Soft storey and other weak points

Earthquakes find the weakest link. Surveys after every major earthquake show the same patterns of collapse, and most come from features that could have been avoided at the planning stage. The best-known is the soft storey: an open ground floor, used for parking or shops, with walls above. The upper storeys, stiffened by their infill walls, move as a block, and almost all the sway concentrates in the bare columns of the open storey, which then fail.

A weak storey is related but different: a storey whose strength, rather than stiffness, is much lower than the one above. Other vertical irregularities include a sudden setback, a heavy mass such as a rooftop tank or swimming pool, and columns or walls that stop at one level and are carried on beams, called floating columns or discontinuous walls. Each one interrupts the smooth path of inertia forces down to the foundation.

Plan irregularities cause torsion. If stiff walls or cores sit on one side of a building, its centre of stiffness is far from its centre of mass, and the building twists as it sways, overloading the columns on the flexible side. Re-entrant corners in L, T or U shapes concentrate damage at the junction. Short columns, created by partial-height walls or mezzanine beams, attract very high shear because stiffness grows with the cube of the shortened length.

IS 1893 (Part 1) defines these irregularities and restricts or penalises them; some are not permitted in higher zones. The remedies are simple and cheap if adopted early: continuous walls or bracing from foundation to roof, symmetric layouts, separation joints between wings, and no floating columns.