4a. Moment Ratios

Conditions for Strong-Column and Weak-Beam (SC/WB)

For the highly ductile steel frames or reinforced concrete structural system with shearwalls and steel frames, the beam-column connections must satisfy the condition of the Strong-Column and Weak-Beam (SC/WB) approach depending on the earthquake direction. In beam-to-column connections, the following relationship must be fulfilled:

where ΣM_pc^* is the sum of the nominal flexural strength of the columns above and below the joint to the beam centerline with a reduction for the axial force in the column. The determination of ΣM_pc^* from the Eq (E3-2) is permitted. When the centerlines of beams in opposite direction in the same joint do not coincide, the mid-line between centerlines should be used. For the ΣM_pb^*, the sum of the expected flexural strengths of the beams at the plastic hinge locations to the column centerline. The determination of ΣM_pb^* from the Eq (E3-3) is permitted.

The nominal compressive strength, P_c, shall be determined as follows:

and the required axial strength is P_rc = P_uc (LRFD) or P_rc = Pac (ASD), as applicable.

In the calculation of column flexural strength capacities, the required compressive force P_ac (ASD) or P_uc (LRFD) are obtained from the load combinations, which makes these bending moment capacities the lowest in accordance with the direction of the earthquake.