Hybrid systems to enhance the seismic performance of steel soft-story structures

Recent global seismic events have shown that most destroyed structures were irregularly designed, especially those with a soft story. A “soft story” refers to a building floor significantly weaker or more flexible than the others, often because of large openings such as windows or parking spaces. This reduction in stiffness creates a vulnerable point where excessive deformation can occur during an earthquake, leading to potential structural failure. This study proposes a practical hybrid connection strategy that combines fully rigid and semi-rigid joints to address this case and mitigate the failure mechanism affecting the overall seismic response of steel structures. The objective is to smooth stiffness discontinuities and improve earthquake performance in a 12-story steel moment-resisting frame with a mid-height soft story. Semi-rigid joints are modeled as zero-length rotational springs with a fixity factor (α), applied above the soft story in two layout configurations. Compared to the fully rigid frame, introducing semi-rigid joints increases the fundamental period (by ≈10% for α = 0.5 and up to ≈60% for α = 0.1 in Case 1; more moderate increases of ≈5% and ≈12% in Case 2), thereby shifting seismic demand toward lower spectral accelerations. Intermediate fixity (α ≈ 0.5–0.75) offers a balanced performance: roof displacements and inter-story drifts remain controlled while the soft-story stiffness criterion is satisfied. Overall, strategically placing semi-rigid connections above the soft story reduces drift concentration, smooths the stiffness profiles, and enhances seismic resilience without compromising global stability. These findings support hybrid connection detailing as a cost-effective and practical strategy for improving the seismic behavior of steel structures.