Abstract
The eccentric bracing system equipped with vertical links is capable of providing high levels of stiffness, strength and ductility, and therefore, can be efficiently used for seismic retrofit of existing structures. This study aims to investigate the seismic reliability of steel moment-resisting frames retrofitted by this system using a novel combined series–parallel system approach. The seismic response of 4, 8 and 12-storey steel moment-resisting frames (MRFs) are evaluated under a set of design basis earthquakes (DBE) before and after retrofitting intervention. Adopting an engineering demand parameter approach (EDP-Based) for reliability assessment and development of analytical models for the frames using systems consisting of the series–parallel elements are the major distinctions between the present study and the other similar works. To estimate the global reliability of the frames, first, the reliability of each storey is individually derived based on various probable damage levels for the lateral-load resisting members. Then, the seismic reliability of the frame is globally obtained by combining the reliability of each storey for different damage levels in the lateral load-resisting subsystems. The results indicate significant impact of this type of bracing system on improvement of the performance level and load-carrying capacity of the frames along with reduction of the lateral displacements. It is shown that application of the vertical links can reduce the maximum inter-storey drifts by at least 60%, while it leads to at most 17% increase in the base shear. All retrofitted frames exhibited a performance level higher than the Life Safety (LS) when subjected to the DBE hazard level records (earthquakes with return period of 475 years). At the same level of earthquake intensity, in the cases when the drift corresponding to the LS performance level is used as the target, the reliability of the retrofitted frames was improved by more than 90% compared to the original frames for all damage states developed in the vertical links.
Highlights
Substandard structures with deficient lateral load-carrying systems may exhibit large nonlinear displacements and extensive damage during strong earthquake events
This study presented the detailed design process of the bracing system equipped with vertical links and investigated the impact of this system on the seismic performance of the existing steel moment-resisting frames (MRFs)
Based on the results obtained from a comprehensive reliability assessment analysis on 4, 8 and 12-storey frames, it was concluded that the proposed system can efficiently improve the seismic performance of the weak MRFs under the target hazard level
Summary
Substandard structures with deficient lateral load-carrying systems may exhibit large nonlinear displacements and extensive damage during strong earthquake events. Given the fact that vertical links can act as a seismic fuse, the majority of the earthquake energy is absorbed by these elements, and the other structural members generally remain in their linear phase of behaviour (Zahrai and Mahroozadeh 2010; Mohsenian and Mortezaei 2018a; Mohsenian and Nikkhoo 2019) This implies that, unlike conventional EBF systems, the beam elements and slabs are not affected by the link rotation. Mohsenian et al (2021) showed the efficiency of these structural systems under sequential earthquakes In another relevant study, Mohsenian et al 2020a demonstrated that the seismic performance of low to medium rise steel moment resisting frames can be considerably improved by using vertical link elements. The results highlight the capabilities of this bracing system to reduce the displacement demands and promote the seismic reliability of MRFs under different seismic hazard levels
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