Glossary
Soft Storey
Plain-Language Definition
A building storey that is significantly less stiff or weaker than the storeys above it, typically the ground floor with open parking, large openings, or fewer walls. Soft storeys collapse first during earthquakes.
Technical Context
A soft storey is defined as having lateral stiffness less than 70% of the storey above, or less than 80% of the average of the three storeys above (ASCE 7, BNBC). Common causes: ground floor open parking (no infill walls), large shop-front openings, transfer beams where structural system changes. BNBC 2020 requires special analysis and detailing for buildings with soft storey irregularity.
Why It Matters
Soft storey collapse is the single most common cause of building failure in earthquakes. The ground floor buckles while upper floors remain intact — the building "pancakes." Many Dhaka buildings have open ground floor parking with no structural compensation for soft storey effect. This is a critical structural vulnerability.
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