Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems (2012) / Chapter Skim
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123 A p p e n d i x e s Appendixes A through D of the contractors' final report are not published herein but are available on the TRB website by searching for NCHRP Report 711. Appendix E is published herein and follows.
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124 In NCHRP Project 22-25, Development of Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems, a comprehensive review of the state of the art relative to cable barrier systems was undertaken. These efforts included a review of the literature, a survey of highway agencies, and discussions with knowledgeable professionals from government, industry, and academia.
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Summary of Recommended Guidelines 125 1.2. Facility Features 1.2.1.
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126 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 1.6.2. Drainage needs and the median/roadside soil conditions relative to post and anchorage requirements should be taken into consideration.
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Summary of Recommended Guidelines 127 2.3.3. An anchorage designed for the local soil conditions is needed for each run of cable barrier.
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128 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems • Parallel runs of cable barrier may be appropriate for situations such as differential profile grades, narrow medians, or when objects such as high-mast light poles are located in the middle of the roadway median. • A minimum clear distance equivalent to the likely deflection of cable barrier should be maintained between the cable barrier system and any obstruction.
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Summary of Recommended Guidelines 129 3.2.3. Cable barrier design deflection should not be based solely on deflection data from full-scale crash tests.
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130 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 3.4.6. The barrier should be placed as close as practical to the convex side to allow more space behind the barrier for the added deflection.
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Summary of Recommended Guidelines 131 • Adequate offset (flare) should be used between the cable barrier and the other system to prevent vehicle instability during the impact.
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132 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 4.1.3. Initial cost estimates should be based upon a clear understanding of the placement of the cable barrier installation, obstacles, environmental factors, associated anchorages, interconnections, and other factors.
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Summary of Recommended Guidelines 133 5. Construction Proper construction of any barrier system is necessary for it to perform properly.
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134 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems cable systems that are susceptible to maintenance needs, such as cable tensioning or post pull out on installations on vertical curves. Guidance for maintenance needs is provided below.
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Abbreviations and acronyms used without definitions in TRB publications: AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TEA-21 Transportation Equity Act for the 21st Century (1998)
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TRANSPORTATION RESEARCH BOARD 2012 EXECUTIVE COMMITTEE* OFFICERS CHAIR: Sandra Rosenbloom, Professor of Planning, University of Arizona, Tucson VICE CHAIR: Deborah H
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N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP REPORT 711 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Dhafer Marzougui Umashankar Mahadevaiah Fadi Tahan Cing Dao (Steve) Kan GEORGE WASHINGTON UNIVERSITY Washington, D.C.
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NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed research provides the most effective approach to the solution of many problems facing highway administrators and engineers. Often, highway problems are of local interest and can best be studied by highway departments individually or in cooperation with their state universities and others.
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The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. On the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters.
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C O O P E R A T I V E R E S E A R C H P R O G R A M S ACKNOWLEDGMENTS The George Washington University project team wishes to acknowledge the NCHRP Project Panel and staff for their demonstrated confidence in the team's capabilities and the regular feedback provided on the research materials. It also wishes to cite its gratitude for contributions to this effort of the support consultants Mr.
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This report provides guidance for the selection, use, and maintenance of cable barrier systems. While cable barrier systems have been in use for more than 70 years, their use has been on the rise and is expected to continue in the future.
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conditions. Given the results of previous research, the variety of cable barrier systems available, and the inadequacy of past deployment practices for new systems, there was a need to establish better guidance for highway engineers.
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1 Chapter 1 Introduction 3 1.1 Project Objectives and Scope 5 1.2 Report Organization 6 Chapter 2 Literature Review 6 2.1 History and Usage 7 2.2 Cable Barrier Designs 8 2.3 Performance of Cable Barriers 12 2.4 Placement of Barriers 14 2.5 Cable Heights 16 2.6 Deflection, Post Spacing, and Anchor Spacing 18 2.7 Horizontal Curves 18 2.8 Maintenance Issues 22 2.9 Review of Existing State DOT Guidelines 24 2.10 International Practices 28 2.11 Summary 30 Chapter 3 Cable Barrier Current Practices 30 3.1 Extent of Cable Barrier System Use 33 3.2 Median Conditions for Cable Barrier Installation 35 3.3 Factors Considered in Selecting Cable Barriers 37 3.4 Cable Barrier Impacts, Penetrations, and Crashes 39 3.5 Repair or Maintenance Concerns 41 3.6 Other Critical Issues 43 3.7 Summary 45 Chapter 4 Descriptions of Available Cable Barriers 45 4.1 Weak-Steel Post Cable (Three-Strand) Guardrail 46 4.2 Brifen Wire Rope Safety Fence 47 4.3 Gibraltar Cable Barrier System 48 4.4 Nucor Steel Marion Cable Barrier System 49 4.5 Safence Cable Barrier System 50 4.6 Trinity CASS Cable Barrier System 51 4.7 Other Designs 53 Chapter 5 Analyses and Results 54 5.1 Cable Barrier Lateral Placement 67 5.2 Cable Barrier Deflection 85 5.3 End-Anchoring and Post-Anchoring Systems 87 5.4 Interconnection with Other Systems 89 5.5 Horizontal Curvature 91 5.6 Installation Costs 98 5.7 Cable Barrier Maintenance: Tolerances, Repairs, and Systemwide Maintenance C O N T E N T S
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103 Chapter 6 Guidelines for Cable Barriers 103 6.1 Warrants 103 6.2 Structural Details and Crash Performance Characteristics 104 6.3 Cable Barrier Guidelines 112 Chapter 7 Summary and Conclusions 113 7.1 Research Findings 114 7.2 Development of Guidance, Guidelines, and Procedures 114 7.3 Future Research Needs 116 References and Bibliography 119 Glossary 123 Appendixes Note: Photographs, figures, and tables in this report may have been converted from color to grayscale for printing. The electronic version of the report (posted on the Web at www.trb.org)
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1Cable barriers are longitudinal roadside devices used to contain and/or redirect errant vehicles that depart the roadways. These barriers gradually redirect or arrest an impacting vehicle by stretching of the cables, minimizing forces on the vehicle and its occupants.
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2 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Figure 1.1. Typical generic, 3-cable, low-tension median cable barrier installation.
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Introduction 3 factors included the terrain geometry and shape, speeds and angles of the vehicles as they leave the road, lateral placement of the barrier, barrier system configuration (heights of cables) , vehicle type (front geometry and mass)
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4 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems panel as possible topics for more detailed study. The findings of the research completed under Tasks 1 and 2 and the recommendations for more detailed research were summarized in the interim report.
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Introduction 5 1.2 Report Organization This report is organized into seven chapters as follows: • Chapter 1 -- Introduction • Chapter 2 -- Literature Review • Chapter 3 -- Cable Barrier Current Practices • Chapter 4 -- Descriptions of Available Cable Barriers • Chapter 5 -- Analyses and Results • Chapter 6 -- Guidelines for Cable Barriers • Chapter 7 -- Summary and Conclusions The contractors' final report also includes several appendices to provide details relative to the cable median barrier systems studied, approaches employed, detailed results, and similar materials. Appendix E is included in this report.
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6Literature Review At the outset of the project a detailed literature review was performed to gather information related to cable barriers with a focus on the design, performance, evaluation, maintenance, and application of cable barrier systems. Many types of documents were collected, organized, and reviewed including cable barrier research papers and reports, presentations, DOT guidelines, in-service evaluations, international studies, usage and safety performance statistics, and success stories.
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Literature Review 7 Table 2.3 gives an estimate of installed miles of cable barrier for four different points in time, April 2006, September 2006, January 2008, and April 2008. These data gathered by the Texas Transportation Institute (TTI)
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8 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems impact conditions. Thus weak-post barriers can be used safely only when adequate clear area exists behind the barrier to accommodate the dynamic deflection.
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Literature Review 9 guardrail, 43 miles of W-beam/cable mix, 31 miles of weak post W-beam, and 18 miles of W-beam/weak post W-beam mix)
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10 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 49 to 100; however, the average annual rate of fatal crashes dropped 89 percent from 3.00 to 0.33. Likewise, the average annual rate of disabling crashes dropped 51 percent from 3.60 to 1.76.
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Literature Review 11 shown have wide statistical confidence intervals. Table 2.8 shows the effectiveness of cable barriers in preventing cross-median penetrations.
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12 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 2.4 Placement of Barriers The TTI report entitled "Guidelines for the Selection of Cable Barrier Systems (Generic Design vs. High Tension)
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Literature Review 13 Ray's cable barrier summary [3] contains a table with data from ten states on their guidelines for installing cable median barriers.
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14 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Test 1 Placement Test 2 Placement GOOD BAD 3 feet Vehicle Direction Figure 2.3. Sample trace envelope of vehicle crossing a sloped median.
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Literature Review 15 16 ft Median 24 ft Median 32 ft Median 40 ft Median 48 ft Median Figure 2.4. Vehicle trajectories from computer simulations of a pickup truck traversing a V-shaped 6:1 sloped median at different impact speeds and angles [19]
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16 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 2.6 Deflection, Post Spacing, and Anchor Spacing The dynamic deflection of a cable barrier during impact is an important characteristic for many reasons. Compared to semi-rigid W-beam barriers and rigid concrete barriers, cable barriers have much greater deflections, which is the reason that cable barriers typically are more forgiving to the impacting vehicle's occupants.
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Literature Review 17 (weaved) 4-cable barrier and a non-woven (parallel)
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18 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems spacings. Simulations were not conducted on anchor spacings longer than 1,000 m (3,300 ft)
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Literature Review 19 Low-tension cable barrier systems lose their effectiveness after a crash because of the lack of tension in the cables, which causes the cables to droop, or even lie, on the ground. On the other hand, high-tension systems maintain their effectiveness after crashes as long as the anchors remain in place and a limited number of posts is destroyed.
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20 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems On-Going Maintenance Most of the reported non-crash-related maintenance issues have involved soil conditions, post foundations, and/or anchors. Some barriers located along the median centerline have experienced problems with weak, saturated soils.
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Literature Review 21 Figure 2.9. High-tension cable barrier after its anchor failure due to crash [28]
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22 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems researchers recommended the use of Field Swage as a retrofit for low-tension systems because it is easier to install than the Epoxy Socket and has higher maximum strength than the other terminations. The importance of proper installation was also emphasized in the study to ensure that full strength is reached and premature cable pull-out is avoided.
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Literature Review 23 Area # Guidelines Selection 1 Utilize the recommended guidelines for installing me dian b arriers on h igh-speed roadways in Texas. 2 Cable barrier is for use only in roadway medians in Texas.
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24 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 2.10 International Practices The literature review included an attempt to determine the evolution of the technology and application of cable barriers in other parts of the world. These are summarized relative to use, evaluations, maintenance, and other concerns in the paragraphs below.
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Literature Review 25 evaluations of safety performance discovered, but nothing of a large scale. For example, Marsh and Pilgrim analyzed the performance of wire rope for the Centennial Highway in New Zealand [33]
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26 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems noted that there were changes made in the 1987 AustRoads "Safety Barriers" guide that include flexible systems incorporating wire ropes [41]
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Literature Review 27 Other Concerns Schermans and Van der Hoek indicated that the 2+1 concept was being considered for the Netherlands. They noted that there are more motorcyclists in the Netherlands than in Sweden, which may cause a problem [45]
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28 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 2.11 Summary The literature review covered a broad spectrum of information sources and provided a viable snapshot of the current state of the practice. From the literature review, it is possible to conclude the following: • Cable barriers have a long history of use on highways.
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Literature Review 29 • The few evaluations of systems deployed in other parts of the world generally have concluded that wire rope systems have been effective in reducing the number and severity of crossover crashes. They have been judged to be "cost effective." • The success with median applications has inspired the use of cable barriers in other applications.
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30 In order to establish an up-to-date picture of current agency efforts to utilize the emerging cable barrier technology, a survey was conducted at the outset of the project. To maximize the potential to capture information, the survey was similar to the one used in an earlier study.
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Cable Barrier Current Practices 31 The difference could be a result of errors in the approximations made by some states and/or the lack of data from 10 states that did not respond to the survey. Ninety-seven percent of cable barrier systems were placed in the median while only 3 percent were used on the side of the road.
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32 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Figure 3.3. Total miles of cable barrier by manufacturer as reported by 37 states.
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Cable Barrier Current Practices 33 3.2 Median Conditions for Cable Barrier Installation The survey next posed four questions about the conditions or configurations of the medians where cable barrier systems were installed. The detailed versions of these questions are provided in Appendix A of the contractors' final report, but the basic questions were • Question 2: Please estimate the median slopes that exist in your state for which cable median barriers have been, or are planned to be, installed to reduce cross-median crashes.
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34 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems and average median widths reported from the 23 states by side slope are shown in Table 3.1. The minimum median widths ranged from 3.5 to 8.5 m (12 to 28 ft)
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Cable Barrier Current Practices 35 3.3 Factors Considered in Selecting Cable Barriers The survey posed four questions about the factors or criteria considered in determining that cable barriers would address the safety needs for a given situation. The intent was to determine if warrants, state experience with cable barriers, and/or existing standards provided an impetus or direction for specific decisions.
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36 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Selection criteria for cable barriers over other longitudinal barriers in roadside applications were provided by 33 states as follows: • Allowable deflection (10 states) • Distance to hazard (8 states)
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Cable Barrier Current Practices 37 for tasks such as retensioning, anchor monitoring, and reinstallation and for keeping the cable barrier system in operating condition. Figure 3.8 shows the number of states having cable barrier standards for system design, alignment, construction, and maintenance.
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38 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems per mile per year. Additionally, two states, which have a total cable barrier length of less than 15 miles, reported a higher rate of about 25 crashes per mile per year.
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Cable Barrier Current Practices 39 • One state had one crash where the motorcyclist came upon stopped traffic and decided to go into the median rather than hit the rear of a stopped vehicle. The driver was thrown off the motorcycle and took out two of the support posts.
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40 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems • Posts shearing off instead of bending during crashes cause some difficulty in removing the remaining post from the socket (1 state) • Frequent hits make it difficult to keep up with repairs by contractors (1 state)
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Cable Barrier Current Practices 41 3.6 Other Critical Issues The survey next posed two questions regarding general concerns about the deployment of cable barrier systems. These addressed basic issues as well as concerns related to procurement, design, and addressing regional conditions.
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42 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems point for steep slopes (such as 2H:1V)
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Cable Barrier Current Practices 43 • Use construction requirements included in the contract specifications and state's standards (5 states) • Use only the manufacturer's recommendations (3 states)
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44 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems • Less than half the states provided information about the possible factors attributed to barrier penetration. The factors cited included wet conditions, vehicle type and shape, severe angle and speed, lateral position of the barrier, the slope conditions, soil conditions, and commercial vehicles.
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45 Six types of NCHRP Report 350-accepted cable barrier systems are currently available for use on U.S. highways, as follows: • Weak-Steel Post Cable (3-Strand)
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46 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Generic cable barrier systems are available in different configurations. Three different posts are used with these systems: S75 × 8 steel I-beam (Acceptance Letter B64 -- SGRO1-a)
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Descriptions of Available Cable Barriers 47 both configurations, the top cable is placed in a slot at the center of the post. The other two or three cables are woven around the posts.
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48 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems used with the Gibraltar cable barrier system. Spacing from 3 m (10 ft)
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Descriptions of Available Cable Barriers 49 with these systems. The cables in both systems are attached to 6 kg/m (4 lb/ft)
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50 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems This system is intended for non-median applications (roadside application only)
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Descriptions of Available Cable Barriers 51 separated by plastic spacers. The posts can be anchored to the ground using steel sockets that are cast into concrete cylinders, steel tubes driven in soil, or posts directly driven in the soil.
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52 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems in the tests, are imbedded 1.0 m (3.3 ft) in the ground.
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53 As noted in Chapter 1, the panel directed the contractor to pursue analyses in the seven areas outlined in Table 1.1. This chapter will provide the details about the approaches used, the factors considered, and the results obtained for the following seven areas: • Cable barrier placement • Cable barrier deflection • End-anchoring and post-anchoring systems • Interconnection with other systems • Horizontal curvatures • Construction and maintenance tolerances • Installation and maintenance costs The intent of conducting the analyses in these areas was to derive science- and data-based insights on the influence of various factors with the subsequent translation of these findings into guidelines that would facilitate the deployment of effective cable barrier systems for median and roadside applications.
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54 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems – Tangent and curved roadway sections – Obstacles in the median or roadside – Shoulder/slope design – Drainage requirements – Soil conditions – Seasonal effects – Transitions to other barriers – Installation operations • Maintenance considerations – Probability of nuisance hits – Anchor movement – Post foundation failures – Retensioning – Maintenance cycles – Ease of repair – Requirements for lane closure These factors can have different types and degrees of effects on cable barrier performance. The need exists to assess the implications of the factors to identify the critical ones and translate them into generic guidance.
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Analyses and Results 55 vehicle will land (or return to a distribution of weight on all wheels) , and the suspension will compress to absorb the dynamic load.
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56 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems mathematical models provide a detailed description of a motor vehicle that considers the influence of weight, suspension system, and other vehicle factors. Available databases include a wide range of high-fidelity vehicle models that can be used in dynamic reconstructions and simulations.
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Analyses and Results 57 These vehicle models included a pickup truck (Chevrolet C2500) and a small car (Honda Civic)
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58 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems • Defining the impact conditions to be considered. One approach would be to follow NCHRP Report 350 or MASH requirements.
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Analyses and Results 59 line goes above the highest yellow line there is an opportunity for an override to occur. Where the green line falls below the lowest yellow line, the possibility of an underride exists.
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60 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems The lower portion of Figure 5.9 shows the profile or cross-section of the median related to the upper graph. The green hatched portions indicate the lateral positions where this specific barrier will be effective.
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Analyses and Results 61 • Initial velocity occurs when the vehicle leaves the shoulder. Some deceleration is expected to occur (3–5 mph was noted in the research)
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62 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems for the same medians with varied median widths (5 to 17 m, 16 ft to 56 ft) and the combined maximum curve (thick blue curve)
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Analyses and Results 63 for systems where the top cable is at least 838 mm (33 in.) and the bottom cable is no higher than 533m (21 in.)
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64 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems sloped profiles, the underride region is smaller (−1.2 m to 1.5 m [−4 ft to 5 ft]
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Analyses and Results 65 6.7 (22 ft) from the edge of the median, two cables would be engaged.
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66 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Zero at Edge of Median Figure 5.18. Override limit plots for 4H:1V slope medians at varied superelevations.
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Analyses and Results 67 5.2 Cable Barrier Deflection The dynamic deflection of a cable barrier during impact is an important characteristic for many reasons. Compared to semi-rigid W-beam barriers and rigid concrete barriers, cable barriers have greater deflections, which is the reason that cable barriers typically are more forgiving to the impacting vehicle's occupants.
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68 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems systems that are most commonly installed and have multiple full-scale crash test data available. This process ensured that the computer models could be validated fully with crash test data and that the analyzed systems would represent the majority of installed systems.
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Analyses and Results 69 shows the details in the finite element model of the Gibraltar cable barrier system. The geometry and design details of all components were obtained from FHWA acceptance letters.
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70 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Figure 5.21. Comparison of sequential plots; Gibraltar Test 1.
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Analyses and Results 71 Figure 5.22. Comparison of sequential plots; Gibraltar Test 2.
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72 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems (11.7 ft) and for the crash test, 3.7 m (12.1 ft)
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Analyses and Results 73 Figure 5.24. Comparison of sequential plots; Safence cable barrier system.
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74 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Figure 5.25. Finite element model of the Brifen wire rope system.
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Analyses and Results 75 Figure 5.26. (Continued)
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76 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Figure 5.28. Comparison of sequential plots; Brifen Test 2.
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Analyses and Results 77 a good correlation with the test. Maximum dynamic deflection observed in the test was 2.1 m (6.9 ft)
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78 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems In both the test and simulation, the top two cables engaged the vehicle and redirected it. The maximum dynamic deflection for the simulation was 2.5 m (8.1 ft)
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Analyses and Results 79 Figure 5.31. Comparison of sequential plots from CASS simulation.
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80 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Figure 5.32. Finite element model of the Nucor cable barrier system.
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Analyses and Results 81 Figure 5.33. (Continued)
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82 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems hooks to hold the cables to the posts)
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Analyses and Results 83 3 5 4 6 7 8 9 10 11 12 13 0 500 1000 1500 2000 2500 3000 3500 4000 1.5 2.0 1.0 2.5 3.0 4.0 3.5 0 200 400 600 800 1000 1200 B ar rie r D ef le ct io n (ft ) End-anchor Spacing (ft)
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84 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 3 5 4 6 7 8 9 10 11 12 13 0 500 1000 1500 2000 2500 3000 3500 4000 1.5 2.0 1.0 2.5 3.0 4.0 3.5 0 200 400 600 800 1000 1200 B ar rie r D ef le ct io n (ft ) End-anchor Spacing (ft)
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Analyses and Results 85 5.3 End-Anchoring and Post-Anchoring Systems Adequate anchoring of the cables is critical to ensure satisfactory barrier performance during impact. Anchor movements lead to lower tension in the cables, which results in larger deflection of the system.
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86 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Table 5.1. Recommended end-anchor sizes from MwRSF study [62]
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Analyses and Results 87 5.4 Interconnection with Other Systems There have been accepted systems for the interconnection of cable barriers with strong post guardrail systems. These designs are reflected in FHWA acceptance letters B-147 and B-147A.
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(a) Generic (b)
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Analyses and Results 89 Similarly, the connection between each cable and W-beam rail should be designed such that it can withstand 90 kN (20 kips) , the maximum load on the cables observed in the simulations under Section 5.2 of this study.
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90 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Figure 5.46. Post-crash picture of overlapping cable barrier to W-beam guardrail transition.
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Analyses and Results 91 Figure 5.50 shows the percentage increase in deflection due to horizontal curvature for the three different post spacings. The base deflection is from the simulations using a straight alignment, i.e., zero degree curvature.
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92 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 0.4 sec 0.6 sec 0.8 sec Figure 5.49. Sequential plots from convex-side impacts for different horizontal curvatures.
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Analyses and Results 93 Figure 5.50. Influences of horizontal curvature and post spacing on deflection in convex-side impacts.
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94 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems state DOTs have not collected this information or accumulated enough data to have meaningful averages. However, it is possible to provide information on some aspects of cable barrier costs that can be used to estimate life-cycle costs.
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Analyses and Results 95 The data in Table 5.4 is shown graphically in Figure 5.51. This figure shows that for higher discount rates the average annual cost is insensitive to changes in service life beyond 20 years.
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96 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems ranged from $2,000 (winning bidder)
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Analyses and Results 97 Posts are a major component of overall barrier installation costs, but they are less variable than anchor costs. For the highlighted values in Table 5.6, which represent typical values, the highest value is 2.4 times the lowest value.
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98 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems for very little extra cost. Penetrations of barriers are caused when vehicles go under, through, or over cable barriers.
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Analyses and Results 99 The lateral position of the barrier relative to the roadway or median centerline is important for several reasons. These reasons were explained in detail in Section 5.1.
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100 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems tension in the repaired system should be checked. If any of the individual cable wires has been broken or if significant kinks in the cable are visible, the cable should be evaluated to determine if it needs to be replaced.
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Analyses and Results 101 The lower repair costs appear to be mostly associated with repairs done by highway agency crews rather than by private contractors. It is possible that some of the states reporting costs included only materials costs and not labor costs.
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102 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems which lowers its tension. Each cable barrier manufacturer has a temperature-tension table that gives the required tension as a function of cable (not air)
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103 Although strong-post cable barriers were in use in some states as early as the 1950s, New York pioneered the development and testing of the weak-post design in the 1960s. This design, included in the 1977 AASHTO Guide for Selecting, Locating, and Designing Traffic Barriers as the G1 Cable Guardrail, may have been the first design guidance for a cable barrier system.
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104 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems run-off-the-road fatalities and severe injuries. High-tension cable barrier systems are available from five manufacturers: Brifen, Gibraltar, Nucor Steel Marion, Gregory Industries (Safence systems)
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Guidelines for Cable Barriers 105 • The gentler the slope, the lower the effects on the trace of the vehicle interface area and the greater the potential for an effective vehicle-to-barrier interface. • Wider medians offer more effective lateral placement options.
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106 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems • To avoid override of larger vehicles (SUVs and pickup trucks) , the barrier should not be placed in a region between 1.2 m (4 ft)
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From page 252... ...
Guidelines for Cable Barriers 107 results, as expected, indicated that in impacts with cable barriers, the maximum dynamic deflection is affected significantly by the end-anchor spacing and post spacing. Greater end-anchor spacing leads to increased barrier deflections.
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108 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems caused by lower temperatures, or as a result of crashes into the barrier. Movement of the anchor in the soil decreases the tension in the cables which, when extreme, may result in unsatisfactory barrier performance.
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Guidelines for Cable Barriers 109 post or end anchor. Under this study, simulations were conducted to identify the minimum length needed for strong-post W-beam systems, and it was found to be 23 m (75 ft)
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110 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems • Shorter post spacings should also be used to reduce the bending of the posts over time due to lateral forces applied by the cables on the posts. • The lateral placement guidelines developed under Section 5.1 should be followed to ensure adequate vehicle-to-barrier interface.
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Guidelines for Cable Barriers 111 • Because of the small marginal cost of adding an extra cable and the likelihood of improved safety performance of the barrier with an extra cable, benefit/cost analyses should be used to determine when a fourth or fifth cable is justified. • Periodic inspection schedules based on expected frequency of impacts should be established for each cable barrier installation.
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112 Following the tasks prescribed under this project, a comprehensive effort to identify and develop guidelines for the use, selection, and maintenance of cable barriers was undertaken. The focus of the effort was to undertake analyses that would provide a sound basis for the guidelines.
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Summary and Conclusions 113 • After review of the preliminary guidelines by the panel, presented in this draft report, a workshop was held to solicit feedback on the draft guidelines from state agencies, industry representatives, and other practitioners who would be the primary users of the guidelines. The feedback and comments from the workshop participants serve as the basis for updates to the guidelines.
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114 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems • Crash simulations were conducted to analyze the influences of post spacing on dynamic deflection. There was a uniformity of increase in deflection for all systems with greater post spacing.
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Summary and Conclusions 115 • Conduct further investigations for connecting cable barriers to other systems such as concrete and weak post barriers (transitions)
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116 [1] Alberson, D
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References and Bibliography 117 [28] Focke, D., "Ohio DOT's recent experience with Cable Median Barrier," presentation at AASHTO TIG, June 2008.
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118 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems [61] Baxter, J
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119 Acceptance Testing -- Crash tests reported to FHWA to obtain a letter accepting a roadside hardware item to be considered acceptable for use on the National Highway System. Anchor -- a mass designed to counter the tension forces in a cable barrier system at either end.
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120 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems deformation, or failures. Systematic updates of the changes to each element allow replication of impacts or other physical changes.
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Glossary 121 NCAC -- National Crash Analysis Center at George Washington University. NCHRP Report 350 -- Report 350 was adopted as the national standard for roadside hardware crashworthiness testing.
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122 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems Validation -- The process of determining how well a model represents the real situation. Vaulting -- See Override.
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123 A P P E N D I X E S Appendixes A through D of the contractors' final report are not published herein but are available on the TRB website by searching for NCHRP Report 711. Appendix E is published herein and follows.
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124 In NCHRP Project 22-25, Development of Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems, a comprehensive review of the state of the art relative to cable barrier systems was undertaken. These efforts included a review of the literature, a survey of highway agencies, and discussions with knowledgeable professionals from government, industry, and academia.
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Summary of Recommended Guidelines 125 1.2. Facility Features 1.2.1.
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126 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 1.6.2. Drainage needs and the median/roadside soil conditions relative to post and anchorage requirements should be taken into consideration.
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From page 272... ...
Summary of Recommended Guidelines 127 2.3.3. An anchorage designed for the local soil conditions is needed for each run of cable barrier.
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128 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems • Parallel runs of cable barrier may be appropriate for situations such as differential profile grades, narrow medians, or when objects such as high-mast light poles are located in the middle of the roadway median. • A minimum clear distance equivalent to the likely deflection of cable barrier should be maintained between the cable barrier system and any obstruction.
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Summary of Recommended Guidelines 129 3.2.3. Cable barrier design deflection should not be based solely on deflection data from full-scale crash tests.
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130 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 3.4.6. The barrier should be placed as close as practical to the convex side to allow more space behind the barrier for the added deflection.
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Summary of Recommended Guidelines 131 • Adequate offset (flare) should be used between the cable barrier and the other system to prevent vehicle instability during the impact.
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132 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems 4.1.3. Initial cost estimates should be based upon a clear understanding of the placement of the cable barrier installation, obstacles, environmental factors, associated anchorages, interconnections, and other factors.
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Summary of Recommended Guidelines 133 5. Construction Proper construction of any barrier system is necessary for it to perform properly.
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134 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems cable systems that are susceptible to maintenance needs, such as cable tensioning or post pull out on installations on vertical curves. Guidance for maintenance needs is provided below.
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Abbreviations and acronyms used without definitions in TRB publications: AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TEA-21 Transportation Equity Act for the 21st Century (1998)
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Key Terms
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