Engineers explain the earthquake

March 7, 2011

The Institution of Professional Engineers of New Zealand has a fact sheet on the Christchurch earthquake.

Among the observations are:

. . . Earthquake records show that some buildings may have experienced shaking more than two times more intense than a new building would be currently designed for, but perhaps for a lesser duration than envisaged by the loadings code (NZS 1170.5). The intensity of shaking appears to have died out rapidly as it travelled westwards from the fault. . .

. . . Non-residential buildings designed before 1976 were not explicitly required to have ductility incorporated in them. In the early 1980s, the design standard for reinforced concrete was revised significantly to ensure nonbrittle behaviour under design-level earthquake loadings, and the strong-columns/weak beams philosophy was introduced so that life safety could be achieved under design-level earthquake shaking. . .

. . . Buildings are not designed to be earthquake-proof. Two design levels are considered. A building of ordinary importance is designed for a level of shaking that has a 10 % probability of being exceeded in its design life of  50 years. The design standards are formulated to ensure that life safety is achieved during that shaking, but the building might be an economic write-off because of the damage. It must not collapse at this level. . .

. . . This Serviceability Level is set to correspond with shaking that has a 10% probability of being exceeded in one year. To put it another way, The Life Safety design level can be expected to be exceeded on average (over a very long period of time) once every 500 years, and the Serviceability Level once every 20 years.

Note that no mention has been made of earthquake (Richter) Magnitude, as the building responds the same way to shaking that comes from a small close earthquake or a large distant one. . .

. . .  The two buildings which have catastrophically collapsed (the Pyne Gould Corproration and CTV buildings), while described by the press as modern, are understood to have been constructed in about 1963 and 1986 respectively. Many buildings designed before the early 1980s may have experienced earthquake loads significantly above that for which they were designed. Nevertheless, many of them have experienced no or minimal structural damage. A number of experienced structural engineers have observed that buildings with well-conceived and simple structural systems with minimal irregularities have exhibited superior performance to those which may have only nominally or theoretically met codified requirements. . .

. . . The propensity for buried services to be disrupted and uplifted by the buoyancy of the liquefied material is well-known from the experiences of other earthquakes around the world, but the scale of the damage experienced in Christchurch may be the greatest ever recorded anywhere in a modern city. . .


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