Repair versus retrofit; configuration factors

Repair versus retrofit

Configuration factors for seismic safety

1. Simple, symmetrical plan

Square or rectangular plans resist earthquakes best. Irregular shapes (L, T, U, H) twist and concentrate stress at re-entrant corners; where such shapes are needed, separate them into rectangular blocks with seismic joints.

2. Plan aspect ratio

Length : width should not exceed about 3. Long buildings receive different ground motion at the two ends and develop large diaphragm stresses.

3. Symmetry of mass and stiffness

The centre of mass and the centre of rigidity should coincide to avoid torsion. Place stiff walls and cores symmetrically.

4. Regular elevation and avoiding a soft storey

Avoid sudden changes in stiffness, mass or strength along the height (open ground storey, set-backs, heavy top floor). Walls and columns should be continuous from foundation to roof.

5. Light weight and low height

Earthquake force is proportional to mass, so use a light roof and floors, keep the centre of gravity low, and limit the number of storeys.

6. Openings in walls

Openings should be small and centrally placed, away from corners: total openings not more than about 50% of the wall length in a storey, and not closer than about 0.5 m (or one-fourth of the wall height) to a corner.

7. Seismic gap

Adjacent buildings should be separated by a gap (about 15-30 mm per storey as per IS 4326, depending on the structural system, and more in high seismic zones) to avoid pounding.

8. Strong foundation and continuous load path

Use the same foundation type and depth throughout, tie the structure with a plinth band or tie beams, and provide a rigid diaphragm to transfer loads to the walls.