Severity of ground shaking at a given location during an earthquake can be minor, moderate and strong. Thus relatively speaking, minor shaking occurs frequently; moderate shaking occasionally and strong shaking rarely. For instance, on average annually about 800 earthquakes of magnitude 5.0-5.9 occur in the world while about 18 for magnitude range 7.0-7.9. So we should 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, even though the life of the building may be 50 or 100 years?
Engineers do not attempt to make earthquake proof buildings that will not get damaged even during the rare but strong earthquake; such buildings will be too robust and also too expensive. Instead the engineering intention is to make buildings earthquake-resistant; such buildings resist the effects of ground shaking, although they may get damaged severely but would not collapse during the strong earthquake. Thus, safety of people and contents is assured in earthquake-resistant buildings, and thereby a disaster is avoided. This is a major objective of seismic design codes throughout the world.
- Under minor but frequent shaking, the main members of the buildings that carry vertical and horizontal forces should not be damaged; however buildings parts that do not carry load may sustain repairable damage.
- Under moderate but occasional shaking, the main members may sustain repairable damage, while the other parts that do not carry load may sustain repairable damage.
- Under strong but rare shaking, the main members may sustain severe damage, but the building should not collapse.
Earthquake resistant design is therefore concerned about ensuring that the damages in buildings during earthquakes are of acceptable variety, and also that they occur at the right places and in right amounts. This approach of earthquake resistant design is much like the use of electrical fuses in houses: to protect the entire electrical wiring and appliances in the house, you sacrifice some small parts of electrical circuit, called fuses; these fuses are easily replaced after the electrical over-current. Likewise to save the building from collapsing you need to allow some pre-determined parts to undergo the acceptable type and level of damage.
Earthquake resistant buildings, particularly their main elements, need to be built with ductility in them. Such buildings have the ability to sway back-and-forth during an earthquake, and to withstand the earthquake effects with some damage, but without collapse.