Which of the following best defines a soft storey?

A soft storey refers to a level in a building that has significantly lower stiffness than the levels above it, which can compromise the structural stability during an earthquake or substantial wind forces. Specifically, the definition highlights that a soft storey has less than 70% of the stiffness compared to the storey above. This reduced stiffness can lead to increased lateral movement and deformation, making the building more vulnerable to damage during seismic events.

In the context of the other options, the percentage thresholds related to strength and uniformity do not accurately reflect the characteristics that define a soft storey. The focus is not on strength alone (as mentioned in the first and fourth choices) but rather on comparative stiffness, which is the critical factor in assessing a soft storey condition. The average stiffness referenced in the third choice is irrelevant in defining a soft storey, as the concept is based on the relationship between the stiffness of a particular storey and that of the storey directly above it.