence of a Guidelines for the use of curbs and guardrails were devel- curb under the face of the guardrail (i.e., at 0-m offset) for all oped by reviewing the results of crash tests in the open road- operating speeds of 100 km/h or less. side safety literature, curbguardrail FEAs, bumper position There were no full-scale tests found in the literature for time-histories in curb-traversal FEAs and live-driver curb- offsets greater than zero, so the design chart was developed traversal tests, and full-scale crash tests of selected curb primarily using information from FEA simulations of curb guardrail combinations. These analyses are discussed in guardrail impacts. In addition, simulations and live-driver tests Chapter 5. of curb traversals (i.e., with no guardrail behind the curb) Six types of curbs were considered in the analyses: were used to assess the bumper height time history in order to AASHTO Types A, B, C, D, and G, and New York's T100 determine when the bumper would be positioned correctly curb, referred to as NY in the figures. These curbs are shown in with respect to a guardrail. As shown in Figure 44, there is a Figure 29. The barrier system considered in these analyses was region between 0 and 2.5 m in front of the guardrail where the the G4(1S), a strong-post W-beam guardrail with steel posts. FEA results were unacceptable. The single exception to this Figure 44 summarizes the results of the analyses of curb was the G curb at 2.5 m in front of the guardrail at 70 km/h. guardrail combinations. Solid-filled shapes in the figure indi- For the general case of vehicles leaving the roadway with a cate failed tests or simulations and open shapes indicate suc- broad range of speeds and angles, the bumper is likely to be cessful tests or simulations; the shading in the figure marks too high for acceptable guardrail performance in the region the different types of curbs used. Tests or simulations were up to a lateral distance of 2.5 m for typical 685-mm tall considered successful if they passed the criteria established guardrail systems. Guardrails should not be located any closer in NCHRP Report 350 (i.e., the occupant risk criteria were than 2.5 m from the curb line to minimize the chance of a satisfied and no rollover, vaulting, or underride was observed). vehicle vaulting over the barrier due to the bumper and sus- Circles indicate tests described in the literature, squares rep- pension system being too high. resent full-scale crash tests performed as a part of this project, FEA simulations did indicate, however, that once the sus- and triangles represent simulations performed in this project. pension and bumper had time to recover from the effects of The points are located near but not necessarily at the nomi- the curb traversal, placing a guardrail may be acceptable. The nal test condition. For example, there are five points near the necessary offset depends on the operating speed. For exam- 100 km/h 0-m offset point; all five tests were performed at ple, guardrails can be placed at a lateral offset of 2.5 m or these nominal conditions and are shown in a grouping sim- greater from 150-mm tall sloping curbs as long as the oper- ply because there are too many points to locate at the exact ating speed is 70 km/h or less. The reason for the restriction positions. The letters next to the shapes (e.g., C and NY) refer on the operating speed is that higher speeds create more sus- to the curb type. pension system disturbance and therefore require more time The offset distance in Figure 44 refers to the distance and distance for the bumper to return to the correct position. between the face of the curb and the face of the guardrail, illus- Guardrails can be placed at offsets of 4.0 m or greater from trated in Figure 45. As shown in Figure 44, a successful crash curbs that are not more than 100-mm tall as long as the oper- test is likely when the curb is positioned under the face of the ating speed is less than 85 km/h. Smaller curb heights cause guardrail for all speeds up to an operating speed of 100 km/h. less suspension system disturbances so these smaller curbs The majority of full-scale crash tests in the literature were can be used at higher speeds. performed with these impact conditions. Two of the four tests found in the literature that were performed at 100 km/h resulted in failures, but these failures are believed to be the DESIGN GUIDELINES result of problems with the guardrails in those particular tests rather than a problem with the interaction between the curb The recommendations that were developed can be sum- and guardrail. Further information on these failed tests can marized as follows.

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86 Roads with Operating Speeds of 71 to 85 km/h Any combination of a sloping-face curb that is 150 mm or shorter and a strong-post guardrail can be used at a lateral offset of 0 m (i.e., the curb is flush with the face of the guardrail) up to an operating speed of 85 km/h. In cases where guardrails are installed behind curbs, a lat- eral distance of at least 4 m is needed to allow the vehicle sus- pension to return to its predeparture state at these operating speeds. Once the suspension and bumper have returned to their normal position, impacts with the barrier should pro- ceed successfully. Guardrails may be used with 100-mm high or shorter sloping-face curbs as long as the face of the guardrail is located at least 4 m behind the curb. Vehicles traveling at speeds greater than 85 km/h may vault over the guardrail for some departure angles. Figure 44. Summary of crash tests for curbguardrail combinations. Roads with Operating Speeds Greater than 85 km/h Roads with Operating Speeds of 60 to 70 km/h Above operating speeds of 85 km/h, guardrails should only be used with 100-mm high or shorter sloping-faced Any combination of a sloping-faced curb that is 150 mm curbs, and the curbs should be placed at 0 m offset (i.e., the or shorter and a strong-post guardrail can be used at a lateral curb is flush with the face of the guardrail). Above operating offset of 0 m (i.e., the curb is flush with the face of the speeds of 90 km/h, the sloping face of the curb must be no guardrail) on roads with operating speeds of 85 km/h. more than 1:3 and must be no more than 100 mm high. Guardrails installed behind curbs should not be located Guardrails should not be located behind a curb on roads closer than 2.5 m for any operating speed in excess of 60 km/h. with operating speeds greater than 85 km/h. The vehicle bumper may rise above the critical height of the guardrail for many road departure angles and speeds in this region, making vaulting the barrier likely. A lateral distance Design Chart of at least 2.5 m is needed to allow the vehicle suspension to return to its predeparture state. Once the suspension and The recommended guidelines for the use of curbguardrail bumper have returned to their normal position, impacts with combinations are shown in Figure 46. The chart shows regions the barrier should proceed successfully. For roadways with operating speeds of 70 km/h or less, guardrails may be used with 150-mm high or shorter sloping-face curbs as long as the face of the guardrail is located at least 2.5 m behind the curb. Vehicles traveling at speeds greater than 70 km/h may vault over the guardrail for some departure angles. Offset Figure 46. Design chart for curbguardrail combinations Figure 45. Curb and barrier placement along roadways. by operating speed and offset distance.