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5  This chapter provides an overview of the scope of NCHRP Project 12-113 with an emphasis on the work conducted in Stage 2, which considered WT sections. The chapter provides a brief description of the experimental methods, FEA validation, and parametric studies that were con- ducted in Stage 2 of the investigation. Results from the experiments, FEA validation, and para- metric studies are covered in more detail in subsequent chapters. To accomplish the objectives of Stage 2, two additional phases, Phase V and Phase VI, were systematically carried out by the research team following the four phases completed in Stage 1. The organization of these additional phases and the general workflow of the research conducted in Stage 2 are presented with a workflow diagram in Figure 2-1 for reference. Phase V and Phase VI contain four major tasks. In Phase V, laboratory tests on WT sections and validation of the FEA models were performed, which allowed the parametric FEA studies in Phase VI to commence. In total, Phase V consisted of four tasks, which are briefly summa- rized below: Phase V ⢠Task 16. Laboratory Experiments: Based upon information obtained from bridge owners, designers, and fabricators, specimens were experimentally tested with representative WT sections, gusset plates, and connection plates. The tests provided validation data for FEA models of WT cross-frame members that were used in Task 17. ⢠Task 17. Validation of FEA Models: Data from the experiments conducted in Task 16 were used to validate FEA models of the WT sections, gusset plates, and idealized connection plates. ⢠Task 18. Develop Scope of Parametric Studies: A comprehensive plan for the parametric studies was developed considering the practical range of WT/plate sizes as well as the depth and spacing of the bridge girders to be evaluated. ⢠Task 19. Phase V Interim Report: An interim report was prepared summarizing the work from Tasks 16 through 18. The research team received feedback from the review panel on an interim report documenting the findings from Phase V and were given permission to proceed to Phase VI. In Phase VI, based on the experimental results from Phase V, parametric FEA studies were conducted to investigate the appropriate R-factors for both construction and in-service conditions of bridges. Phase VI included four major tasks, as summarized below: Phase VI ⢠Task 20. Parametric FEA Studies: The parametric studies outlined in Task 18 were executed. ⢠Task 21. Develop Proposed Changes to AASHTO LRFD Bridge Design Specifications: Based upon the results from Task 20 and the recommendations of the review panel, the C H A P T E R  2 Research Approach
6 Improved Cross-Frame Analysis and Design: Wide-Flange T-Shape Sections previously balloted and accepted AASHTO LRFD bridge design specifications and commentary language in Stage 1 will be edited to incorporate the recommendations from Stage 2 for WT sections used as cross-frame members. ⢠Task 22. Develop Ballots for AASHTO LRFD Bridge Design Specifications: An agenda item ballot will be developed for the AASHTO LRFD Bridge Design Specifications reflecting the final proposed revisions recommended in Task 21. ⢠Task 23. Final Report Preparation: A final report documenting the results and findings from all tasks in Phase V and Phase VI was prepared and submitted for review and publication. 2.1 Experimental Program and Model Validation In the experimental program, specimens consisting of the WT sections, gusset plates, and a simulated connection plate were utilized in laboratory experiments conducted in a hydraulic testing machine. The machine had a capacity of 220k, which was satisfactory for the range of expected loads. The specimens were subjected to both tension and compression to obtain a mea- sure of the axial and flexural deformations that occur as a function of eccentricity. The research team considered cross-frame details from several state departments of transportation to ensure that the specimen sizes were representative of WT sections, connection plates, and gusset plates utilized in practice. In total, 14 WT specimens were fabricated and tested that are representative of common WT members utilized in cross-frames commonly found in bridges. A bridge fabricator was used to fabricate the specimens to ensure the details were representative of details found in practice. The experiments included connections between the gusset plate and connection plate that add to the eccentricity (referred to as âadditive eccentricityâ herein) as well as reduce the effective eccentricity (referred to as âsubtractive eccentricityâ herein). To determine whether connection methods were a significant consideration for the parametric studies, the research team considered the difference Figure 2-1. General flow of work in Stage 2 of NCHRP Project 12-113.
Research Approach 7 between welded and bolted connections. The bolted specimens allowed direct comparisons between additive and subtractive eccentricities without increasing the number of specimens that had to be fabricated. Figure 2-2 provides a sketch of details with additive and subtractive eccentricities for cases with a bolted connection. By shifting the WT from one side to the other and rotating the shape 180 degrees, the additive or subtractive eccentricities caused by the orientation of the WT was considered. The data collected from the laboratory tests were used to validate the FEA models of the WT sections, gusset plates, and connection plates. The effect of the shift in the eccentricity due to con- necting the gusset to the front and back side of the connection plate was reflected in the valida- tion studies. The validation studies consisted of comparing the behavior of individual members subjected to compression and tension. After validation of the model, the research team developed a comprehensive plan for the parametric FEA studies including the range of geometry and member sizes for the parametric studies. The selected WT sizes were representative values used in practice. Additional parameters such as girder depth and girder spacing were also determined. Feedback was obtained from the NCHRP Project 12-113 review panel on the plan for the parametric studies. 2.2 Parametric Finite Element Analytical Studies The parametric FEA studies included panel-level and system-level studies. The analyses were carried out on the range of variables (i.e., WT size, girder depth, girder spacing) determined in Task 18. In the panel-level studies, the research team compared cross-frame stiffness between shell-element and truss-element models. Figure 2-3 depicts the general modeling approach for shell- element and truss-element models developed for the panel-level parametric studies. In the panel- level parametric studies, the research team applied equal rotation loading to the cross-frame panels to investigate the appropriate R-factor for the construction condition of bridges. Based on the panel-level parametric study, the research team obtained the best R-factor for construction conditions from the equal rotation loading case. However, cross-frames in com- posite bridges (in-service condition) experience not only equal rotation loading but also com- binations of other deformation patterns depending on the location of the truck loading on the bridge relative to the cross-frame. Although panel-level studies were considered to ensure that the modeling could capture the various deformational modes that occur in the composite system, Additive Eccentricity Subtractive Eccentricity Figure 2-2. Connection details with eccentricities for the experimental program.
8 Improved Cross-Frame Analysis and Design: Wide-Flange T-Shape Sections the primary analyses reflecting the stiffness behavior of cross-frames in-service come from full bridge models. Therefore, R-factors reflecting the behavior for in-service bridges were investi- gated in the system-level parametric studies. Whereas the panel-level studies evaluated an isolated cross-frame stiffness for bridges during the construction condition, the system-level studies investigated the accuracy of the simpli- fied modeling approaches for 3D composite bridges by comparing cross-frame member forces between shell-element models and the R-factors assigned truss-element models. More spe- cifically, the appropriate R-factor for in-service bridges was sought for a variety of bridge and cross-frame geometries, as well as the sensitivity of the cross-frame response due to the assigned R-factor. The accuracy of eccentric beam approaches for WT sections for cross-frame systems was also evaluated in the system-level studies. Chapter 3 provides a detailed overview of the experimental studies along with results for all the specimens that were tested. The FEA validation process is also covered. Chapter 4 highlights the parametric studies that were conducted including both the panel-level and system-level studies. Shell Model Truss Model Figure 2-3. Shell-element and truss-element models in panel-level parametric studies.
