This paper presents a new strengthening method for existing frames by using outside sub-structures, namely, precast bolt-connected steel-plate reinforced concrete (PBSPC) frame-braces. The working principles, numerical methods, and design procedures are presented, and case studies are carried out. The simulation results were consistent with the results from previous experiments, indicating the effectiveness of the numerical model. The design objective and procedure were put forward, and during the process, a coefficient η that considers the precast influences was considered. A practical engineering retrofitting was performed based on the proposed numerical model and design process, where the structure was subjected to the design basis earthquake (DBE) and maximum considered earthquake (MCE). The analyses demonstrated that the structural demands were clearly reduced within the thresholds, and the inner force was transferred from the existing building to the external substructure after strengthening. The incremental dynamic analysis (IDA) and fragility curves were plotted, illustrating the greater reliability in structural capacity and the lesser possibility of structural damage with the new upgrading form.

Introduction

Surveys of historical earthquakes have shown that old buildings without seismic upgrades are prone to severe damage during the hazards, and the collapse probability is extremely high, especially in highintensity areas. In contrast, slight damage and lower collapse probability have been observed for structures after retrofitting, greatly reducing the property loss and casualties caused by strong earthquakes. In view of the large amounts of old structures in rural areas and the frequent occurrences of earthquake disasters in recent years, the importance of seismic performance upgrading as well as seismic capacity improvement has been put forward to a new height [1–۳]. The existing upgrading techniques tend to focus more on component-level performance enhancements (e.g., wrapping with fibre-reinforced polymer (FRP) [4], enlarging section areas [5], bonding steel plate externally [6], replacing concrete and reinforcement [7]), which have played a role in improving the structural stiffness and strength and have been validated by practical projects. However, these retrofitting methods generally cause individual components to be stronger, and the integrity improvement of the whole structure (e.g. the overall force/ displacement pattern change) is not obvious, especially for the structures considering the soft storey effect [8,9].