Sep 23, 2023
Sep 23, 2023
India's growing economic might is making many countries look at it with a sense of envy. This newfound economic prosperity has also left India more vulnerable and susceptible to natural calamities. It is imperative that we start following the global best practices and stop living in a sense of denial.
Two countries have learnt their lessons the hard way ï¿½ the US and Japan. The colossal damage caused by the quakes of Northridge in 1994 and Kobe in 1995 forced the two governments to spend millions on research. The research helped to reach a better understanding of how structures perform in devastating scenarios. This led to classification of buildings depending on their performance. Now there exist internationally accepted documents issued by the two governments.
It will come as a surprise that till date there is no definition in this country for what can be dubbed earthquake-resistant. Neither the national disaster management guidelines nor the Indian seismic code has even attempted to address the issue. This has led the misuse of the term.
Internationally, earthquake protection is classified into four categories, each having a detailed definition. The categories are: operational, immediate occupancy, life-safety and collapse prevention.
'Operation' is the highest level and is adopted for important structures like hospitals, administrative centres and essential infrastructure. Operational buildings are seldom more than 5 or 6 storey high.
The second level is 'immediate occupancy'. Buildings designed to these standards sustain minimal structural damage even during a major earthquake. They are safe for occupation and for use immediately after a major earthquake.
'Life-safety' buildings are those that are so designed that their primary aim is to save lives; the building per se will sustain severe structural damage. After the earthquake the kind and level of damage that the columns suffer will determine if the building is repairable or will have to be demolished and rebuilt.
The lowest category 'collapse prevention' buildings are those that will sustain extensive structural damage. The damage will render the building unfit for use and will have to be immediately vacated, demolished and subsequently rebuilt.
In case the builder/developer has followed the Indian seismic building code to the fullest, the building would achieve 'collapse prevention' level of safety. The term 'collapse prevention' is disguised in our country by using alternative words: earthquake resistant.
The detailed definitions of various categories of protection are:
Operational Level: The lowest level of overall damage to the building (highest performance). The structure will retain nearly all of its pre-earthquake strength and stiffness. Expected damage includes minor cracking of facades, partitions and ceilings as well as structural elements. All mechanical, electrical, plumbing and other systems necessary for normal operation of the buildings are expected to be functional, possibly from standby sources.
Negligible damage to non-structural components is expected. Under very low levels of earthquake ground motion, most buildings should be able to meet or exceed this performance level. Typically, however, it will not be economically practical to design for this level of performance under severe levels of ground shaking, except for buildings that house essential services.
Immediate Occupancy Level: Overall damage to the building is light. Damage to the structural systems is similar to the Operational Performance Level; however, somewhat more damage to non-structural systems is expected. Non-structural components such as cladding and ceilings, and mechanical and electrical components remain secured; however, repair and clean up may be needed. It is expected that utilities necessary for normal function of all systems will not be available, although those necessary for life safety systems would be provided.
Many building owners may wish to achieve this level of performance when the building is subjected to moderate levels of earthquake ground motion. In addition, some owners may desire such performance for very important buildings, under severe levels of earthquake ground shaking. This level provides most of the protection obtained under the Operational Building Performance Level, without the associated cost of providing standby utilities and performing rigorous seismic qualification to validate equipment performance.
Life Safety Level: Structural and non-structural damage is significant. The building may lose a substantial amount of its pre-earthquake lateral strength and stiffness, but the gravity-load bearing elements function. Out-of-plane wall failures and tipping of parapets are not expected, but there will be some permanent drift and select elements of the lateral-force resisting system may have substantial cracking, spalling, yielding, and buckling.
Non-structural components are secured, but many architectural, mechanical and electrical systems are damaged. The building may not be safe for continued occupancy until repairs are done. Repair of the structure is feasible but it may not be economically attractive to do so. This performance level is generally the basis for the intent of code compliance.
Collapse Prevention Level or Near Collapse Level: The structure sustains severe damage. The lateral-force resisting system loses most of its pre-earthquake strength and stiffness. Load-bearing columns and walls function, but the building is near collapse. Substantial degradation of structural elements occurs, including extensive cracking and spalling of masonry and concrete elements, and buckling and fracture of steel elements. Infills and unbraced parapets may fail and exits may be blocked. The building has large permanent drifts.
Non-structural components experience substantial damage and may be falling hazards. The building is unsafe for occupancy. Repair and restoration is probably not practically achievable. This building performance level has been selected as the basis for mandatory seismic rehabilitation ordinances enacted by some municipalities, as it results in mitigation of the most severe life-safety hazards at relatively low cost.
(Sandeep Donald Shah is a M.Sc. in Earthquake and Civil Engineering Dynamics from the University of Sheffield, UK.)
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