Design of Roadside Barrier Systems Placed on MSE Retaining Walls (2010)

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11.1 Research Problem Statement Millions of square feet of mechanically stabilized earth (MSE) retaining wall are constructed annually in the United States. When used in highway fill applications in conjunction with bridges, these MSE walls are typically constructed with a roadside barrier system supported on the edge of the wall. This barrier system generally consists of a traffic barrier or bridge rail placed on a continuous footing or structural slab. The foot- ing is intended to reduce the influence of barrier impact loads on the retaining wall system by distributing the load over a wide area. The proper design of the roadside barrier, the struc- tural slab, and the MSE wall system requires a good under- standing of relevant failure modes, how barrier impact loads are transferred through the structural slab into the wall system, and the magnitude and distribution of these loads. Current design procedures and standard details for place- ment of roadside barriers on retaining walls vary widely among state highway agencies. Most current designs are believed by engineers to be overly conservative. This conservative design is in part due to an inadequate understanding of how barrier impact loads are transferred and distributed to the slab and wall system. There is a need to develop standardized proce- dures for use by state highway agencies in designing economi- cal roadside barrier systems placed on MSE retaining walls. 1.2 Research Objective The objective of this research is to develop, in a format suitable for adoption by American Association of State High- way and Transportation Officials (AASHTO), procedures for designing roadside barrier systems placed on MSE retaining structures. 1.3 Research Approach The research plan for accomplishing the project objective to develop procedures for designing roadside barrier systems placed on MSE retaining structures consisted of 10 tasks divided into three distinct phases: • Phase I – Task 1. Conduct literature review. Analyze, describe, and critique pertinent domestic and international research, on the basis of applicability, conclusiveness of find- ings, and usefulness in developing a methodology for the design of roadside barrier systems placed on MSE retain- ing walls. Critically review the AASHTO Standard Spec- ifications for Highway Bridges (1) and the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Spec- ifications (2) and relevant interim updates. – Task 2. Conduct survey. Survey state highway agencies, federal agencies, crash testing facilities, and retaining wall manufacturers to determine existing methodologies and details used in designing roadside barrier systems placed on MSE retaining walls. Identify the barrier impact loads and crash-test level [as defined in NCHRP Report 350 (3)] associated with particular barrier systems. Find and doc- ument lessons learned from known failures of existing systems. – Task 3. Summarize state of practice. Summarize the state of practice for roadside barrier systems placed on MSE retaining walls. Identify successful design procedures and details that warrant further investigation. In addition, identify promising design procedures and details for fur- ther investigation that are not necessarily used in current practice. – Task 4. Develop detailed work plan. Based on the informa- tion developed in Tasks 1, 2, and 3:  Propose design procedures for further investigation that address barrier impact loads and the transfer of these loads to the MSE retaining wall.  Prepare a detailed plan for developing a recommended design procedure. The plan shall include proposed analytical methods, component testing, and full-scale C H A P T E R 1 Introduction

crash testing needed for development and validation of the design procedure.  Update and elaborate on the work plan. – Task 5. Submit first interim report. Submit an interim report documenting the results of Phase I. The interim report shall include as a separate appendix the updated work plan for completing Phases II and III. Meet with the NCHRP 22-20 panel to discuss the interim report and the updated work plan. Work on Phase II will not begin with- out prior approval of NCHRP. • Phase II – Task 6. Develop preliminary design procedures. Develop preliminary procedures for the design of roadside barrier systems placed on MSE retaining structures. Evaluate the preliminary procedures using the analytical methods and component testing. Modify and re-evaluate the prelimi- nary procedures as necessary. – Task 7. Develop full-scale crash test plan. Develop the jus- tification for and the details of a full-scale crash testing plan for validating the preliminary procedures. The test- ing plan shall include a detailed description of the instru- mentation to be used in the barrier, the load transfer system, and the MSE retaining wall system. – Task 8. Submit second interim report. Submit the proce- dures, modeling results, and full-scale crash test plan for review by the NCHRP 22-20 panel. Meet with the NCHRP 22-20 panel to discuss the procedures and crash testing plan. • Phase III – Task 9. Validate procedures. Validate the procedures by executing the full-scale crash testing plan agreed on dur- ing the Task 8 interim meeting. Modify the procedures as necessary. – Task 10. Submit final report. Submit a final report docu- menting the entire research effort. Include the proce- dures in a stand-alone appendix, in a format suitable for adoption by AASHTO. 1.4 Report Scope This report documents the research efforts, findings, and recommendations of this project. The report includes details of the state-of-practice survey, the engineering analyses, finite element modeling, and full-scale testing conducted in support of the guideline development process, and presents guidelines for designing barriers supported on the edge of MSE walls. 2