Seismic design of plane steel MRFS, EBFS and BRBFS by improved direct displacement-based design method

An improved direct displacement-based design (DDBD) method for the seismic design of three types of plane steel frames, i.e., moment-resisting frames (MRFs), steel eccentrically braced frames (EBFs), and buckling-restrained braced frames (BRBFs) is developed. The improvement lies in the fact that it uses a MDOF equivalent system instead of a SDOF one used by the original DDBD method. Thus, higher modes and P-Δ effects are considered more rationally and with higher accuracy than in the conventional method. The proposed method employs deformation-dependent equivalent modal damping ratios and design modal displacements for the first few modes significantly contributing to the response, as functions of target inter-storey drift ratios representing various performance levels. Employment of a displacement design spectrum for high amounts of damping in conjunction with the equivalent modal damping ratios and design modal displacements can provide the required modal periods, the modal stiffness values, and the modal base shear forces. The design base shear force is obtained by a modal combination rule, such as the SRSS rule, and is distributed to the stories of the frame for member dimensioning. Numerical examples illustrate the method and demonstrate its advantages over the original DDBD method. •Seismic design of plane steel MRFs, EBFs and BRBFs is considered.•Improved direct displacement-based design (DDBD) method is proposed.•Improved DDBD method is based on a MDOF equivalent system.•Improved DDBD method takes care of P-D and higher mode effects in a rational way.•Improved DDBD method results in more accurate and economical designs than the original DDBD method.