How is a weak storey defined in terms of strength?

A weak storey is defined primarily in the context of structural engineering and seismic design as a level in a building that has significantly less strength than the storey above it. The defining characteristic of a weak storey is that its strength is less than 80% of the strength of the storey directly above it. This disparity can lead to potential failure during seismic events because the weak storey cannot effectively resist lateral forces that may be applied to the building. Thus, understanding this definition is crucial for engineers and architects to ensure structural integrity and safety in multi-storey buildings.

The other choices do not accurately capture the essence of a weak storey. For instance, stating that strength exceeds a certain percentage of the storey above contradicts the fundamental definition of a weak storey. Similarly, defining strength as equal to the room's effective mass or exceeding the total weight of seismic-resisting elements does not relate to the comparative strength needed to qualify a storey as weak in seismic design. These factors may relate to other aspects of structural behavior but do not specifically identify a weak storey situation.