Recommended Guidelines for Curb and Curb-Barrier Installations (2005)

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

Roads with Operating Speeds of 60 to 70 km/h Any combination of a sloping-faced 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) on roads with operating speeds of 85 km/h. Guardrails installed behind curbs should not be located closer than 2.5 m for any operating speed in excess of 60 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 of at least 2.5 m is needed to allow the vehicle suspension to return to its predeparture state. Once the suspension and bumper have returned to their normal position, impacts with 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. 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. Roads with Operating Speeds Greater than 85 km/h Above operating speeds of 85 km/h, guardrails should only be used with 100-mm high or shorter sloping-faced curbs, and the curbs should be placed at 0 m offset (i.e., the curb is flush with the face of the guardrail). Above operating speeds of 90 km/h, the sloping face of the curb must be no more than 1:3 and must be no more than 100 mm high. Guardrails should not be located behind a curb on roads with operating speeds greater than 85 km/h. Design Chart The recommended guidelines for the use of curb–guardrail combinations are shown in Figure 46. The chart shows regions Offset Figure 44. Summary of crash tests for curb–guardrail combinations. Figure 45. Curb and barrier placement along roadways. Figure 46. Design chart for curb–guardrail combinations by operating speed and offset distance.

where it is acceptable to use a curb–guardrail combination as a function of the lateral offset from the guardrail and the oper- ating speed of the roadway; the shading in the figure marks the different types of curbs. VALIDATION OF DESIGN GUIDELINES The foregoing design guidelines and chart were developed almost entirely with FEAs so it was necessary to validate the results with some full-scale crash tests. A series of full-scale crash tests were performed in this project to validate the design chart, as discussed in Chapter 5. The tests are indi- cated on Figure 44 with square shapes. The purpose of these tests was to validate the design chart by confirming that test failures and successes were observed in appropriate regions of the chart. E-TECH Test 52-2556-001 was an 85 km/h, 25-degree impact of the guardrail with a 150-mm high B curb located under the face of the rail. The test was a success and is plotted in the acceptable region of the design chart. Test 52-2556-002 was an 85 km/h, 25-degree impact of the guardrail located 2.5 m behind a 150-mm high B curb. Unfortunately, there was an installation error: the guardrail was 100 mm too short. The vehicle vaulted over the guardrail, so the test failed. The test conditions are plotted in the unacceptable section of the chart, although the incorrect rail height casts some uncertainty on this result. Test 52-2556-005 was a success, using a NY curb 4.5 m in front of the guardrail and impact conditions of 80 km/h and 25 degrees. This test is plotted in the acceptable region of the chart since the NY curb is a 100-mm high curb. The objective of Test 52-2556-006 was to validate the cor- ner of the 2.5-m offset, 150-mm high curb block. The guardrail was placed 2.5 m behind a 100-mm high NY curb, and the test was run at 70 km/h and 25 degrees. The test was a success and the impact conditions plotted in the acceptable region of the design chart, validating that portion of the chart. The last test, 52-2556-007, involved the same installation (i.e., a 100-mm high NY curb 2.5 m in front of the guardrail), but at a higher speed of 85 km/h. The FEAs and the design chart suggested that this test should be a failure, since it plots in the failing portion of the design chart. The crash test results, however, indicated it was a success. As mentioned earlier, the NY curb is characterized by a very low tripping risk index, so it seems likely that some very flat-faced, low-height curbs can be used 2.5 m in front of a guardrail even on some higher speed road- ways. In general, however, guardrails should be placed at least 4 m behind the curb on roads with operating speeds between 71 and 85 km/h unless testing or analysis of a spe- cific curb indicates that it will perform satisfactorily. Except when the guardrail was installed with the incorrect height, the design chart correctly predicts the results of all the full-scale tests. This indicates that the design guidelines and chart are valid based on a comparison of five full-scale crash tests performed at a variety of locations on the design chart. 87 TRIPPING RISK INDEX Development of the Tripping Risk Index The information obtained from the full-scale testing and the finite-element parametric analysis performed in this project was also used to develop a tripping risk index (TRI) for mount- able curbs. This index indicates the probability of a rollover based on events observed during the impact. The complexity of the problem under analysis makes the identification of the causes and effects difficult and probably impractical. It is pos- sible that two full-scale tests under the same nominal impact conditions could lead to dramatically different results (i.e., the vehicle may or may not overturn). Several events were identified that can be correlated to vehicle rollover during a curb impact: failure of one or two tires, rim-curb snagging, and rollover or outrigger engage- ment. A TRI value for each test or simulation was generated by assigning risk points to each adverse event recorded dur- ing the curb traversal. Points were also added based on the sta- bility ranking, a subjective value recorded by the driver that indicated the stability of the vehicle during the impact. Ta- ble 40 shows the points assigned to each event and parameter. The TRI for each test or simulation was then calculated as (1) where 33 is the maximum number of risk points possible, 3600 is a normalization factor in kilometers per hour squared, and V is the impact velocity in km/h. Note that the TRI can never be equal to zero since there is always the possibility that a curb may act as a tripping mech- anism due to some parameters or event not explicitly included in the TRI definition. The TRI is weighted by the inverse of the squared impact velocity (proportional to initial kinetic energy) to allow comparison of heterogeneous tests con- ducted at different speeds. TRI RiskPts V= × ⎛ ⎝⎜ ⎞ ⎠⎟ × ∑ 33 100 3600 2 , Event/parameter Risk pts Single tire failure 3 Double tire failure 5 Rim-curb snag 6 Rollover 10 Stability Ranking: Excellent Good Fair Poor 3 6 9 12 TABLE 40 Risk points for definition of the TRI

Table 41 shows the TRI values for the studied impact sce- narios. The TRI for each curb type is the arithmetic average of the TRI values for all the tests and simulations conducted on that particular curb type. Relationship of TRI to Design Variables The data in Table 41 suggest a correlation between the TRI and two geometric curb design variables: curb height and curb slope. To find an analytical approximate relation between the TRI and these two geometric variables, the method of least squares was used. (2) where a1 and a2 are regression coefficients, H is the curb height, and S the gross curb slope, computed as the curb height divided by slope base. For each tested curb type, (3) The problem is overdetermined, but it was solved using the least squares method: (4) This linear model has a coefficient of determination, R2, of 0.793. Figure 47 shows the TRI plane as a function of curb height and slope; it was plotted by substituting the correlation coef- ficients of Equation 4 into Equation 2. Curbs that are in the lower one-third of the chart are considered the safest. The tripping risk increases as the curb slope and height increase. The black stars represent the curbs studied in this research. The linear model was not able to correctly compute the TRI under all circumstances. For a very low curb with a nearly vertical face, the computed TRI indicated an unrealistic pos- sibility that the curb might trip the vehicle. This is contrary to intuition; if the height of the curb approaches zero, there is no curb to trip the vehicle. Since the linear model was not always appropriate, a nonlinear model was sought to better describe the TRI as a function of the two geometric parameters. (5) was assumed and linearized by taking the natural logarithms of both sides. With a simple transformation of the variables, TRI′ = ln(TRI), H′ = ln(H), S′ = ln(S) (6) A relation of the form ,TRI H Sa a= 1 2 a A A A TRI a a T T[ ] = [ ] [ ]( ) [ ] [ ] ⎡⎣⎢ ⎤⎦⎥ = ⎡⎣⎢ ⎤⎦⎥ −    1 1 2 0 0432 50 793 . . TRI H S a a TRI A ai i i nx nx x= [ ] ⎡⎣⎢ ⎤ ⎦⎥ [ ] = [ ] [ ]  1 2 1 2 2 1 A linear relation was first assumed of the form =TRI a H a S1 2 + , 88 a linear expression was obtained: TRI′ = H′ ⋅ a1 + S′ ⋅ a2 (7) The problem was then solved in transformed space follow- ing the procedure presented for the linear model. Solving Equation 5 after the applicable substitution yields (8) The coefficient of determination for this nonlinear model, computed in the transformed space, is 0.9912. Figure 48 is a perspective view of the surface described by Equation 8. Equation 8 was also used to develop the design diagram shown in Figure 49, identifying three areas of tripping risk. These areas were determined by tracing the isolevel lines of Equation 8, using a TRI of 20 for the first boundary and TRI of 45 for the second boundary. The diagram shows three rollover tripping risk regions: low risk, moderate risk, and high risk. The boundaries of the three regions are not defined uniquely since there is a certain degree of arbitrariness in TRI threshold values. The threshold values of 20 and 45 were selected after analysis of the data available. Figure 49 can be used as a design tool. For example, if a certain road needs a curb height of 120 mm for hydrological reasons and the curb must be placed within the clear zone for the roadway, the diagram suggests that the curb slope be less than 0.3 for a low risk of tripping errant vehicles. Conclusions This section has presented an approximate method to numerically evaluate the tripping risk offered by different types of curbs. The method was used to rank the different curbs studied in this research as shown in Table 42. Correlation between the TRI and two geometric curb design variables allowed the development of an approximate ana- lytical relationship of the TRI as a function of the curb height and curb slope, defined in Equation 8. This relationship fits the data both visually and statistically with a coefficient of determination of 0.99, which is exceptionally good for exper- imental data. Equation 8 was then used to develop the design diagram shown in Figure 49, which identifies three regions of low, moderate, and high risk of vehicle tripping offered by a curb characterized by its height and front face slope. Based on the tripping risk areas identified in Figure 49, the following can be concluded: • Curbs with an experimental or estimated (i.e., by Equa- tion 8) TRI above 45 should not be used on higher-speed roadways. a a TRI H S1 2 0 8333 0 79760 8333 0 7976 ⎡ ⎣⎢ ⎤ ⎦⎥ = ⎡ ⎣⎢ ⎤ ⎦⎥ → = . . , . .

89 Test name Impact Tire damage Rim-curb Rollover Stability Risk Percentile Tripping speed no. failed snag rating points risk points risk index Curb Type B V1-01_B 60.0 0 1 16 76.19 76.19 V1-02_B 60.0 1 0 9 42.86 42.86 V1-03_B 60.0 2 1 27 81.82 81.82 V2-01_B 80.0 0 1 28 84.85 47.73 V2-02_B 80.0 1 1 31 93.94 52.84 V2-03_B 80.0 2 1 27 81.82 46.02 603XB0135A 56.3 0 1 28 84.85 96.30 603XB0135B 56.3 1 0 18 54.55 61.91 603XB0235A 56.3 0 0 9 27.27 30.95 603XB0235B 56.3 0 1 1 0 1 1 0 1 1 0 0 0 4 4 4 4 4 3 3 3 9 27.27 30.95 56.76 Curb Type C V1-01_C 60.0 0 1 22 66.67 66.67 V1-02_C 60.0 1 1 25 75.76 75.76 V1-03_C 60.0 0 1 22 66.67 66.67 V2-01_C 80.0 1 1 25 75.76 42.61 V2-02_C 80.0 1 0 9 27.27 15.34 V2-03_C 80.0 2 1 27 81.82 46.02 530XC0135A 56.3 0 1 22 66.67 75.66 530XC0135B 56.3 0 0 9 27.27 30.95 530XC0235A 56.3 0 0 6 18.18 20.64 530XC0235B 56.3 0 0 0 0 0 0 0 0 0 0 0 0 4 4 4 4 2 4 4 3 2 2 6 18.18 20.64 46.10 Curb Type D 602XD0125A 40.2 0 0 0 3 9.09 20.22 602XD0130A 48.3 0 1 0 18 54.55 84.26 602XD0130B 48.3 0 1 0 18 54.55 84.26 603XD0135A 56.3 1 1 0 18 54.55 61.91 603XD0135B 56.3 0 1 0 18 54.55 61.91 603XD0135C 56.3 2 1 1 33 100.00 113.50 603XD0235A 56.3 0 0 0 6 18.18 20.64 603XD0235B 56.3 0 0 0 1 4 4 3 4 4 2 2 6 18.18 20.64 58.41 Curb Type G V1-01_G 60.0 2 1 27 81.82 81.82 V1-02_G 60.0 1 0 9 27.27 27.27 V1-03_G 60.0 0 0 9 27.27 27.27 V2-01_G 80.0 1 1 25 75.76 42.61 V2-02_G 80.0 2 0 11 33.33 18.75 V2-03_G 80.0 1 1 0 4 0 2 0 3 0 4 0 2 0 4 25 75.76 42.61 40.06 Curb Type NY V1-01_NY 60.0 0 3 9.09 9.09 V1-02_NY 60.0 0 3 9.09 9.09 V1-03_NY 60.0 0 3 9.09 9.09 V2-01_NY 80.0 0 3 9.09 5.11 V2-02_NY 80.0 0 3 9.09 5.11 V2-03_NY 80.0 0 6 18.18 10.23 527XN0120A 32.2 0 3 9.09 31.60 529XN0135A 56.3 0 6 18.18 20.64 530XN0135B 56.3 0 6 18.18 20.64 530XN0235A 56.3 0 3 9.09 10.32 530XN0235B 56.3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 2 1 2 2 1 1 3 9.09 10.32 12.84Tripping Risk Index for the Curb Type (Average of the tripping risk of each test): Tripping Risk Index for the Curb Type (Average of the tripping risk of each test): Tripping Risk Index for the Curb Type (Average of the tripping risk of each test): Tripping Risk Index for the Curb Type (Average of the tripping risk of each test): Tripping Risk Index for the Curb Type (Average of the tripping risk of each test): TABLE 41 TRI values by curb type

• Curbs that are located in the moderate risk area of the diagram should be avoided on higher-speed roadways. Their use may be acceptable where nontracking impacts are not probable (e.g., tangent section, warm climate, wide shoulder, or fenced roads) and on roads with 85th percentile speeds below 110 km/h. 90 • The use of low-tripping-risk curbs is recommended for roads with 85th percentile speeds above 110 km/h, where winter weather conditions (e.g., icing, snow, or mist) are expected and on poorly paved or drained roads. Low- tripping-risk curbs should always be used at access ramps and curves. Figure 48. TRI as a nonlinear function of curb height and slope. Figure 47. TRI as a linear function of curb height and slope.

91 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 20 40 60 80 100 120 140 160 180 Curb Slope Cu rb h ei gh t [ mm ] NY G C B D Area 3: High Tripping Risk Area 2: Moderate Tripping Risk Area 1: Low Tripping Risk Figure 49. Curb geometric design diagram with respect to the tripping risk in nontracking impacts. Safety rank Curb type TRI 1 NYDOT NY 12.48 2 AASHTO G 40.06 3 AASHTO C 46.10 4 AASHTO B 56.76 5 AASHTO D 58.41 TABLE 42 Curb safety in nontracking impact scenarios