Which condition indicates a stiffness irregularity related to a soft storey?

A stiffness irregularity related to a soft storey is identified when the lateral stiffness of that storey is less than a defined threshold compared to the storey above it. Specifically, the correct condition indicates that the lateral stiffness of the soft storey is less than 70% of the stiffness of the storey directly above it. This situation commonly arises in buildings where the ground or lower floors are designed to be more flexible, such as in cases where there are large openings for parking or other reasons, leading to potential vulnerabilities during lateral loading events like earthquakes.

In contrast, a stiffness that exceeds 70% would imply that the storey in question is adequately stiff relative to the storey above, indicating a more uniform distribution of lateral stiffness. A uniform stiffness throughout would suggest that there are no irregularities present, and the building is expected to perform better under lateral loads. Lastly, a scenario where the storey strength exceeds average stiffness does not directly correlate with the stiff or soft storey condition but instead focuses on strength versus stiffness, which is a different aspect of structural analysis. Thus, identifying a stiffness irregularity necessitates looking for the criterion where the lower storey's stiffness is significantly less than that of the upper storey.