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14 Introduction Highways consist of tangent and curved roadway sections for which there are well-established geometric design criteria. Curved roadway sections on higher speed roads are gener- ally constructed with superelevation to compensate for the centripetal forces exerted on the vehicles, making it easier for the driver to control the vehicle through the curved section. Guidelines for the design of CSRS are found in the Green Book. These indicate superelevation rates for varying degrees of cur- vature for two speeds. They do not provide design guidance for shoulder features or barriers for superelevated curves. There is limited guidance for the selection, design, and installation of barriers for CSRS of the road network. While it is well known that crashes occur proportionately more often on curves than tangent sections, the influence of curvature, superelevation, and roadway features on crash propensity or severity is not well understood. Barriers are often deployed on CSRS as a continuation of barriers on adjacent sections or to address situations created by the superelevated curve (i.e., the protection from a drop to the backslope, often asso- ciated with the embankment needed to provide the super- elevation slope). Guidance for the deployment and testing of barriers on CSRS is limited. The need exists for a better understanding of the behavior of vehicles that leave the trav- eled way in such situations and the associated performance requirements for barriers deployed on the CSRS. This report provides an overview of efforts and findings under two NCHRP projects: NCHRP Project 22-29, âPerfor- mance of Longitudinal Barriers on Curved, Superelevated Roadway Sections,â and NCHRP Project 22-29A, âEvaluating the Performance of Longitudinal Barriers on Curved, Super- elevated Roadway Sections.â These were initiated to address the limited knowledge and guidance on barrier performance on CSRS through reviews of current agency practices, pub- lished research and guidance, accumulated knowledge about crashes and safety issues, and a three-phase effort to analyze the influences of specific CSRS conditions on barrier perfor- mance. The research developed insights from comprehensive VDA, crash simulation, and full-scale testing to develop guid- ance for improved selection, design, and deployment practices for common longitudinal barriers [e.g., NJ concrete, G4(1S) W-beam guardrail, and the MGS] for CSRS situations. 1.1 Background The safety performance of longitudinal barriers under current and past crashworthiness evaluation criteria has been assessed under idealized impact conditions where a linear section of the barrier is installed on level terrain and the impacting vehicle is freewheeling with minimum roll and pitch effects. This protocol has evolved to provide a âpractical worst-caseâ impact condition that is reproducible and com- parable. In reality, barriers are rarely installed and impacted under ideal conditions, and installations and impacts on CSRS are examples of conditions that are far from ideal. State DOTs have addressed the installation of barriers for CSRS in varying ways because there is limited guidance in both the Green Book and the Roadside Design Guide (AASHTO 2011b). Figure 1.1 shows examples of concrete and steel W-beam longitudinal barriers installed on CSRS. Curved road- way sections are generally constructed with superelevation to compensate for the centripetal forces exerted on the vehicles, making it easier for drivers to control their vehicle at higher speeds through the curved section. Both curvature and superelevation can affect vehicle dynamics and the vehicleâs trajectory, orientation, weight distribution, and speed. Recent research has noted that the dynamic effects can significantly affect the interface between the vehicle and the barrier as it leaves the road (Marzougui et al. 2008a, 2010a, 2012a). On curved sections, the vehicle can leave the road at a sharper angle and consequently hit the barrier with higher impact severity. The higher impact severity can lead to increased forces on the occupants, more intrusion into the occupant compartment, ruptured barriers, or unusual interactions between contacted components. In C H A P T E R 1
15 addition, a sharper impact angle can increase vehicle instabil- ity and may lead to vehicle rollover, override, or penetration of the barrier. The sharper angles increase vehicle climb for rigid barriers and tire/post snagging for semi-rigid strong- post barriers. Furthermore, the road superelevation may cause the vehicle to approach the barrier at a different orien- tation (i.e., roll, pitch, and yaw) and hence impact at a height higher relative to the barrier than would be the case for a flat surface. This is particularly critical when a shoulder has a negative slope relative to the roadway surface. Background for this research was gathered from reviewing the literature, conducting a state DOT survey, and inves- tigating crash data in search of issues associated with safety performance of CSRS. These efforts revealed that there had been little previous research for longitudinal barriers on CSRS, no common barrier selection, design, or installation practices for barriers on CSRS across the United States, and limited opportunity to discern safety issues due to limited data for impacts with barriers on CSRS. Thus, there was a need to assess barrier safety performance as a function of curvature, superelevation, shoulder configuration (i.e., width and slope), and impact conditions. The MASH crash- worthiness requirements served as the benchmark for the assessment. 1.2 Project Objectives and Scope The objectives of this research were to (1) evaluate the crash performance of standard longitudinal barriers installed on CSRS; (2) determine if the curvature and superelevation details used by state DOTs degrade the performance of the barriers to the extent that they will no longer meet the crash test criteria for MASH; and (3) develop guidance for the design, selection, and installation of barriers on CSRS. The effort applied state-of-the-art analysis tools to enhance the understanding of the influencing factors and identify possible Figure 1.1. Example CSRS with typical longitudinal barriers.
16 future research to study barrier modifications, changes to roadway geometrics, or both in response to any safety issues identified. Under the second phase of the research, the evalu- ation of crash performance included (1) a review of the devel- opment and validation of the crash simulations in NCHRP Project 22-29 using LS-DYNA FE models of four vehicles and three barrier types (G4-1S, MGS, and vertical concrete) and (2) completion of one planned simulation from NCHRP Project 22-29. The Research Team met these objectives by using simula- tion analyses and validating the results using crash tests. A wide spectrum of cases was analyzed in detail and cases where the barrier type, design, or placement did not meet require- ments were isolated. The following sections describe perti- nent accomplishments from NCHRP Project 22-29 and the efforts that were undertaken in this project. 1.3 Research Approach Curvature and surface slope are known to affect 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 impact the barrier with greater force, which could potentially result in a higher impact severity. The degree of superelevation in combination with the shoulder slope can lead to a higher interface with the barrier, which can increase vehicle instability, barrier climb, vehicle rollover, or override. Further, the superelevation with a negative shoulder slope might cause the vehicle to impact the barrier at a different orientation (roll and pitch). Thus, an important starting point for analyses of barriers on CSRS is establishing an understanding of the dynamics of vehicles leaving the roadway and traversing the shoulder and side slope before impacting the barrier. Much effort has 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 trajectories in all three dimen- sions. For example, trajectory data in the vertical direction is directly related to the height of the interface of the vehicle and the barrier. This effect is more likely to occur when there is a change in the surface slope between the roadway and the shoulder leading to a shift in the distribution of the vehicleâs weight, which could lead to an override or underride of the barrier due to poor interface. The combined effect of the superelevation of the roadway, the slope of the shoulder, and the side slope of the roadside for a vehicle leaving the roadway on a curve can be explicitly analyzed using VDA tools. These tools readily allow the range of combinations of roadway, shoulder, and side slope design features to be analyzed for varying types of vehicles and their paths or trajectories determined. Thus, a VDA approach was proposed as the starting point for this research to cover a broad range of CSRS conditions. Because VDA only provides insights on the vehicle-to- barrier interface, the second phase of the analyses was to use crash simulation analysis to understand barrier strength and behavior for impacts on varying CSRS. Simulations of crashes into barriers have been shown to effectively replicate actual events and therefore, can provide useful metrics on safety performance. An array of FE models for vehicles and barriers was available to support such analyses. Simulations allow variations of the vehicle-to-barrier interface to be con- sidered, as well as the necessary aspects of barrier strength as a function of its detailed design and deployment. Since simu- lation runs are time-consuming, it was planned that a subset of CSRS conditions would be selected for simulation based on the VDA results. Various metrics can be derived from the simulations, including the MASH TL-3 crashworthiness measures related to vehicle stability and occupant risk. The simulation analysis supports the generation of many crash metrics as well as digital views of the vehicle-to-barrier impacts. These were considered essential to understanding barrier safety performance on CSRS. These simulation results also provide a means to explain the nature of vehicle-to-barrier interactions, as well as compare the effects of various factors on behavior and performance. Full-scale crash testing was the last step to verify and validate the simulation results. Tests were conducted to deter- mine the validity of the simulation results. The tests showed outcomes that were similar to the simulations for similar con- ditions, leading to the conclusion that the simulation results were valid. Since guidelines for the testing and deployment of roadside safety barriers on sloped surfaces and curved sec- tions do not exist, it was recognized that innovative efforts would be needed. Crash testing protocols for barriers have evolved to provide a practical worst-case impact condition that is reproducible and comparable. Barriers had been tested under idealized impact conditions, with the tested barriers being installed on a straight section, having a flat approach terrain, and the impacting vehicle freewheeling with mini- mum roll and pitch effects. These protocols have evolved to provide important assessments that determine whether safety hardware is âcrashworthy.â While crash testing pro- tocols 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 that only a few efforts had addressed the safety of designs or provided guidance for placement on CSRS. The need existed to understand performance of longitudinal barriers along the CSRS to develop effective barrier designs and appropriate placement guidelines for such locations.
17 The research approach involved summarizing the research findings in each of the facets to allow translation of the results into guidance on the design, selection, and installation of longitudinal barriers on CSRS. It is planned that guidance derived from these efforts will be presented to AASHTO and FHWA for their critical review and possible integration into the appropriate documents. NCHRP Research Report 894 describes the findings and explains the rational for the pro- posed guidelines. 1.4 Report Organization This report is organized as follows: â¢ Summary â¢ Chapter 1âIntroduction â¢ Chapter 2âLiterature Review and State DOT Survey â¢ Chapter 3âCrash Data Analysis â¢ Chapter 4âVehicle Dynamics Analysis for Vehicles Leaving the Traveled Way on CSRS â¢ Chapter 5âCrash Simulation Analysis of Impacts into Longitudinal Barriers on CSRS â¢ Chapter 6âFull-Scale Testing and Results â¢ Chapter 7âDevelopment of Guidance for Improved Longi- tudinal Barrier Design, Selection, and Installation on CSRS â¢ Chapter 8âConclusions The report also includes Appendices A through E as follows: Appendix A: State DOT Survey Instrument and Instruc- tions; Appendix B: Vehicle Dynamics Simulation Results; Appendix C: Finite Element Model Validations; Appendix D: Finite Element Simulation Results; and Appendix E: Full-Scale Crash Testing Report. These appendices can be found on the TRB website (www.trb.org) by searching for âNCHRP Research Report 894â.