Seismic behavior, strength and retrofit of exterior RC column -to -steel beam connections

Hybrid systems consisting of the combination of steel and reinforced concrete elements have been used at an increasing rate during the last thirty years. One of the most efficient types of mixed structures is represented by frame structures made of steel beams and reinforced concrete columns, known as RCS composite systems. However, the use of RCS frame structures has been limited to non- or low-seismic risk zones due to the lack of design provisions for RCS beam-column connections subjected to severe earthquake loading.;A comprehensive experimental and analytical program was undertaken at the University of Michigan to develop new information on the inelastic cyclic response of RCS connections, especially exterior joints. The lateral load testing of nine exterior RCS beam-column subassemblies is reported in this research study. The specimens were constructed in an approximately 3/4-scale and subjected to cyclic displacements larger than those expected during a major earthquake. The primary joint details investigated were: two-part U-shaped stirrups passing through holes in the steel beam web, steel band plates or cover plates surrounding the joint region, steel fiber concrete or engineered cementitious composite (ECC) material in the connection, and bearing bars attached to the beam flanges. All the test specimens showed good overall inelastic cyclic response. They were able to maintain their strength throughout the tests with a good stiffness retention capacity.;The retrofit of two composite joints severely damaged during the original tests is also reported. The wrapping of the column regions just above and below the steel beam with carbon fiber bands effectively restored the strength and stiffness of the joints. An analytical model was developed to predict the shear strength, and stirrup and concrete strains in RCS connections at any level of joint shear deformation. A design model was also proposed for estimating the ultimate shear strength of the connections. In addition, seismic damage indexes were determined for the test specimens and calibration values are given to evaluate the level of damage that a composite connection will sustain during a seismic event.