502 dead. A flat-slab roof punched through its supporting columns after a rooftop AC unit was relocated and the columns were undersized for the revised loads.
During construction the original architect was replaced after refusing structural changes. The fifth-floor slab was changed from a beam-supported design to a flat slab to save cost. Column diameters were reduced from 800 mm to 600 mm without re-running shear checks. When 30-tonne air-conditioning units were dragged across the roof to relocate them, the punching shear capacity was exceeded and the slab collapsed within seconds.
Two-way (punching) shear demand at the column-slab interface exceeded the reduced column's capacity by ~40%. The smaller columns provided a smaller critical perimeter b₀, reducing punching capacity proportionally. No recalculation was performed when columns were resized.
Two-way (punching) shear at column-slab interface — flagged the moment the column diameter is reduced.
Open Foundation Design