Cui, Jilong (2021-12). Investigation of Local Failure Mechanisms of Inverted-T Bent Caps and Strengthening Using CFRP Sheets. Doctoral Dissertation.
Thesis
Prevalence cracks on in-service inverted-T bent caps were reported over the past decades. These un-desirable cracks formed at the re-entrant corner of inverted-T bent caps are an indication of the local structural deficiencies. To address such a concern, this study presents an experimental and analytical investigation on local failure mechanisms of inverted-T bent caps with emphasis on punching shear, strengthening ledge- and hanger-deficiencies with CFRP, and an analytical study on the impact of various reinforcement layouts on the local behavior of inverted-T bent caps. Punching shear failure is critical to the ledge of inverted-T bent caps as the ledge is sustaining a concentrated load from the girders with a shallow section. The punching failure mechanisms of the inverted-T bent cap ledges were experimentally investigated with bearing pad size as the primary variable. A nonlinear finite element model was also developed to perform correlative studies between analytical and experimental investigation. The analysis results revealed that enlarging the size of the bearing pad can improve the serviceability and the ultimate punching shear resistance of the inverted-T bent cap ledges. The capacity was also found to be affected by the eccentricity of the bearing pad from the face of the web. The code given estimation on the punching shear capacity was examined by the experimental results and revealed to be conservative. A practical modification to improve the accuracy of capacity estimation was proposed. Prevalence of cracks in many of in-service inverted-T bent caps was observed and raised a concern to the structural sufficiency of the inverted-T bent caps that were built in early days. These bent caps may be considered structurally deficient when evaluated using the current design criteria and/or have insufficient strength to accommodate an increase in traffic. To this end, FRP strengthening techniques for in-service inverted-T bent caps with local weakness were developed and experimentally validated. The experimental results showed that the FRP retrofits were able to eliminate the local deficiencies of inverted-T bent caps. The retrofit schemes also benefited the bent caps by improving the serviceability, displacement ductility, and ultimate strength. The design approach of the developed FRP retrofit was provided as a guidance. Nonlinear finite element model developed for the punching failure mechanism simulation was further calibrated to capture the remaining local failure mechanisms - ledge and hanger failures. A parametric study was carried out to investigate the impact of four different reinforcement layouts on the local behavior of inverted-T bent caps. The variations were given to the configuration of the ledge reinforcement and the spacing of the ledge and hanger reinforcement around the loading region. The analysis results demonstrated that the reinforcement layouts have a slight impact on the strength response of the bent cap while the impact on the crack control of the section was significant. Use of diagonal ledge bar instead of the horizontal ledge bar or combined use of the two layouts around the loading region was recommended by means of crack control.
