Seismic design methodology for self-centering reinforced concrete frames

This paper presents a new design approach for self-centering reinforced concrete frames (SRCFs). SRCF is a seismic resilient structure characterized by minimal structural damage and little residual deformation under seismic excitations. This remarkable seismic performance is achieved by specially designed self-centering beam-column joints and column-base joints. These joints connect beams and columns by clamping force provided by unbonded post-tensioning steel. By this means, when the lateral load exceeds serviceability level (i.e. gravity load, frequent earthquake and wind load), the gaps at self-centering joints are allowed to open and the columns are allowed to uplift at the base. The opening and uplifting behavior significantly mitigates the damage in "plastic hinge area", where conventional concrete frame intends to sacrifice in exchange for ductility and energy dissipating capacity. After unloading all the gaps between components will close under clamping force, and the components thus restore their original position with negligible residual deformation. This state change of opening and close makes the conventional design methodology no longer applicable for SRCFs. Despite its extraordinary seismic performance, the absence of applicable design method hinders the application of SRCFs. In this paper, key configurations of SRCFs are summarized based on existing research, and load capacities of different limit states are analyzed. A two-phase design approach is presented based on these load capacities. In the first phase, the elastic performance of the structure is designed to accommodate the gravity load and meet the displacement requirement under frequent earthquakes. The detailed design of post-tensioning steel, damping devices and the reinforcement of the components are determined in the second phase to achieve the performance target under strong earthquakes. Design examples are given to illustrate the design approach. Time history analyses of design examples are conducted to verify the validity of this approach. Analysis result indicates that this approach can be used to design structures to achieve predefined performance targets with reasonable conservative.