Behavior of circular tubed-RC column to RC beam connections under axial compression

The circular tubed-reinforced concrete (TRC) column is a kind of special concrete-filled steel tube (CFST) columns, in which the outer thin-walled steel tube does not pass through the beam-column joint and thus can avoid the direct transfer of an axial load and maximize the confinement effect from the steel tube. Although the columns possess high load-carrying capacities and good ductility performance in seismic zones, there is a possible decrease in the axial bearing capacity of the TRC column to RC beam connections due to the discontinuity of the column tube, which is a particular concern to engineers. To compensate for the discontinuity of the column tube, strengthening stirrups, a tube with rectangular openings, and horizontal haunches are adopted in the connection zone as Type A, Type B, and Type C connections, respectively. Nine connections aforementioned and four reference circular TRC columns were tested under axial compression. The experimental results show that Type B and Type C connections are effectively strengthened, while Type A connection needs further strengthening because of the lower axial bearing capacity than the reference columns. A finite element (FE) model was developed to simulate the behavior of connections under axial compression. The predicted load-stain curves are in good agreements with the measured ones. A theoretical model for predicting the axial bearing capacities of the connections which is based on the confined concrete theory and local compression theory is proposed.