In the design of lateral load-resisting systems, Soft Storey irregularity occurs when
- AThe lateral stiffness of a storey is less than 70% of that in the storey above.Correct
- BThe floor height is very small compared to the storey above.
- CThe building is painted with soft-texture colours.
- DThere is an excess of partition walls in a particular floor.
Solution
Concept
Soft Storey Irregularity
Formula
N/A
Step-by-step solution
1. Define the criteria for Soft Storey irregularity in seismic design. 2. Recall that a soft storey occurs when the lateral stiffness of any storey is less than 70% of the storey above it. 3. Select Option A as the correct choice.
Explanation
A soft storey irregularity happens when one floor of a building is significantly weaker or less rigid than the floor directly above it. Specifically, if a floor's ability to resist sideways forces like earthquakes is less than 70% of the floor above it, it is classified as a soft storey.
Shortcut
To remember Soft Storey Irregularity, use the rule "Soft = Stiffness < 70%": - Soft Storey: Lateral Stiffness is less than 70% of the storey above (or \< 80% of average of 3 storeys above). - Weak Storey: Lateral Strength is less than 80% of the storey above.
Common mistake
A common mistake is confusing a soft storey (which relates to lateral stiffness being less than 70% of the storey above) with a weak storey (which relates to lateral strength) or incorrectly assuming that soft storey refers to physical features like smaller floor heights or soft architectural finishes. Students also frequently assume that adding partition walls creates a soft storey, whereas infill walls actually increase storey stiffness.
Correct answer: A) The lateral stiffness of a storey is less than 70% of that in the storey above.