What is referred to as a condition where the lateral stiffness is less than 70 percent of the stiffness of the story above?

A condition where the lateral stiffness is less than 70 percent of the stiffness of the story above is known as a soft story. In structural engineering, a soft story is characterized by a significant reduction in lateral load resistance when compared to the floors above it. This can lead to poor performance during seismic events, as the building may experience excessive movement or even failure due to the insufficient lateral stiffness.

Identifying a soft story is crucial for earthquake-resistant design because the reduced stiffness can make a building vulnerable to overturning or collapse under lateral loads, such as those caused by winds or seismic activity. In contrast, the other terms like story drift, toughness, and elasticity refer to different structural characteristics and behaviors that do not specifically describe the condition of lateral stiffness relative to the structure above. Story drift pertains to the lateral displacement of a building’s stories, toughness refers to the ability of a material to absorb energy, and elasticity relates to the ability of a material to return to its original shape after being deformed.