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44 Vehicle Dynamics Analysis for Vehicles Leaving the Traveled Way on CSRS 4.1 Background The curvature and surface slope on a roadway are known to effect vehicle dynamics and influence vehicle trajectories, orientation, and speed. On curved sections, the vehicle is more likely to leave the road at a sharper angle and consequently impact the barrier with greater force that could potentially result in higher impact severity. The degree of superelevation in com- bination with the shoulder slope can lead to variations in the vehicle-to-barrier interface which can increase vehicle insta- bility, barrier climb, vehicle rollover, or override/underride. Further, the superelevation with a negative shoulder slope might cause the vehicle to impact the barrier at a different ori- entation (roll and pitch). Thus, an important starting point for the analysis of barriers on CSRS is understanding the dynamics of vehicles as they leave the traveled way on CSRS and interface with barriers. A considerable amount of effort has recently been devoted to analyzing the dynamic effects of vehicles on non-level terrain and the subsequent effects on their trajectories and interfaces with barriers. VDA has been shown to provide new insights on the effects of a vehicleâs suspension system on tra- jectories in all three dimensions. For example, trajectory data in the vertical direction is directly related to the interface of the vehicle and the barrier. The slope changes from the road- way to the shoulder could affect the vehicleâs trajectory and cause it to contact the barrier too high, which may lead to undesirable override or underride conditions. The combined effect of the superelevation of the roadway, the slope of the shoulder, and the side slope of the roadside for a vehicle leav- ing the roadway in a curve can be explicitly analyzed using VDA tools. These tools readily allow the range of combina- tions of roadway, shoulder, and side slope design features to be analyzed for varying types of vehicles, and their paths or trajectories can be determined. Guidelines for the testing and deployment of roadside safety barriers on sloped surfaces and curved sections are limited. For example, crash testing protocols for barriers have evolved to provide a practical worst-case impact condition that is reproducible and comparable. Thus, barriers are tested under idealized impact conditions, with the barrier being tested installed on a straight and level section minimizing the roll, pitch, and yaw effects on the impacting vehicle. These protocols have evolved to determine whether safety hardware is âcrashworthy.â While crash testing protocols have evolved to include tests for a variety of angular impact conditions, one aspect that is not fully addressed is the crashworthiness of barriers installed on CSRS. A review of the literature revealed only a few older efforts address the safety of designs or pro- vide guidance for placement on CSRS. The need exists to systematically analyze a typical set of curved, superelevated roadway situations and the possible paths of errant vehicles to understand (1) the trajectories along the possible vehicle paths, (2) the associated vehicle-to- barrier interfaces for various barrier types and placement, and (3) how the stability of the vehicle (i.e., functions of induced roll, pitch, and yaw effects) may affect the engagement with the barrier and its crashworthiness. VDA results provide a convenient means to understand trajectories and interface scenarios, as well as indicate those critical scenarios that may warrant crash simulation analyses. The analysis of the overall motion of a vehicle can be very complex, especially at higher speeds. However, vehicle motion is primarily governed by the forces and moments generated by the interaction of the tires and the ground. In vehicle dynamics studies, six degrees of freedom are studied: longi- tudinal, lateral, and vertical displacement; and roll, pitch, and yaw angles. Generally, the vehicle fixed coordinate system is associated with the CG of the vehicle, but it is possible to generate metrics that allow the frontal interface region for each vehicle to be determined. The data allows the evaluation of potential barrier effectiveness given road departure speed and angle for the surface conditions associated with the road- way, shoulder, transition to the side slope, and the side slope. C H A P T E R 4
45 Such metrics are important for understanding the position of the frontal region of the vehicle relative to the barrier. 4.2 Objective The objective of the research reported in this chapter was to apply vehicle dynamics tools to assess the trajectories of vehicles leaving the traveled way on CSRS. The intent was to develop a better understanding of the influence of vari- ous roadway curvatures, superelevation, shoulder/roadside designs, and barrier features and placement on the dynamic response of vehicles and to assess the safety performance of barriers used in these situations. 4.3 Research Approach Vehicle dynamics simulations were performed to assess vehiclesâ trajectories as they crossed from the traveled way to varying shoulder and side slope conditions for different road- way curvatures and superelevation. Simulations were con- ducted with varied vehicles, speeds, and departure angles. The following sections describe the VDA set ups, the software tool used, factors considered, and the cases selected for analyses. 4.3.1 Vehicle Dynamics Analysis Applications The concept of using vehicle dynamics simulation software to analyze run-off-road vehicle behavior and motion is gain- ing popularity. In 1997, McMillan et al. conducted simula- tion studies to analyze driver response to roadway departure. This analysis was used to evaluate the ability of collision counter measure systems to prevent run-off-road accidents. Similar analyses have been performed by Pape et al. (1996) and Hadden et al. (1997) where they extended the VDANL (vehicle dynamics analysis, nonlinear) model of the vehicle/driver to assess the effectiveness of the counter measure system. Other studies have focused on the results of an off-road crash. Day and Garvey (2000) used EDVSM (Engineering Dynamics Vehicle Simulation Model) to perform rollover simulations. They described the limitations of rollover simulation for on-road and off-road accident reconstruction. The use of simulation software for the analysis of off-road crashes has been broad. Claar et al. (1980) concentrated on suspension modeling for improving off-road ride comfort, whereas some studies have focused on friction influences in the case of water or snow on the road surface, as did Mancosu (2002). There has been little research using vehicle dynamics sim- ulation software to analyze and enhance the roadway design itself. Sicking and Mak (2004) presented a paper which sug- gested that efforts should focus on developing better vehicle and roadside safety hardware models. Also, they indicated that significant effort must be devoted to improving the capability of computer simulations to model run-off-road crashes. The NCAC (National Crash Analysis Center) staff used the HVE simulation program to study the effect of edge drops on guard- rail roadside barrier performance (Marzougui et al. 2007). They used varied initial conditions and different vehicles to analyze the behavior of the vehicle encountering various edge drops. The NCAC used VDA to trace two critical points on impacts with W-beam guardrails to determine barrier effectiveness relative to vehicle underride or vaulting. Similarly, the NCAC made extensive use of VDA to analyze the effects of median configurations on the effectiveness of cable barrier placement (Marzougui et al. 2008a, 2009a, 2010a). A major use of VDA that provided the basis for guidelines for the placement of cable median barriers was reported in NCHRP Report 711 (Marzougui et al. 2012a). Last, a study conducted at Penn- sylvania State University showed the utilization of commer- cially available VDA software as a tool to analyze the effect of highway median width and slope on vehicle stability. The researchers used the CarSim programs to run thousands of simulations using different vehicles, median widths and slopes, steering conditions, and initial conditions to gener- ate various metrics, including roll and lateral velocity. The resulting data was used to provide a preliminary assessment of tradeoffs in the size and slope of median profiles versus the types of accidents observed (Brennan and Hamblin 2007). 4.3.2 Analyzing Vehicle Dynamics There is a well-developed body of knowledge about the physics of vehicles that has evolved with the automotive industry. Detailed VDA has been packaged into commercially available software tools. The VDAs in this effort were under- taken with the CarSim software. CarSim is a nonlinear vehicle simulation program capable of analyzing vehicle-roadway interaction and providing a detailed description of the vehicleâs trajectory taking into consideration speed, weight, suspension system, surface features, and other factors. It is readily linked to development tools such as MATLAB to extend its function- ality. It also allows batch inputs to reflect ranges of conditions that define performance enveloped. 4.3.3 Critical Vehicle Interface Analysis Approach The findings of the literature review, the state DOT sur- vey, reviews of design documents like the Green Book, and discussions with the NCHRP Project 22-29A panel led to the identification of factors believed to affect the safety perfor- mance of longitudinal barriers placed on CSRS. The initial
46 set of factors and specific parameters associated with them are indicated below: â¢ Barrier type â Concrete barrier [height â¤ 32 in. (813 mm)]: NJ concrete barrier â Strong-post W-beam guardrail [height < 31 in. (787 mm)]: G4(1S) â Strong-post W-beam guardrail [height â¥ 31 in. (787 mm)]: MGS â¢ Vehicle type â 2270P pickup truck: 2007 Chevrolet Silverado Model â 1100C small car: 2010 Toyota Yaris Model â¢ Curvature/superelevation combinations â 614 ft (187 m)/12% â 2,130 ft (649 m)/12% â 758 ft (231 m)/8% â 2,670 ft (814 m)/8% â 833 ft (254 m)/6% â 3,050 ft (930 m)/6% â¢ Shoulder width and slope â 4 ft (1.22 m), 8 ft (2.44 m), and 12 ft (3.66 m) shoulder widths â 0%, 3%, 6%, and 8% shoulder angles â¢ Roadside slope â 12H:1V (negative) from the edge of shoulder for all shoulder slopes â¢ Impact conditions â Three impact angles: 20Â°, 25Â°, and 30Â° â Three impact speeds: 57 mph, 62 mph, and 67 mph (90 km/h, 100 km/h, and 110 km/h) â¢ Barrier placement relative to road section â Lateral position: at edge of shoulder, 4 ft (1.22 m) offset, and 8 ft (2.44 m) offset â Vertical orientation: normal to road and parallel to true vertical VDA software was used to model vehicle behavior when traversing the shoulder and side slope for the above range of conditions to obtain trajectories for each case. Aggregat- ing the results across subsets of these parameters allowed the generation of maximum and minimum trajectory traces that provide a means for analyzing the vehicle-to-barrier interface for varying lateral placement. These results provide a basis for identifying critical scenarios for the FE simulations, as well as providing insights useful to generating proposals for improved practices. 4.4 VDA Considerations Undertaking VDA requires information about vehicles, the barriers to be studied, the effective interface areas, and the terrain or surface conditions associated with CSRS. The fol- lowing sections describe these aspects as they were defined for this research. 4.4.1 Vehicles Considered The research focused primarily on two types of vehicles typically found on U.S. highways: a Chevrolet Silverado pickup truck (2,270 kg) and a Toyota Yaris sedan (1,100 kg). These vehicles correspond to test vehicles defined in MASH. The specific weight, size, frontal geometry, and suspension systems of these vehicles were incorporated into the VDA. In these analyses, two points were defined for each type of vehicle considered to represent the primary interface (engage- ment) region on the vehicle. These are labeled Point 1 and Point 2 in Figure 4.1. The points are located at positions on Point 1 Point 2 Figure 4.1. Vehicle models used in VDA and their interface points.
47 the front of the vehicles that represent the engagement point that differentiates between tendencies to override or under- ride a barrier. Point 1 for the small vehicle is located at a height of 21 in., while Point 2 for the pickup is at a height of 25 in. These point positions were defined by examining the frontal profile of the vehicles and reviewing full-scale crash tests conducted using similar vehicles. The traces of these points are critical in determining the interface with barriers for any vehicle trajectory. 4.4.2 Vehicle-to-Barrier Interface Regions Three barriers were selected for analysis and an inter- face region was defined such that if the two critical points (Point 1 and Point 2) are inside this region at the start of the impact, the barrier is considered likely to redirect the vehicle. If Point 1 (from the small car) falls below the interface region, an underride or significant snagging is likely to occur. Simi- larly, if Point 2 (from the pickup truck) is above the inter- face region, vehicle override is likely to occur. The interface regions are shown with a shaded box in Figure 4.2 as the maximums and minimums. These regions are based on the geometry of the barrier and a review of full-scale crash tests conducted on these barriers. For the concrete barrier, only the override condition is considered, so there is no minimum. It is important to note that these interface analyses accounted for the effects of vehicle orientation (changes in roll, pitch, and yaw angles) in computations to determine the positions of Points 1 and 2 relative to the vehicle CG. Further, varia- tions in the designs of these barriers, such as the inclusion of rub rails, increased heights, or different shapes for the con- crete barrier were not considered. The evaluations based on these interface regions were only used in the VDA as prelimi- nary criteria to identify the set of cases to be simulated in the FE analysis. The actual impact is simulated in the FE evalua- tions, and the barrier performance is assessed based on these results. 4.4.3 Roadway Curve Conditions Various degrees of roadway curvature were considered reflecting the range of superelevation applications commonly found on highways. These range from tight curves used on ramps to gentle sweeping curves. Figure 4.3 provides exam- ples of the range of curves considered in the simulation. A total of six roadway curve conditions with different curvature and superelevation were used in the VDA. These conditions were selected based on the Green Book design supereleva- tion tables. The analyses incorporated three superelevations (6%, 8%, and 12%). For each superelevation, two curvatures were selected representing the minimum radii at the 50-mph (80-km/h) and 80-mph (130-km/h) design speeds. 4.4.4 Analysis of Vehicle Trajectories on CSRS Figure 4.4 shows the typical path or trajectory (via sequen- tial vehicle images) of a vehicle attempting to negotiate a curve before departing the roadway, as marked by the red line. The cross section of a superelevated curve perpendicular to the centerline (as indicated by the black line) is depicted in the figure. In this case, the banking of the roadway surface is exaggerated. The shoulders can be designed to have the same slope relative to the roadway cross section or a negative slope for drainage purposes. The red line shows the typical path or horizontal trajectory of an errant vehicle leaving the road on a CSRS. It shows a rising surface reflecting a diagonal crossing of the superelevation, followed by diagonally traversing the negative shoulder and side slope. In the VDA, the vehicle was run a distance of about 1,000 ft (300 m) on this surface to be in a âcurve operationâ equilibrium state before it was directed off the road. Several predefined departure paths were input into the software to represent various departure angles. Repeated simulations of vehicles traversing such paths were conducted. These were varied to (a) G4(1S) (b) MGS (c) NJ Concrete Barrier Note: The shaded boxes represent the interface regions. Figure 4.2. Interface regions for the three barriers selected.
48 reflect exit angles of 20Â°, 25Â°, and 30Â° for the vehicles travel- ing at 57 mph 62 mph, and 67 mph (90km/h, 100 km/h, and 110 km/h). In this research, the roadway to shoulder slopes that were analyzed are depicted in Figure 4.5 with a 12H:1V roadside slope. It is important to note that these cross sections are consistent with the guidance provided in the Green Book. The Green Book defines cross slope in Figures 4-2A and 4-2B, which define âroll-overâ as the algebraic difference in rate of cross slope. It also notes that âroll-oversâ should not exceed 8%. The scheme defined for this research is consistent with these requirements. A number of different possible conditions for road depar- tures were considered in the VDA with the following underly- ing assumptions: â¢ The vehicle carries one average-sized male occupant. â¢ The roadside has a firm surface, meaning tire furrowing into the surface is negligible. â¢ Vehicles are âtrackingâ as they enter the roadside (i.e., vehicle initial speed is in the same direction as its longitudinal axis). â¢ There are no driver inputs (e.g., steering, braking) that affect the vehicle. â¢ The tire-to-road friction was made identical in all runs using a friction coefficient of 0.9. â¢ The simulation software provided dynamics analysis results every thousandth of a second as the vehicle traversed the roadway, shoulder, and side slope. â¢ There is a smooth transition between the pavement and shoulder, and between the shoulder and side slope, to limit any other effects that might alter vehicle stability. 4.4.5 VDA for a Worst-Case Departure Scenario The dynamic effects on a vehicle traversing a worst-case path for a CSRS without a barrier was undertaken to better (a) Sharp Curvature 614 ft (187 m) (b) Gentle Curvature 3,050 ft (930 m) Figure 4.3. Sample variations in roadway curvature. Cross Section at CSRS Path or Horizontal Trajectory of the Vehicle Figure 4.4. Sample VDA perspective of a vehicle leaving the road.
49 Figure 4.5. Vertical surface cross sections analyzed for superelevated curves.
50 understand the effects as reflected in changes in the vehicleâs trajectory (i.e., x-, y-, and z-coordinates, and the roll, pitch and yaw angles). The effects were considered to be the greatest where the higher slopes and inflection changes took place. The worst case is represented by the cross section in Figure 4.6. The analyses also consider that the vehicle is on a diagonal path, so the right front tire will incur a change before the left front tire and so forth. Such changes imply that the changes at Points 1 and 2 located on the right front will be different for similar points on the left front. The VDA results shown in Figure 4.7 reflect the differences observed between four cases with two vehicles (1100C small car and 2270P pickup) and two road profiles (with and with- out superelevation). Figure 4.7(a) shows the effect on the roll angle of the vehicle. This plot covers a duration of 12 s, but the critical period is between 4 s and 7 s (as indicated by the verti- cal lines) where the vehicle is reaching the shoulder, traversing it, and then encountering the side slope. Similar patterns are noted for both vehicles and for both the superelevated and the non-superelevated cases. A negative roll begins when the tire encounters the shoulder slope, but it is countered as more of the vehicle gets on the shoulder. The roll effect becomes con- stant once the vehicle gets onto the side slope. The variation between the sets of curves reflects the roll effect induced by the superelevation. Figure 4.7(b) shows the changes in pitch angle in travers- ing the cross section with the greatest amount of deviation associated with the shoulder. It must be noted that while the deviations are great, the scale reflects small changes in pitch. The inflection points occur when the shoulder and the side slope are reached for either vehicle. The effect on the pickup is greatest for the pickup without superelevation. Figure 4.7(c) shows the changes in yaw angle. The dynam- ics of both vehicles is similar for all cases as the vehicle tra- verses the shoulder and the reaches the side slope. The pickup shows more change in yaw on the side slope than the small vehicle due to its longer wheelbase. Figure 4.7(d) shows the effect of the x-value of the vehi- cle CG, Figure 4.7(e) shows the effect on the y-value, and Figure 4.7(f) shows the changes in z-value. There is little dif- ference in the x- and y-values for the horizontal trajectory. The z-value, while appearing different, only reflects the dif- ference in height associated with the superelevation. These metrics for the worst-case scenario show that the vehicle is relatively stable as it traverses the shoulder and ini- tial part of the side slope. It also suggests that the VDA tool is reflecting the variations in surface conditions. It is apparent that there are differences in the vehicle trajectories associated with superelevated and level curves and for various vehicles. It also suggests that there is not likely to be much extraneous variance in the results, leading to the conclusion that there was value to pursuing VDA for the various conditions of interest. 4.5 VDA Simulation Results The VDA software was used to generate trajectories for each of the vehicles at the selected exit angles and speeds for each road departure condition. The vertical trajectories or trace paths of Point 1 for the 1100C vehicle (brown) and Point 2 for the 2270P vehicle (blue) negotiating a curve and departing onto the roadside of a given configuration are shown in Figure 4.8 by line color and type (note the various vehicle weights, speeds, and exit angles in the legend). These trace paths can be visual- ized as standing on the roadside downstream from the point a vehicle leaves the roadway and observing the change in eleva- tion of Point 1 or 2. Multiple curves reflect variations in depar- ture speed and angle for each of the vehicles (as noted in the legend). The differences in basic vehicle heights are reflected by the relative positions of the two sets of curves. There is a con- sistency in the heights with the road profile shown by the black line at the base of the graph. Dynamic effects of the sprung mass cause the curves to vary for the changes in cross section conditions. A similar graph was generated for each set of the conditions in the analysis matrix. Figure 4.9 provides an example of the normalized repre- sentation of the vertical trajectory for the same conditions. In the normalized view, the variations in trajectory are indicated relative to a horizontal plane as opposed to the actual cross section surface. The curve on the bottom shows the road pro- file or cross section as a reference for the vehicle dynamics traces. The normalized view provides a convenient means to analyze and compare vehicle dynamics effects for different conditions simultaneously. The normalized version is also useful to translate the vertical trajectories to a common plane to allow the aggregation of groups of results to define limits. Figure 4.10 depicts a primary use of the normalized graphs of the trajectory data. All trajectory traces for a given set of CSRS conditions were plotted from which maximum and minimum limit curves can be derived. In this case, the bold red line represents the maximum trajectory height limit across the entire path. Similarly, the bold green line indicates the minimum trajectory height. These limits indicate require- ments for any barrier system in that roadside configuration for all lateral positions beyond the shoulder. This approach Figure 4.6. Typical profile for path of a vehicle leaving the traveled way.
51 (a) (d) Effect on X-Coordinate of CGRelative Change in Roll Angle Relative Change in Pitch Angle Effect on Y-Coordinate of CG (b) (e) (c) (f) Relative Change in Yaw Angle Effect on Z-Coordinate of CG Figure 4.7. Variations of roll, pitch, and yaw angles and x-, y-, and z-coordinates.
V er tic al ( in ) Lateral (ft) Figure 4.8. Sample plot of non-normalized vehicle trajectories on CSRS. V er tic al ( in ) Lateral (ft) Figure 4.9. Sample plot of normalized vehicle trajectories on CSRS.
53 can be used to determine the potential effectiveness for varying barrier systems across all possible lateral positions for a given roadside configuration. Figure 4.11 shows more specific examples of how the plot of maximums and minimums can be applied. For a given super- elevated curve and roadside configuration [e.g., 614-ft (187-m) radius curvature and 12% superelevation], the limits can be plotted along with the interface area provided by a specific barrier. These interface areas are represented by the blue and green lines that reflect the maximum and minimum vertical position of the vehicleâs critical points as it leaves the roadway and moves onto the roadside. For the barrier to be effective, it must have a good interface for both large and small vehicles at any given lateral position. The two graphs show the limits for the G4(1S) and MGS barriers, respectively, as yellow lines across the graph for various positions where each type of bar- rier can be placed. If the maximum and minimum limits fall within the yellow lines, then the barrier will have a good inter- face for both types of vehicles. Where the blue line goes above the top yellow line, there is the opportunity for an override to occur. Where the green line falls below the lowest yellow line, the possibility of an underride exists. The lower portion of Figure 4.11 shows the profile or cross section of the road related to the upper graph. Effec- tive placement areas are shown in this pane. The red hatched area defines the lateral positions where the specific barrier V er tic al ( in ) Lateral (ft) Max Min Figure 4.10. Example use of normalized view to show limiting conditions. has an interface area above the maximum lower height limit (green curve) and/or below the minimum height limit (blue curve). Effective lateral placement occurs where both criteria are met, and this is shown in shaded green. The differences in the effectiveness of the G4(1S) and MGS barriers (by virtue of their design differences) is reflected when the effectiveness areas are compared. These maximum and minimum limits are a unique function of vehicle dynamics for the given con- figuration, but the yellow barrier isobars reflecting the effec- tive range would depend on the barrier shape/type. These indicate the effective lateral placement options that can serve as guidance for specific CSRS conditions. Table 4.1 provides a sample summary reflecting the effec- tiveness results for a barrier (NJ Concrete Barrier) across var- ious CSRS conditions. Plots of this type for all different curve and roadside configurations selected were generated and are presented in Appendix B. The VDA simulations were used to determine the maxi- mum and minimum heights of the critical points (Points 1 and 2) on the bumper as the vehicle first comes in contact with the barrier. Barrier lateral placement in these evalua- tions was 1 ft off the shoulder for each of the three barrier systems selected. All combinations of curvature, supereleva- tion, and shoulder width and slope for the different speeds and impact angle were used in the evaluations. The maxi- mum and minimum heights are tabulated in Table 4.2. Each
54 Figure 4.11. Typical barrier interface and effectiveness for given profiles.
55 Case Parameters Profile Diagram 1 Curvature: 3,050 ft Superelevation: 12% Shoulder Width/Angle: 4 ft/0% Roadside Slope: 12H:1V 2 Curvature: 3,050 ft Superelevation: 12% Shoulder Width/Angle: 4 ft/3% Roadside Slope: 12H:1V 3 Curvature: 3,050 ft Superelevation: 12% Shoulder Width/Angle: 8 ft/6% Roadside Slope: 12H:1V 4 Curvature: 3,050 ft Superelevation: 8% Shoulder Width/Angle: 8 ft/0% Roadside Slope: 12H:1V 5 Curvature: 3,050 ft Superelevation: 8% Shoulder Width/Angle: 8 ft/3% Roadside Slope: 12H:1V 6 Curvature: 3,050 ft Superelevation: 8% Shoulder Width/Angle: 8 ft/6% Roadside Slope: 12H:1V 7 Curvature: 3050 ft Superelevation: 6% Shoulder Width/Angle: 12 ft/0% Roadside Slope: 12H:1V 8 Curvature: 3050 ft Superelevation: 6% Shoulder Width/Angle: 12 ft/3% Roadside Slope: 12H:1V 9 Curvature: 3050 ft Superelevation: 6% Shoulder Width/Angle: 12 ft/6% Roadside Slope: 12H:1V Table 4.1. Sample profile comparisons: NJ concrete barrier. cell represents the barrier height for the specific conditions. If the value is red, then it implies that the height is outside the limits (e.g., too high or too low) and hence indicates that there is not a good interface. These tables, as well as other interface plots shown in Appendix B, are used to provide the basis for determining those cases or types of cases that need to be analyzed with crash FE simulation. In Table 4.2, the critical heights range from just under 19 in. to almost 30 in. Examining the results for each type of barrier the following the insights are noted: â¢ NJ Concrete Barrier â Since the concrete barrier has a 0-in. minimum inter- face height, this barrier works for all minimum cases for all the curvature, superelevation, shoulder, and place- ment conditions. Observe that there are no âredâ values in any of the minimum rows. â Similarly, this barrier provides a good interface for all 1-ft offset placements (no âredâ values). â The highest maximum height value is 29.83 in., which suggests that the use of a concrete barrier with a critical interface higher than 30 in. would provide good inter- face for all the conditions considered here. â¢ G4(1S) W-Beam Guardrail Barrier â The G4(1S) barrier appears to meet the minimum interface requirements for all cases, as there are no âredâ values for any of the Min rows, indicating less susceptibility to underride on the CSRS road profile.
Table 4.2. Vehicle interface results for various CSRS and barriers.
57 â There are cases where the maximum requirement is not met (the âred-boldâ values), indicating that there is increased chance of override due to the CSRS road pro- file. These are more noticeable with the higher shoulder slope angles (6% and 8%). â¢ MGS W-Beam Guardrail Barrier â The greater height of the MGS barrier accounts for greater number of good maximum interface indications across a range of conditions, indicating less suscepti- bility to override due to the CSRS road profile than the G4(1S) system. â There is not a corresponding meeting of the minimum requirements. Several of the cells do not meet this cri- terion, indicating susceptibility to a vehicle going under the barrier and its potential for snagging posts. These and other insights demonstrate the value of the VDA results. It is important to note here that the VDA gives an indication of the barrier performance based on the vehicle dynamics and geometry of the barrier. It does not account for the increased or decreased severity of the impact resulting from a change in vehicle orientation and speed. FE analyses were performed to investigate these additional effects. 4.6 Conclusions In this effort, trajectories for vehicles departing from CSRS were determined using VDA tools. VDA tools allowed the entry of data for specific vehicles that reflected differences in size, weight, suspension features, and other factors as well as the cross sectional surface for various conditions under which a vehicle can leave the roadway (i.e., speed, angle). The trace plots generated as the vehicle traverses the various cross sections reflect the effects of the suspension and provide use- ful insights into effects on the vehicleâs interface area relative to the barrier. The latter aspect is a critical metric for assess- ing the barrierâs potential ability to capture and redirect the vehicle. The results from this analysis provide useful insights for identifying critical cases for investigation using FE simu- lations, as well as proposing guidance on selecting and placing barriers on CSRS. The VDA results provide some useful insights about the potential effectiveness of different types of barriers on CSRS: â¢ Barriers offering increased height and depth of their cap- ture area should be used. This is more important for sharper curves and higher levels of superelevation. â¢ Clear zones beyond the shoulder may be an option where sufficient runout area is available. This analysis only con- sidered nearly level 12H:1V roadside slope conditions. It is important to remember that these analyses focus strictly on the vehicle-to-barrier interface. This is a neces- sary condition, but not sufficient to ensure that the bar- rier will meet crashworthiness requirements. This is where further analyses using FE models and crash simulation become useful. There are not clear choices for selecting specific cases for crash simulation. The differences in barriers necessitates that crash simulations be conducted for each of them. For each barrier type, the following crash simulations should be considered: â¢ The most common acceptable interface scenario. â¢ The most divergent case for comparison of crashworthi- ness metrics and considerations of options for varying the design. Based on the results of these crash simulations, decisions can be made on the value of additional simulations, for example, simulations with the following: â¢ Impacts at shallower impact angles. â¢ Selected cases where poor interface might suggest a pro- pensity to cause rollovers. â¢ Variations in the orientation of the barrier to true vertical. â¢ SUTs to understand higher interface and vehicle weight impacts. The benefits of these additional simulations will be weighed in the context of providing needed insights or support for the proposals that are to be developed.