Column-to-beam flexural strength ratio for performance-based design of RC moment frames

The concept of strong column – weak beam is frequently used in seismic design to secure the safety of framed buildings, and for RC special moment frames, current design codes require the minimum column-to-beam flexural strength ratio at joints to reduce the likelihood of yielding in columns. However, the requirement for the strength ratio may not be satisfactory to prevent column hinging due to inelastic moment distributions and higher mode effects of dynamic response. In the present study, to investigate the required strength ratio, a numerical study was performed considering various parameters (plastic rotations of beams and columns, design earthquake loads and vertical distributions, gravity loads, stiffness ratios, design methods for beam strength, joint locations, and frame heights). The numerical study results showed that the strength ratio varies with the parameters and its vertical distribution is generally in a wine-glass shape along the frame height. On the basis of the numerical investigation, a design method for the strength ratio was developed as a function of the height ratio for performance-based design. For verification, nonlinear static and dynamic analyses were performed for regular and irregular frames designed by the developed method, and the results showed that the use of the developed method can improve seismic performance in a more economical way.