During earthquake the behavior of a building is mainly governed by its overall shape, size and geometry. Also, the way how earthquake forces are carried to the ground affects its behaviour. So it is the prime duty of structural engineers and architects to ensure that the features chosen for the building are favorable to resist earthquake.
Severity of ground shaking during an earthquake can be minor, moderate or strong. Relatively speaking, minor earthquakes occur frequently, moderate shaking occasionally and strong shaking rarely. For example: On an average, 800 earthquakes of magnitude 5.0-5.9 occur annually in the world while only about 18 of magnitude range 7.0-7.9 occur annually.
So, the main question which arises is “Should we design and construct a building to resist that rare earthquake shaking that may come only once in 500 years or even once in 2000 years at the chosen project site, even though the life of the building itself may be only 50 to 100 years? Or should we do away with the designs of buildings for earthquake effects?” The later approach may lead to disaster while the former approach will be very expensive. So the design philosophy should lie somewhere in between these two extremes.
Earthquake Design Philosophy
- Under minor but frequent shaking, the main members of the building that carry vertical and horizontal forces should not be damaged. However, building parts that do not carry load may sustain repairable damage.
- Under moderate but occasional shaking, the main members may sustain repairable damage. However, other parts of the building may be damaged such that they may even have to be replaced after the earthquake.
- Under strong but rare shaking, the main members may sustain severe (even irreparable) damage, but the building should not collapse. An earthquake-resistant building should have four major qualities in it. These are:
- Good Structural Configuration: Its size, shape and structural system carrying loads should be such that they have a smooth and direct flow of inertia forces to the ground.
- Lateral Stiffness: The maximum lateral (horizontal) force that it can resist should not result in the collapse of the building.
- Adequate Stiffness: Its lateral load resisting system is such that the earthquake-induced deformations in it do not damage its contents under low moderate shaking.
- Good Ductility: It should be capable to go through deformations under severe earthquake shaking even after yielding, is improved by favorable design and detailing strategies.