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1 S U M M A R Y Performance of Longitudinal Barriers on Curved, Superelevated Roadway Sections Background and Objectives NCHRP Project 22-29, âPerformance of Longitudinal Barriers on Curved, Superelevated Roadway Sections,â and Project 22-29A, âEvaluating the Performance of Longitudinal Barriers on Curved, Superelevated Roadway Sections,â were initiated to develop a better understanding of the safety performance (i.e., crashworthiness) for barriers used on curved, superelevated roadway sections (CSRS) and to suggest options and guidance for improving barrier selection, design, and deployment in pursuit of enhanced highway safety. CSRS are most commonly found on major interstate-type highways, and they exist on both tight and gentle curves (see Figure 1). The most critical CSRS situations occur on the tight curves associated with interchanges. This research involved a four-phase effort to systematically and comprehensively consider safety for varying CSRS situations. The research (1) reviewed the practices and available knowledge of barriers on curves and their safety performance; (2) analyzed issues associated with vehicle-to-barrier interfaces; (3) simulated crashes with various types of barriers for varying curvature, shoulder configurations, and superelevation conditions; and (4) conducted crash tests to confirm the simulation results. The project resulted in the development of proposed enhancements to barrier design, selection, and deployment for varying CSRS situations. Research Approach During the first phase of the project, the Research Team gathered relevant information pertaining to the safety performance of longitudinal barriers on CSRS from reviewing technical literature and conducting a survey of state DOTs (state DOT survey) to obtain their design standards and practices. The review of domestic and international literature (from TRID) revealed that very little research has been conducted to analyze the safety of designs for barriers used in CSRS situations. For example, current crashworthiness evaluation criteria only apply to straight or tangent sections of barriers, and there have been very few efforts to test barriers on curves. The state DOT survey revealed varying practices for the selection and deployment of barriers on CSRS, but these were essentially similar to those for barriers on tangent sections. Analyses of crash data indicated that there are more crashes on curved road sections, but the details in the data are not sufficient to discern differences by features of the curve (e.g., degree of curvature, superelevation) or the type and placement of barriers where crashes occurred. It was concluded that a clear need exists to develop a deeper understanding of longitudinal barrier safety performance for CSRS. The research began with applying vehicle dynamics analysis (VDA) to study the effects of surface changes of the roadway, shoulder, and side slopes on the trajectory and orientation
2 of a vehicle at its interface with varying barrier types and their placement. The under- lying premise for these analyses was that a good vehicle-to-barrier interface is necessary for adequate safety performance. Vehicles undergo changes in their roll, pitch, and yaw as they traverse a banked roadway, a shoulder (most likely having a different slope), and then the graded side slope before encountering a barrier. The design features for the Manual for Assessing Safety Hardware (MASH) test vehicles (i.e., the small sedan and large pickup truck) were input into commercially available VDA software to generate trajectory plots for a broad set of conditions. The analyses covered a representative set of conditions as summarized in Table 1 for three common types of longitudinal barriers. Various plots and summaries were generated in the assessment of interface effectiveness. Figure 1. Typical CSRS with longitudinal barriers. Barrier Type o Concrete barrier [height 32 in. (813 mm)]: NJ concrete barrier o Strong-post W-beam guardrail [height < 31 in. (787 mm)]: G4(1S) o Strong-post W-beam guardrail [height 31 in. (787 mm)]: MGS Vehicle Type o 2270P pickup truck: 2007 Chevrolet Silverado model o 1100C small car: 2010 Toyota Yaris model Curvature/Superelevation Combinations o 614 ft (187 m) radius/12% superelevation o 2,130 ft (649 m) radius/12% superelevation o 758 ft (231 m) radius/8% superelevation o 2,670 ft (814 m) radius/8% superelevation o 833 ft (254 m) radius/6% superelevation o 3,050 ft (930 m) radius/6% superelevation Shoulder Width and Slope o 4 ft (1.22 m), 8 ft (2.44 m), and 12 ft (3.66 m) widths o 0%, 3%, 6%, and 8% shoulder angles Roadside Slope o 12H:1V (negative) relative to shoulder for all simulations Impact Conditions o Impact angle: 25Â° o Impact speed: 62 mph (100 km/h) Barrier Placement Relative to Road Section o Lateral position: at edge of shoulder o Vertical orientation: normal to road, normal to shoulder, or true vertical Table 1. Factors considered in the research.
3 The research considered variations in shoulder width and slopes and organized the results based on shoulder angle as noted in Figure 2. This metric reflects the cross section slope changes on the superelevated roadway to the adjacent shoulder. A negative side slope of 12H:1V relative to shoulder was used for all simulations. The second phase of the analyses involved crash simulation analysis using finite element (FE) models. Simulations of crashes into barriers have been shown to effectively replicate actual events and can provide useful metrics on safety performance. Simulations allow variations of the vehicle-to-barrier interface to be considered as well as the necessary aspects of barrier strength as a function of its detailed design and deployment. A subset of CSRS conditions were selected as candidates for simulation based on results from the VDA. Since detailed crash simulations each take 20 h to 40 h of computer processing time, the VDA simulation results were used to minimize the number of FE cases that needed to be simulated. Approximately 200 FE simulations were run. Various metrics were derived from the simulations, but the focus was on the MASH Test Level 3 (TL-3) crashworthiness measures related to vehicle stability and occupant risk (MASH 2009). Full-scale crash testing was undertaken as the last step in this research to verify and validate the simulation results. Three tests were conducted to provide a basis for asserting the validity of the simulation results. The tests showed outcomes similar to the simulations for similar conditions, leading to the conclusion that the simulation results were valid. More details on these analysis efforts are provided in the following sections. Vehicle Dynamics Analysis The VDA research efforts initially involved adapting models to assess the various aspects of vehicle-to-barrier interface performance under different CSRS conditions, as reflected in Table 1. Human-vehicle-environment (HVE) (Engineering Dynamics Corporation 2005) Figure 2. Shoulder slope conditions relative to the roadway surface analyzed.
4 and CarSim (CarSim 2006) programs for VDA were used to generate vehicle trajectories to gain insights on the influences of surface features on the orientation of the vehicle and the likely barrier interface regions for the various superelevation, slope, shoulder, and back- slope conditions. Typical results from these analyses are the plots shown in Figure 3. For the random set of conditions displayed, the VDA-based normalized override (blue curve) and underride curves (green curve) can be noted. The yellow line indicates the âbarrier interface regionâ that would exist for a concrete barrier across positions off the shoulder where they could be placed. The shapes of these curves reflect the maximum and minimum traces of the primary structural regions for the two vehicles and varying speed and impact angles considered. They vary as a result of the differences in curve radius, degree of superelevation, shoulder width and slope, and backslope conditions. It can also be seen that the effective placement areas (shaded green area below curves) vary by conditions. VDA interface curves like these were generated for a range of possible barrier placement practices. Road: 833-ft curve, 6% super Shoulder: 12-ft width, 6% angle Road: 3,050-ft curve, 6% super Shoulder: 4-ft width, 3% angle Road: 3,050-ft curve, 6% super Shoulder: 4-ft width, 0% angle Road: 3,050-ft curve, 6% super Shoulder: 4-ft width, 6% angle Figure 3. Typical override and underride limits on varying CSRS.
5 Tabular summaries were also generated of the interface performance for all the specific conditions of interest. The plots and tabular summaries reflect the physics of a vehicle leaving the road on a CSRS and interfacing with barriers positioned at edge of the shoulder. The VDA results were used to determine the maximum and minimum vehicle bumper heights at first contact with the barrier for all combinations of curvature, superelevation, and shoulder width and slope. In Table 2, the maximum and minimum heights are indicated. Each cell represents the vehicle bumper height for the specific conditions. The cells with values in red type indicate those situations where poor interface conditions are likely to exist. These imply that the height is outside the limits (e.g., too high or too low) and there is potential for vehicle overriding or underriding the barrier. This table as well as the interface plots provided insights about conditions where safety issues may occur and were used to decide which cases to evaluate using FE simulations. The VDA results provide some useful insights about the potential effectiveness of differ- ent types of barriers on CSRS. Some possible implications of the VDA results include the following: â¢ Use barriers offering increased height and depth of their capture area for a CSRS. This may be more important for sharper curves, higher levels of superelevation, and more pronounced changes from roadway to shoulder angles. â¢ Clear zones beyond the shoulder may be an option where sufficient runout area is available. This analysis only considered nearly level 12H:1V roadside slope conditions. â¢ Based on the VDA results, it was observed that there is no vehicle-to-barrier interface issue (i.e., potential for override or underride) with concrete median barriers when used on CSRS. â¢ The simulations show that there may be potential for small vehicle underride with the Midwest Guardrail System (MGS) barriers and potential for override of the pickup truck with the lower height G4(1S) W-beam guardrail systems. The VDA focused on the vehicle-to-barrier interface. This is a necessary condition, but not sufficient to ensure that the barrier will meet crashworthiness requirements. This is where further analyses using FE models and crash simulation became necessary. Finite Element Simulation Analyses Simulation analysis was undertaken to analyze the impact performance of the three barrier types for the various CSRS and barrier placement options. The LS-DYNA software used vali- dated FE models of barriers and vehicles to simulate crashes. These models were validated by comparisons of crash test data to simulated results for tests on tangent, level sections. Validation efforts indicated that the models effectively replicated the crash tests based on similarities of the motion metrics (e.g., yaw, pitch, roll, and associated velocity and accelera- tion profiles for the x-, y-, and z-axes) indicating the viability of the simulations. Figure 4 displays results from the simulation analyses. It shows the predicted behavior of the vehicle and all the pertinent MASH metrics and evaluations. The basic CSRS features, barrier placement, and impact conditions are indicated in the upper part of the table. In this case, a 614-ft-radius curve with a superelevation of 12% is modeled. The curve has a 4-ft-wide shoulder with a 6% slope. At the edge of the shoulder, a New Jersey concrete safety shape barrier (NJ concrete barrier) is placed with a ânormal to roadâ orientation. A MASH 2270P vehicle traverses the CSRS and departs the traveled way and crosses the 4-ft shoulder leading to an impact with the barrier at 100 km/h and 25Â°. The picture shows âsnapshotsâ of the vehicle position at various points of time during the approximately 2-s âcrash event.â
Note: Values in red type indicate situations where poor interface conditions are likely to exist. Table 2. Summary of results of VDA bumper-barrier interface heights.
7 The vehicle approaches as if from the inside travel lane on a departure trajectory. The simu- lation is initiated away from the barrier to allow the vehicle model to be âstabilizedâ before impact. The point of the 25Â° impact has the right front side of the pickup truck making first contact with the barrier. Fractions of a second later, the vehicle yaws, leading to a rear-end impact with the barrier while riding up the barrier and beginning an outward roll. About 0.3 s later, the vehicle has been redirected toward the travel lane and down the barrier side. There is greater outward roll, but further contact with the barrier reverses the roll direction. This visual sequence provides a convenient means to compare performance among various conditions. The lower part of the table provides a summary of the MASH crashworthiness evaluation metrics for the conditions simulated. Similar summaries were generated to allow convenient comparisons of barrier perfor- mance under different conditions. For example, Figure 5 compares the vehicle behavior for MASH 3-10 and 3-11 impacts on curves of 758- and 2,670-ft radius with 8% superelevation, 2270P - NJ Concrete Barrier (102) Radius Super Shoulder Width Shoulder Angle Barrier Orient. Speed Angle 614 ft 12% 4 ft 6% Normal to Road 100 [km/h] 25 [Â°] Evaluation Criteria A Test article should contain and redirect the vehicle; the vehicle should not penetrate, underride, or override the installation although controlled lateral deflection of the test article is acceptable. Pass D Detached elements, fragments, or other debris from the test article should not penetrate or show potential for penetrating the occupant compartment, or present an undue hazard to other traffic, pedestrians, or personnel in a work zone. Pass F The vehicle should remain upright during and after the collision. The maximum pitch and roll angles are not to exceed 75Â°. Max Roll (Deg) 28.54 Pass Max Pitch (Deg) 23.38 H Longitudinal and lateral occupant impact velocities (OIV) should fall below the preferred value of 30 ft/s (9.1 m/s), or at least below the maximum allowed value of 40 ft/s (12.2 m/s). Vx (m/s) â5.29 Pass Vy (m/s) 8.15 I Longitudinal and lateral occupant ridedown accelerations (ORA) should fall below the preferred value of 15.0 g, or at least below the maximum allowed value of 20.49 g. Ax (g) 9.92 Pass Ay (g) 17.65 Figure 4. Typical simulation analysis summary report.
8 and a 4-ft-wide shoulder with an 8% shoulder angle. The barriers were oriented normal to road. The figure shows the vehicle behavior differences observed between the small car and the large pickup truck for these conditions. A considerable amount of information was generated by the simulations. The outcomes are summarized in a series of tables for each barrier and MASH test condition across the range of conditions simulated. An example is given in Table 3. For each CSRS condition (i.e., curve radius and superelevation; shoulder width and shoulder angle), the results of the simulation runs for impacts with specific types of barriers are provided. Some of these cells are marked with an asterisk (*) to indicate that the outcome was based on expert judgment derived from the FE simulations and VDA. The Research Team drew insights from these summaries. The following findings emerged: â¢ Variations in barrier performance were noted for the various conditions, suggesting that the simulation models and approach reflect the physics of barrier impacts on CSRS. â¢ After approximately 60 simulations for the NJ concrete barrier (as shown in this table), most have passed the MASH Test 3-11 (the most critical test) requirements for CSRS conditions when the barrier is installed normal to road or shoulder. The simulations Parameters and Results Case Time Sequence View CSRS: Radius 758 ft, 8% super Vehicle: 1100C A â Containment (Pass) D â Detached Elements (Pass) F â Max Roll â 26.59 (Pass) Max Pitch â 21.99 (Pass) H â OIV â Vx â â5.47 (Pass) Vy â 9.76 (Pass) I â ORA â Ax â â3.19 (Pass) Ay â 14.23 (Pass) 224 1100C CSRS: Radius 758 ft, 8% super Vehicle: 2270P A â Containment (Pass) D â Detached Elements (Pass) F â Max Roll â 33.66 (Pass) Max Pitch â 29.86 (Pass) H â OIV â Vx â â5.39 (Pass) Vy â 8.24 (Pass) I â ORA â Ax â â13.38 (Pass) Ay â 18.43 (Pass) 124 2270P CSRS: Radius 2,670 ft, 8% super Vehicle: 1100C A â Containment (Pass) D â Detached Elements (Pass) F â Max Roll â 25.85 (Pass) Max Pitch â 21.72 (Pass) H â OIV â Vx â â5.49 (Pass) Vy â 9.72 (Pass) I â ORA â Ax â â3.27 (Pass) Ay â â13.54 (Pass) 264 1100C CSRS: Radius 2,670 ft, 8% super Vehicle: 2270P A â Containment (Pass) D â Detached Elements (Pass) F â Max Roll â 35.84 (Pass) Max Pitch â 31.6 (Pass) H â OIV â Vx â â5.57 (Pass) Vy â 8.13 (Pass) I â ORA â Ax â â12.09 (Pass) Ay â 18.28 (Pass) 164 2270P Figure 5. Sample comparison of radius effects for different vehicles.
9 Note: Asterisk (*) indicates that the outcome was based on expert judgment derived from the FE simulations and VDA. â³ Table 3. Sample performance table based on simulations.
10 of impacts with the NJ concrete barrier indicated that it is more prone to fail the crash- worthiness requirements for situations where the superelevation is 8% or greater and the shoulder angle is 6% to 8%. â¢ Performance under less severe CSRS and barrier placement conditions is incrementally improved. This suggests that current applications of the concrete barrier are viable. This also suggests that there may not be compelling reasons to conduct full-scale crash tests on concrete barrier. â¢ Efforts to simulate G4(1S) W-beam barriers for the various conditions showed consistent results associated with barrier height. The 27Â¾-in.-high barriers did not perform as well as higher barriers. â¢ The simulations of vehicle impacts into G4(1S) barriers at a height of 27Â¾ in. for CSRS applications showed a propensity for override, as the VDA results suggested. There were fewer cases of vaulting for the G4(1S) barriers at 29 in. high. There were no cases where underride was indicated to be a problem. â¢ Simulations of the MGS barriers (31 in. high) showed no propensity for underride issues with the small car. â¢ Additional simulations for F-shape concrete barriers indicated improved performance over the NJ concrete barrier. The simulation efforts included some additional runs to add depth to the analyses and provide a better understanding of the underlying physics. They provide additional metrics or different views of the simulated impacts. Full-Scale Testing There was considerable discussion about which crash tests would be most important to conduct. Ultimately, three tests were conducted: â¢ Test 16004. G4(1S) barrier at 29 in. high with a 2270P vehicle at 100 km/h for a 254-m (833-ft) radius curve with a 6% superelevation with a â2% shoulder slope, and a 4-ft shoulder. The vehicle impacted the barrier at the desired speed, but there was less drift than anticipated, causing the vehicle to impact 2 ft to 3 ft from the desired Critical Impact Point (CIP), hitting closer to the first downstream post. Consequently, the rail was more rigid and the vehicle traveled along it near the top, but did not vault the barrier as had been seen in the simulation. This was considered a marginal result. â¢ Test 16010. G4(1S) barrier at 29 in. high with a 2270P vehicle at 100 km/h for a 254-m (833-ft) radius curve with a 6% superelevation with a â2% shoulder slope, and a 4-ft shoulder. The vehicle impacted the barrier at the desired speed near the CIP. Conse- quently, the rail was less rigid, allowing the vehicle to climb and vault over it. This result was considered to confirm the simulation results. â¢ Test 16015. G4(1S) barrier at 29 in. high with a 2270P vehicle at 100 km/h for a 254-m (833-ft) radius curve with a 6% superelevation with a â2% shoulder slope, and an 8-ft shoulder. The vehicle impacted the barrier at the desired speed near the CIP. The rail safely redirected the vehicle. This result was considered to confirm the simulation results for 8-ft shoulders. These tests were conducted for the most common type of W-beam barrier and bracketed the pass/fail limits indicated by the simulations. Because the test results were considered similar, they are believed to provide confirmation that the simulations reflected the real- world safety performance of barriers on CSRS.
11 Development of Guidance for Deployment of Longitudinal Barriers on CSRS A considerable amount of information was derived from the VDA, simulation analyses, and crash testing. The challenge was to translate these results into guidance for the design, selection, and installation of longitudinal barriers on CSRS. Table 4 contains the significant implications and guidance derived for the barriers and CSRS conditions analyzed. These are included along with the critical guidance elements (in bold) that evolved from this research. Note: Critical guidance elements are shown in bold type. Aspect Implications and Guidance Elements Barrier Design General â¢ Poor vehicle-to-barrier interface limits the barrier functions in a crash. â¢ Good interface is a necessary, but not a sufficient condition for selection of a barrier type. The degree of increased impact severity needs to be assessed. â¢ Consider using interface analyses (i.e., VDA) to evaluate special cases or other types of barriers to increase confidence in the design. â¢ Consider higher barriers to better accommodate larger vehicles for CSRS applications. Concrete Barriers â¢ Concrete safety shapes do not have underride problems, but face slopes can induce rollovers. â¢ Use higher concrete barriers where there is a concern about override associated with CSRS features. â¢ Concrete barriers with an appropriate face slope may be considered the most universally effective design for CSRS conditions. â¢ Design concrete barriers with minimum face slope to limit vehicle ride-up and maintain a viable interface area overlap. W-Beam Barrier â¢ The need for a higher barrier is apparent, but increasing the rail height necessitated review of underride potential. â¢ Increases in barrier height are most important for tight curves where excessive speeds are likely to occur (e.g., off-ramps, downhill). â¢ Follow the FHWA Technical Memorandum of May 17, 2010, that recommends the nominal height for new installations of G4(1S) barrier be 29 in. for CSRS (Nicol 2010). â¢ Consider 31-in.-high W-beam barrier designs for CSRS situations. Selection Curvature and Superelevation â¢ Conduct deeper analysis of short-radius, high superelevation CSRS situations. â¢ Limit the use of tight curves with high superelevations. â¢ Consider using higher barriers on CSRS with appropriate underride protection. Shoulder Width and Angle â¢ Limit major changes in shoulder slope to avoid impacting the barrier when the suspension effects can maximize the potential interface area. â¢ Use wider shoulders where slope changes must be large to allow the suspension to stabilize the vehicle before impact. â¢ Limit shoulder angle to comply with the AASHTO recommendation that melting snow flow away from the road. Roadside Slope â¢ Limit the variation of slope change on the roadside for situations where the barrier is not placed adjacent to the shoulder to provide an acceptable interface. Barrier Type â¢ Consider higher (e.g., 31-in.) W-beam barrier designs for CSRS situations. â¢ Select barriers with increased height for tight curves where high speeds are likely to occur. â¢ Consider using concrete barriers with minimum face slope (e.g., F-shape) to reduce the risk of rollover. Installation Orientation â¢ Promote use of barrier orientation perpendicular to the roadway for concrete barriers. Placement â¢ Limit the placement of barriers to the edge of the shoulder on CSRS, particularly where there is a non-trivial slope change going to the roadside slope. â¢ Use wider shoulders with lower shoulder angles relative to the road on CSRS with short radii and high superelevation. Maintenance â¢ Analysis on the effectiveness of damaged barriers on CSRS is needed. â¢ Further analysis of relative priorities for barrier maintenance on CSRS may be needed. Table 4. CSRS implications and guidance derived for the barriers and CSRS analyzed.
12 These are subject to further vetting, rewording, and editing in consultation with AASHTO committees. This construct provides a summary of the findings of the multifaceted analyses that support the proposed guidance for barrier design, selection, and installation. Conclusions In this effort, it was determined that there is limited information on the influences of CSRS features on safety. It was found that there were physics-based criteria for determining appropriate curvature and banking parameters to allow vehicles to safely negotiate curves under varying surface conditions. Criteria for basic curve design are found in the AASHTOâs A Policy on Geometric Design for Highways and Streets (Green Book) (AASHTO 2011a). It was noted, however, that there was very limited guidance available for addressing concerns about vehicles leaving the roadway under CSRS conditions. While there is the basic understanding that crashes occur more often on curves than on tangent sections, the influences of CSRS features on crash propensity were not clear. It was noted that there is a fundamental issue with the level of detail associated with crash reporting that limits analysis options. The usual data captured for crashes falls short on details about the features of the road at or upstream of the crash location. In some cases, there are basic features that are provided on crash reports (e.g., pavement condition), but rarely are details on grade or curvature captured. This limits the ability to analyze CSRS crashes because the necessary data is not routinely captured. The problem occurs even when an agency has the road features data but cannot link it to specific crash sites. There has been a growing understanding of the dynamics of vehicles as they traverse specific surfaces, but such analyses have not typically been undertaken in most crash analysis efforts, despite the availability of software tools for the purpose. Sophisticated simulation tools that allow the physics of vehicle dynamics and vehicle-to-barrier impacts to be analyzed may not be applied due to limited funds. The interest in understanding the safety perfor- mance of barriers on CSRS provides the impetus for using advanced tools when ordinary research approaches are limited. This effort was undertaken in three phases to enhance the understanding of the safety performance of barriers on CSRS and develop guidance for their effective design, selection, and installation. The following insights resulted from this research: â¢ There has been little effort to determine whether longitudinal barriers adjacent to CSRS perform in the way same as those on tangent sections. â¢ Current guidance for barrier design, selection, and maintenance is essentially the same as that for tangent sections. â¢ VDA using commercially available tools provides a means to study the effects of speed, surface features, and vehicle type on the trajectory and orientations of a vehicle departing the traveled way on CSRS. â¢ Vehicle trajectories for two types of vehicles traveling at 62 mph (10km/h) were examined to determine their interface with barriers at various locations along the roadway. â¢ The VDA provided useful information on vehicle-to-barrier interfaces for a range of CSRS conditions. â¢ VDA results were used to determine which situations warranted deeper analyses using simulation. â¢ FE simulations were undertaken to investigate the impact performance (i.e., physics) of selected vehicles impacting one of three types of barriers placed on a CSRS. â¢ The simulation analyses focused on MASH impact conditions to evaluate the performance for NJ concrete, G4(1S) W-beam, and MGS barriers.
13 â¢ The results indicated that there was some potential for failure, but options for address- ing the problems existed. â¢ Three full-scale crash tests were conducted that validated the simulation analyses. These tests also demonstrated approaches for conducting future tests of barriers on CSRS. The findings from all three phases of the research were summarized into proposed actions that could increase barrier safety on CSRS. The proposed actions will be shared with various groups for feedback and refined for possible incorporation into guidance documents and state practices. Needs for future research were also defined. NCHRP Research Report 894 documents in detail the analyses and results from this project. These were synthesized into a series of proposals for effective design, selection, and installation of longitudinal barriers on CSRS.