Development of consistent fish-bone simplified model with energy-based approach for efficient seismic evaluation of irregular steel moment resisting frames

Abstract

This study aims to develop the Consistent Fish-Bone (CFB) simplified model for efficient estimation of Engineering Demand Parameters (EDPs) in irregular steel Moment Resisting Frames (steel-MRFs). To achieve this goal, some modifications based on the energy consistent approach have been applied to the Improved Fish-Bone (IFB) simplified model previously presented for reinforced concrete MRFs. These modifications include: 1) adding truss elements to the IFB model to consider the effect of flexural deformations and determining their areas by balancing the overturning moment and the strain energy due to the axial deformation of columns in the original steel-MRF with the overturning moment and the strain energy of the truss elements in the simplified model and 2) determination of the elastic properties of beams and columns in the simplified model based on the compatibility of the energy absorbed by these elements and the corresponding elements in the original steel-MRF. By applying the mentioned modifications, the proposed CFB model is obtained, which can be used in flexible steel buildings without the limitations of the IFB model. In addition, the proposed CFB model is geometrically similar to the Modified Fish-Bone (MFB) simplified model previously presented for steel-MRF buildings with equal bay lengths. However, the energy-based approach in the CFB model leads to its use in buildings with unequal bay lengths. In order to investigate the accuracy of the proposed simplified model, three 3-, 9-, and 20-story steel-MRFs with irregular bay lengths and three other steel-MRFs with the same number of stories but with vertical geometric irregularity have been used. Then, the efficiency of the proposed simplified model in estimating the static and dynamic behavior of the structure is evaluated using nonlinear static analysis (pushover), Nonlinear Time History Analysis (NLTHA), Incremental Dynamic Analysis (IDA), and seismic fragility analysis. In the seismic evaluation process, 44 far-field and 28 near-field ground motions of FEMA P-695 at different seismic hazard levels are used. The results indicate that the proposed simplified model has high efficiency in low-to high-rise irregular steel-MRFs at all seismic hazard levels up to the collapse stage by properly estimating the EDPs and significantly reducing the Degrees of Freedom (DOFs) of the original steel-MRF. Therefore, this model can be very useful in reducing the computational costs and complexities of the new generation of Performance-Based Earthquake Engineering (PBEE).