Earthquake design philosophy, strength and ductility

Philosophy of design in earthquake-prone regions

It is uneconomical to design buildings to remain elastic in a severe earthquake. The accepted philosophy (IS 1893, NBC 105) is:

• Minor, frequent shaking: the structure should suffer no damage (stays elastic).
• Moderate shaking: structural damage is limited and repairable; some non-structural damage is acceptable.
• Severe, rare shaking (design earthquake): the structure may be heavily damaged but should not collapse, so life is protected.

This is achieved by combining adequate strength, stiffness and, above all, ductility.

Design for strength

The structure is designed for the design lateral force instead of the full elastic force:

$$A_h = \frac{Z}{2}\cdot\frac{I}{R}\cdot\frac{S_a}{g}, \qquad V_B = A_h W$$

where $Z$ is the zone factor, $I$ the importance factor, $R$ the response reduction factor and $S_a/g$ the spectral acceleration. The base shear is distributed over the height ($Q_i = V_B\,W_i h_i^2/\sum W_j h_j^2$) and members are designed with load combinations such as $1.2(DL+LL\pm EL)$, $1.5(DL\pm EL)$ and $0.9DL \pm 1.5EL$.

Design for ductility

Because $R > 1$ allows lower strength, the structure must be able to yield and deform inelastically in a controlled manner. This is ensured by:

• under-reinforced sections and limited steel;
• strong column - weak beam, with hinges forming in beams;
• capacity design for shear (shear capacity greater than flexural);
• confined hoops (closely spaced, $135^\circ$ hooks) at beam and column ends and joints;
• proper anchorage and splice locations (IS 13920);
• regular configuration with no soft storey or short column.