Accounting for Ground‐Motion Spectral Shape in Displacement‐Based Design

ABSTRACT In the direct displacement‐based design (DDBD), a structure is designed to meet a target displacement under a given earthquake intensity. This feature makes it possible to control the amount of structural and nonstructural damage, but the conventional DDBD method displays a strong bias related to structural period. This study demonstrates that this bias is highly correlated with a new measure of the shape of the ground‐motion response spectrum, which can be incorporated in the design procedure to improve its accuracy. Three typical hysteretic systems, representing a reinforced concrete frame, a steel frame, and an elastic‐perfectly plastic system, are adopted to calibrate a displacement‐correction equation. These systems were subjected to numerous nonlinear history analyses using both far‐field and near‐field ground motions. An improved DDBD method was then proposed by implementing the correction equation to estimate the equivalent damping ratio in the design procedure, and its effectiveness was evaluated using a separate ground‐motion set consisting of 208 records. The accuracies of the conventional and proposed DDBD methods were further evaluated with a seven‐story, steel, moment‐resisting frame for three types of design spectra, that is, a two‐period spectrum, a multi‐period spectrum, and a median M9 spectrum generated by a physics‐based model. For each design spectrum, 60 ground motions were selected to perform nonlinear time history analysis. The application of the proposed method demonstrates that it can be implemented with only a small amount of extra effort compared to conventional DDBD. The results of nonlinear time history analyses show that the proposed DDBD method improves the accuracy of the design methodology, both for single‐ and multi‐degree of freedom systems.