Seismic Upgrade of Flat-Plate Slab-Column Connections using Carbon Fiber Reinforced Polymer Stirrups

Flat-plate concrete structures provide an economical structural system that is unique to concrete. However, the slab-column connections can exhibit brittle behavior under lateral displacement reversals imposed by strong ground motions. Brittle behavior is exacerbated when the gravity shear load is greater than or equal to 40% of the connection shear capacity. The addition of carbon fiber reinforced polymer (CFRP) stirrups positioned radially around the slab-column interface is an effective way of increasing the drift capacity and ductility of the connection.

Tests have been conducted at the University of Texas at Austin studying the behavior of flat-plate slab-column connections. Test specimens were square having 2.44 m lengths, 114 mm thickness, and a 305 mm square column. Connections were upgraded by stitching CFRP stirrups in two different patterns around the slab-column interface through cored holes in the slab thickness; a novel technique developed during the course of the research summarized herein. Any remaining voids were filled with two-part epoxy. Specimens were tested under combined gravity and lateral loads. A hydraulic ram and load maintainer were used to simulate gravity load conditions. Lateral displacement excursions were imposed using a hydraulic actuator.

During the tests, flexural cracks were observed in a patterns around the slab-column interface, on each specimen. Punching shear failure initiated in the control specimens at lateral drift-ratios of 2.3% and 2.4%. Upgraded specimens had significant flexural yielding at lateral drifts of 8%, and did not fail in shear. In some specimens, the lateral load displacement ductility factor increased up to 200% and the joint rotation ductility factor increased up to 350%, when the slab-column connection was upgraded. The energy dissipated in CFRP upgraded slab-column connections were more than 10 times greater than that dissipated in the control specimen. It was concluded that the innovative CFRP stirrup upgrade technique of slab-column connections, developed during the course of the study, was structurally effective.