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19 CHAPTER 3 INTERPRETATION, APPRAISAL, AND APPLICATION The overall understanding of the behavior of integral pier gram that does not take into account the effect of the inte- connections between steel I-beam superstructures, steel box- gral connection to the substructure compared well with beam pier caps, and concrete columns was gained from the the forces determined using a 3-dimensional finite ele- questionnaire, literature survey, and results of the analytical ment model of the bridge. The location of the maximum and experimental studies. This understanding was used in positive moments in the girders was different in the two determining the feasibility of the integral connections tested cases; however, the shift in the maximum moment sec- and in developing a design methodology for the integral pier tion location was relatively small to affect the design. caps. This design methodology is presented below. Proposed The live load moments in the single column pier from the design provisions and commentary written in the format of 3-dimensional model compared well with the moment the AASHTO LRFD Bridge Design Specifications were devel- calculated using a 2-dimensional frame representing the oped based on the proposed design methodology. The proposed bridge in elevation. The section properties of the mem- design provisions and commentary are presented in Appen- bers representing the superstructure were taken equal to dix H provided on the accompanying CD-ROM. A detailed the section properties of the entire bridge cross section. design example using the proposed provisions is presented in The maximum live load torsional moment in the pier cap Appendix I provided on the accompanying CD-ROM. and the maximum live load moment at the top of the col- Some of the observations from the analytical and experi- umn are produced by different load cases. The maximum mental studies were extrapolated to accommodate expected variations in geometry and site conditions that were not directly torsional moment is produced when opposite halves of included in the studies. This extrapolation is based on the the two spans next to the pier cap are loaded (see Figure design experience of many of the research team members and 16a). The maximum live load column top moment is sound engineering judgment. produced when the longer of the two adjacent spans is loaded (see Figure 16b). In the prototype bridge, the maximum live load torsional moment in the pier cap on 3.1 FEASIBILITY either side of the column, produced from a load case sim- ilar to that shown in Figure 16a, is approximately equal The experimental studies demonstrated the feasibility of connecting a concrete column integrally to a steel box-beam to the maximum live load moment in the column at its pier cap. They further demonstrated that the bond between top, produced from a load case similar to that shown in the concrete placed inside a box-beam pier cap and the lon- Figure 16b. gitudinal column reinforcement is sufficient to develop the overstrength column moment and no further connection is For the case of maximum torsional moments in the pier required for the column reinforcement. The test results also cap, the column moments and the rotation at the column top indicated that shear connectors installed inside the box-beam and at interior girder intersections with the pier cap are rela- pier cap are sufficient to transfer the column moments from tively small. This results in the exterior girders transferring the concrete to the pier cap. most of the pier cap torsional moment. The maximum seismic torsional moment in the pier cap on either side of the column may be taken equal to one-half the 3.2 PROPOSED DESIGN METHODOLOGY column overstrength moment. For a two-span bridge with 3.2.1 Method of Analysis two girders on either side of the column, the interior girders and the exterior girders transferred 64 and 36 percent of the Based on the analytical studies conducted as part of this torsional moment, respectively. Assuming that 40 percent of project, the following observations were made: the torsional moment on either side of the column is trans- ferred by the exterior girder and the remaining 60 percent is The maximum moments and shears in the girders cal- transferred by the first interior girder is expected to result in culated using a conventional line girder computer pro- adequate design accuracy.