Novel design equation on minimum column depth for interior beam-column joints in special moment frames with large-diameter high-strength rebar

Recently, large-diameter high-strength rebar have been used for the longitudinal reinforcements of beams that penetrate interior beam-column joints, to improve in-situ constructability. The seismic performance of reinforced concrete (RC) interior beam-column joints is decreased by the yield of the rebar, bond failure, and rebar slip. Existing standards of RC special moment frames (SMFs) limit the minimum column depth to control the bond stress of the longitudinal reinforcements penetrating the interior beam-column joint. However, the standard criteria, which is an empirical equation derived in the 1980s based on experiments using mild steel with small-diameter, do not accurately represent the minimum column depth for joints with large-diameter high-strength rebar. This study proposes a new design equation basis on the analytical formulation that determines the minimum column depth of the interior beam-column joint in the SMF with large-diameter high-strength rebar. The proposed design equation considered the effect of large-diameter high-strength rebar on the design of the minimum column depth by formulating the bond strength with respect to the diameter and strength of the rebar from the moment equilibrium condition based on the diagonal strut action. The proposed design equation was verified based on a database of 48 interior beam-column joints and compared with the existing standard criteria. The comparative analysis results confirmed that the proposed design equation presents the minimum column depth for the interior beam-column joint using large-diameter high-strength rebar more accurately than the existing standard criteria. In addition, the proposed equation is expected to address the design feasibilities required to secure the structural safety of the interior beam-column joint in the SMF using large-diameter high-strength rebar and the seismic performance of the structure. •Present the inadequacy of existing design criteria for the minimum column depth using large-diameter high-strength rebar.•Establish a diagonal strut model for interior joints to consider bond mechanism between longitudinal rebar and concrete.•Develop a new design equation on minimum column depth by reflecting diagonal strut actions in interior beam-column joints.•Verify the developed design equation with a comprehensive database.•Suggest applicable limits for diameter and yield strength of longitudinal reinforcements in the new design equation.