National Academies Press: OpenBook

Criteria for Restoration of Longitudinal Barriers, Phase II (2021)

Chapter: Chapter 1. Introduction

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Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
×
Page 3
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Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
×
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Page 5
Suggested Citation:"Chapter 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Criteria for Restoration of Longitudinal Barriers, Phase II. Washington, DC: The National Academies Press. doi: 10.17226/26321.
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1 CHAPTER 1 – INTRODUCTION Guardrails are an important feature of the roadside that are used to shield errant motorists from becoming involved in even more catastrophic crashes by redirecting vehicles away from fixed hazards (trees, poles, etc.) and terrain hazards (steep roadside slopes, fill embankments, etc.). A properly installed and maintained guardrail is a very effective roadside safety device. Occasionally, however, guardrails are struck by errant vehicles and incur damage. Sometimes this damage is extensive and the guardrail must be replaced or reconstructed to bring it back into its fully operational condition. More frequently though, these impacts result in only moderate or minor damage to the device and may only compromise the safety performance a little, if at all. It is not well understood just how much these minor-damage crash events affect the operational performance of the system. Making decisions about the maintenance and repair of these systems are important but challenging tasks for transportation agencies. It is their responsibility to ensure that these safety barriers function as they were originally designed, yet they must make the best use of their constrained maintenance budgets. Repair efforts are costly and often consume a considerable amount of a transportation agency’s resources. Making repairs quickly is also challenging in many circumstances. It is not always clear what constitutes performance altering damage. Damage to these systems is not always characterized by obvious repair signs such as large rail deflections, sheared or twisted posts, or broken components – all of which are characteristic of high severity crashes. The damage is more often the result of low-speed collisions and side-swipes which cause only minor or moderate damage. These systems also suffer deterioration from environmental factors and frequently incur minor impact damage during roadway maintenance operations such as snow plowing and mowing. Unfortunately, very little information has been available to quantify degradation in barrier performance that these seemingly minor damages cause. Regardless of the cause of the damage, failure to repair a system that has been damaged beyond some critical level may lead to serious injuries in a crash and expose the agency to tort liability claims. On the other hand, repairing a system that is still largely functional may unnecessarily consume valuable time and funding resources that could be put to better use elsewhere. One of the oft-touted advantages of strong-post w-beam guardrails, for example, is that they can retain some effectiveness even after having been struck and damaged. Maintenance and repair personnel need to have specific guidance about when a guardrail is so badly damaged that it needs to be repaired and when they can defer the repair of a barrier that is still largely effective even though somewhat damaged. Making such decisions maximizes the overall safety of the roadway by ensuring that barriers are still effective even when damaged while minimizing the overall cost of maintenance and repair by focusing on the guardrails most in need of the limited resources of the Department of Transportation (DOT). Project Objective Historically, each agency has developed its own repair guidelines based on “perceived” level of damage using their own experience and engineering judgment. The primary objective of this study was to develop a Field Guide to assist maintenance personnel in making decisions about repairing damaged guardrail installations. The Field Guide provides quantitative criteria for assessing guardrail damage and repair priority based on measureable damage metrics. The repair priority relates only to the reduced performance of the guardrail for various types and

2 levels of incurred damage modes. There are other benefit-cost and risk factors to consider in deciding which systems most warrant repair (e.g., traffic exposure, operating speeds, site conditions, crash history, etc.). Those aspects were outside the scope of this study; however, the guidance for assessing the need for repair based on quantifiable measures of damage will greatly aid in the decision making process. Background Probably the first widely available guide for the repair of guardrails aimed at DOT maintenance personnel was developed by Iowa State University and published by the FHWA in 1990.[Brewer90] This guide, titled “W-Beam Guardrail Repair and Maintenance: A Guide for Street and Highway Maintenance Personnel,” provides very practical instructions and guidance on repairing and maintaining strong-post w-beam guardrails and has been useful to many DOT maintenance engineers. The guide provided examples including photographs of typical damage, methods for estimating the quantities of materials for repairs, methods for estimating the crew size and time required for repairs, lists of equipment needed for different types of repairs and example forms and worksheets for keeping records of the repairs. The main deficiency of the 1990 Iowa State/FHWA guide is that at the time, no one knew just how effective guardrails with various types of damage might be if struck again. There were no crash tests of damaged guardrails and, at the time, finite element simulations of damaged guardrails were not even possible so the only option for the authors then was to use experience and engineering judgment to assess the degree of damage. Prioritizing which repairs were essential and which could be deferred was based largely on intuition and field experience. In 2006 the National Cooperative Highway Research Program (NCHRP) Project 22-23 was initiated in order to develop specific criteria that would answer these lingering questions on the effectiveness of damaged strong-post w-beam guardrail installations. The result of NCHRP Project 22-23 was NCHRP Report 656.[Gabler10] Report 656 focused on a single barrier type, which was a semi-rigid w-beam guardrail with steel posts and routed wood or plastic blockouts called the modified G4(1S). This system was selected because it is one of the most widely used semi-rigid barriers in the U.S. The Project 22-23 research team used a combination of pendulum impact experiments, finite element simulations, and full-scale crash tests to evaluate performance degradation due to varying degrees of damage for each of these damage modes. Table 1 lists the eleven damage modes that were investigated in Report 656, as well as the methods used to evaluate each damage mode. The performance assessment guidelines for damaged guardrails in Report 656 provide maintenance personnel with a process to determine the repair priority for each damage mode based on quantifiable measures of component damage. The priority levels were defined as high, medium or low and a brief description of each priority level is provided below from NCHRP Report 656:  “High Priority: Indicates damage where the crash performance of the barrier has been compromised to such a degree that a second impact to the damaged barrier would result in unacceptable vehicle and/or barrier performance. This would include vehicle penetration of the barrier (i.e., rail rupture, vehicle override, or vehicle under-ride) and vehicle rollover.”  “Medium Priority: Indicates damage where the crash performance of the barrier has likely been compromised to some degree but the damage is less likely to

3 result in unacceptable vehicle and/or barrier performance than high-priority damage.”  “Low Priority: Indicates damage where the crash performance of the barrier is indistinguishable from the undamaged condition.”[Gabler10] Table 1. Methods for evaluating damage modes in NCHRP Report 656. [Gabler10] Report 656 provided much needed quantitative information on the effectiveness of damaged strong-post w-beam guardrail installations and guidance on determining repair priority. Due to the limited funds available for the project, however, Report 656 only focused on a single guardrail type (i.e., the modified G4(1S) ) and performance assessments were based on the assumption that each damage mode exists in isolation (i.e., there was no attempt to assess the performance of a system that exhibited multiple damage modes). Further guidance was needed in assessing barrier installations with multiple damage modes, as well as further development of the guidelines to include a number of other damage modes and additional barrier systems beyond what is provided in Report 656. A list of additional research needs suggested by Gabler in NCHRP Report 656 is summarized below:  Conduct a more thorough sensitivity analysis for each damage mode.  Include more applicable methodology for assessing component failures due to fracture (e.g., rail tears, fractured blockouts, etc.).  Conduct additional full-scale crash tests of damaged barriers as a means to both evaluate crash performance and to provide additional data for validation of analytical models.  Assess implications of damage based on performance criteria outlined in Manual for Assessing Safety Hardware (MASH) – the current FHWA approved crash testing guidelines.  Assess performance degradation and develop repair criteria for barriers with multiple damage modes.  Assess performance degradation and develop repair criteria for other types of guardrail systems, such as wood post systems, thrie-beam systems, etc.  Assess performance degradation and develop repair criteria for transition systems.  Provide more quantitative assessment and guidance for repair of generic end treatments.

4  Evaluate the effects that damage to barriers near energy absorbing end terminals will have on the ability of those end terminals to activate and function properly. Roadmap of the Report It was not the intent of this study to re-do any of the work already accomplished in Report 656, but rather to expand the work to include damage assessment and repair guidelines for additional strong-post guardrail systems, additional damage modes, and more in-depth sensitivity analyses of select damage modes. The basic approach used in this project was similar to that used in Report 656 and involved a combination of pendulum testing and computational analyses to assess performance degradation of a guardrail for various damage modes. The report is organized into 17 chapters as follows:  Chapter 1 gives an introduction to the report, the study objectives and background information regarding the project.  Chapter 2 provides a summary of current and past testing requirement for obtaining “eligibility” for a roadside device for use on federal-aid projects, and a summary of the design and crash performance of common non-proprietary strong-post guardrails.  Chapter 3 provides a literature review that describes (1) general crash phenomena that lead to poor guardrail performance and (2) the general function and influences of various guardrail components.  Chapter 4 summarizes the results of a survey of practitioners which was conducted to identify which damage modes and system elements should be evaluated in this research.  Chapter 5 discusses which additional guardrail systems should be included in the field guide, evaluates the importance of addressing each damage mode associated with each of those systems, identifies the research methods that could be used in quantifying the effects of various levels of damage for each case, and prioritizes a list of damage modes and system elements for inclusion in the research.  Chapter 6 discusses the research approach and includes the scope of work performed in this study.  Chapter 7 details the development and validation for the finite element model that will be used for assessing the performance of the guardrail with various types and levels of damage.  Chapters 8 through 14 discuss the evaluations and results for assessing the performance of guardrails with various damage modes. The recommended assessment procedures and the criteria for repair priority are provided at the end of each of these chapters. The damage modes include: o Guardrail post deterioration (e.g., rot and insect damage), o End-terminal damages that may affect anchor strength, o Combination damage modes of rail deflection and rail-to-post connection, o Soil erosion around guardrail posts, and

5 o Additional splice damages that were not investigated in Phase I  Chapter 15 discusses the development and formatting for the field guide.  Chapter 16 provides a series of real-world examples performed using the on-line version of the Field Guide at http://www.roadsafellc.com/GCA/index.php to illustrate the basic assessment procedure.  Chapter 17 provides the conclusions and recommendations for future research. The result of this research is a relatively comprehensive field guide for assessing damages for two of the most widely used strong-post w-beam guardrails –the modified G4(1S) and the G4(2W). The field guide was assembled in two basic formats: (1) a field manual, which is provided as Appendix A to this report and (2) an online interactive website which can be accessed at http://www.roadsafellc.com/GCA/index.php. The field manual and the on-line guide are meant to be used together; the online version is intended to facilitate and automate the assessment procedure, while the field manual provides additional details and commentary on how to carry out the assessments.

Next: Chapter 2. Summary of Common Non-Proprietary Guardrails »
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Guardrails are an important feature of the roadside that are used to shield errant motorists from becoming involved in even more catastrophic crashes by redirecting vehicles away from fixed hazards such as trees and poles and terrain hazards such as steep roadside slopes and fill embankments.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 304: Criteria for Restoration of Longitudinal Barriers, Phase II develops a Field Guide to assist maintenance personnel in making decisions about repairing damaged guardrail installations.

Supplementary material to the document is Appendices A-S.

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