INDIAN STANDARDS ON EARTHQUAKE ENGINEERING
Bureau of Indian standards, the National Standard Body of India, is a Statutory Organization under the Bureau of Indian Standards Act 1986. One of the activity is formulation of Indian Standards on different subjects of Engineering through various Division Councils. The Civil Engineering Division Council is responsible for standardization in the field of Civil Engineering including Structural Engineering, Building materials and components, Planning Design, Construction and Maintenance of Civil Engineering Structures, Construction Practices, Safety in Building etc. These standards are evolved based on concensus principle through a net work of technical committee comprising representatives from Research and Development Organizations, Consumers, Industry, Testing Labs and Govt. Organizations etc.
The Civil Engineering Division Council is working towards to achieve the above goal through 35 Sectional Committees covering wide range of subjects and one of the Sectional Committee is Earthquake Engineering Sectional Committee, CED 39.
India is one of the most disaster prone countries, vulnerable to almost all natural and man made disasters. About 85% area is vulnerable to one or multiple disasters and about 57% area is in high seismic zone including the capital of the country. Disaster prevention involves engineering intervention in buildings and structures to make them strong enough to withstand the impact of natural hazard or to impose restrictions on land use so that the exposure of the society to the hazard situation is avoided or minimized.
Himalayan-Nagalushai region, Indo-Gangetic plain, Western India and Cutch and Kathiawar regions are geologically unstable parts of the country and some devastating earthquakes of the world have occurred there. A major part of peninsular India has also been visited by strong earthquakes, but these were relatively few in number and had considerably lesser intensity. It has been a long felt need to rationalize the earthquake resistant design and construction of structures taking into account seismic data from studies of these earthquakes. It is to serve this purpose, Bureau of Indian Standards has rendered invaluable services by producing a number of national standards in the field of Design and Construction of Earthquake Resistant Structures and also in the field of measurement and tests connected therewith. A detail of Indian Standards in the area of mitigation of natural hazard of earthquake is given underneath.
IS 1893:1984 Criteria for Earthquake Resistant Design of Structures
This standard deals with earthquake resistant design of structures and is applicable to buildings; elevated structures; bridges; dams etc. It also gives a map which divides the country into five seismic zones based on the seismic intensity.
IS 1893 was initially published in 1962 as `Recommendations for Earthquake Resistant Design of Structures’ and then revised in 1966. As a result of additional seismic data collected in India and further knowledge and experience gained the standard was revised in 1970, 1975 and then in 1984.
Consequent to the publication of this standard on account of earthquakes in various parts of the country including that in Uttar-Kashi, Latur and Bhuj and technological advancement in the field, the Sectional Committee decided to revise the standard into five parts which deals with different types of structures:
Part 1 : General provisions and Buildings
Part 2 : Liquid Retaining Tanks – Elevated and Ground
Part 3 : Bridges and Retaining Walls
Part 4 : Industrial Structures Including Stack Like Structures
Part 5 : Dams and Embankments
IS 1893(Part 1):2002 `Criteria for Earthquake Resistant Design of Structures : Part 1 General provisions and Buildings’
This standard contains provisions that are general in nature and applicable to all structures. Also, it contains provisions that are specific to buildings only. It covers general principles and design criteria, combinations, design spectrum, main attributes of buildings, dynamic analysis, apart from seismic zoning map and seismic coefficients of important towns, map showing epicenters, map showing tectonic features and lithological map of India.
Following are the major and important modifications made in this revision:
a) The seismic zone map is revised with only four zones, instead of five. Erstwhile Zone I has been merged to Zone II and hence Zone I does not appear in the new zoning; only Zones II, III, IV and V do. The killari area has been included in Zone III and necessary modifications made, keeping in view the probabilistic Hazard Evaluation. The Bellary isolated zone has been removed. The parts of eastern coast area have shown similar hazard to that of the killari area, the level of Zone II has been enhanced to Zone III and connected with Zone III of Godawari Graben area.
b) b) This revision adopts the procedure of first calculating the actual force that may be experienced by the structure during the probable maximum earthquake, if it were to remain elastic. Then the concept of response reduction due to ductile deformation or frictional energy dissipation in the cracks is brought into the code explicitly, by introducing the `response reduction factor’ in place of the earlier performance factor.
c) The values of seismic zone factors have been changed; these now reflect more realistic values of effective peak ground acceleration considering Maximum Considered Earthquake (MCE) and service life of structure in each seismic zone.
d) A clause has been introduced to restrict the use of foundations vulnerable to differential settlements in severe seismic zones.
Here it is worthwhile to mention that it is not intended in this standard to lay down regulation so that no structure shall suffer any damage during earthquake of all magnitudes. It has been endeavoured to ensure that as far as, possible structures are able to respond, without structural damage to shocks of moderate intensities and without total collapse to shocks of heavy intensities.
IS 1893(Part 4):2005 `Criteria for Earthquake Resistant Design of Structures: Part 4 Industrial Structures Including Stack Like Structures
This standard deals with earthquake resistant design of the industrial structures (plant and auxiliary structures) including stack-like structures such as process industries, power plants, textile industries, off-shore structures and marine/port/harbour structures.
In addition to the above, stack-like structures covered by this standard are such as transmission and communication towers, chimneys and stack-like structures and silos (including parabolic silos used for urea storage).
The characteristics (intensity, duration, etc) of seismic ground vibrations expected at any location depends upon the magnitude of earthquake, its depth of focus, distance from the epicenter, characteristics of the path through which the seismic waves travel, and the soil strata on which the structure stands.
The response of a structure to ground vibrations is a function of the nature of foundations, soil, materials, form, size and mode of construction of structures; and the duration and characteristics of ground motion. This standard specifies design forces for structures standing on rocks or soils, which do not settle, liquify or slide due to loss of strength during vibrations.
The design approach adopted in this standard is to ensure that structures possess minimum strength to withstand minor earthquakes (DBE)which occur frequently, without damage; resist moderate earthquakes (DBE) without significant structural damage though some non-structural damage may occur; and withstand a major earthquake (MCE) without collapse.
IS 4326:1993 Earthquake Resistant Design and Construction of Buildings - Code of Practice
This standard provides guidance in selection of materials, special features of design and construction for earthquake resistant buildings including masonry construction, timber construction, prefabricated construction etc. In this standard, it is intended to cover the specified features of design and construction for earthquake resistance of buildings of conventional types. The general principles to be observed in the construction of such earthquake resistant buildings as specified in this standard are Lightness, Continuity of Construction, avoiding/reinforcing Projecting and suspended parts, Building configuration, strength in various directions, stable foundations, Ductility of structure, Connection to non-structural parts and fire safety of structures.
Special Construction Features like Separation of Adjoining Structures, Crumple Section, Foundation design, Roofs and Floors and Staircases have been elaborated in the standard. It also covers the details pertaining to the type of construction, masonry construction with rectangular masonry units, masonry bearing walls, openings in bearing walls, seismic strengthening arrangements, framing of thin load bearing walls, reinforcing details for hollow block masonry, flooring/roofing with precast components and timber construction.
IS 13827:1993 Improving Earthquake Resistance of Earthen Buildings – Guidelines
The guidelines covered in this standard deal with the design and construction aspects for improving earthquake resistance of earthen houses, without the use of stabilizers such as lime, cement, asphalt, etc.
The provisions of this standard are applicable for seismic zones III, IV and V. No special provisions are considered necessary in Zone II. However, considering inherently weak against water and earthquake, earthen buildings should preferably be avoided in flood prone, high rainfall areas and seismic zones IV and V.
It has been recommended that such buildings should be light, single storeyed and of simple rectangular plan. Qualitative tests for the suitability of soil have been suggested.
Guidelines for Block or Adobe Construction, Rammed earth construction, Seismic strengthening of bearing wall buildings, Internal bracing in earthen houses and earthen constructions with wood or cane structures have heen elaborated in this standard.
IS 13828:1993 Improving Earthquake Resistance of Low Strength Masonry Buildings – Guidelines
This standard covers the special features of design and construction for improving earthquake resistance of buildings of low-strength masonry.
The provisions of this standard are applicable in all seismic zones. No special provisions are considered necessary for buildings in seismic zone II if cement-sand mortar not leaner than 1:6 is used in masonry and through stones or bonding elements are used in stone walls.
The various provisions of IS 4326:1993 regarding general principles, special construction features, types of construction, categories of buildings and masonry construction with rectangular masonry buildings of low strength dealt with in this standard. There are however certain restrictions, exceptions and additional details which are specifically included herein.
IS 13920:1993 Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces – Code of Practice
This standard covers the requirements for designing and detailing of monolithic reinforced concrete buildings so as to give them adequate toughness and ductility to resist severe earthquake shocks without collapse.
The provisions for reinforced concrete construction given in this standard apply specifically to monolithic reinforced concrete construction. Precast and/or prestressed concrete members may be used only if they can provide the same level of ductility as that of a monolithic reinforced concrete construction during or after an earthquake.
Provisions on minimum and maximum reinforcement have been elaborated which includes the requirements for beams at longitudinal reinforcement in beams at joint face, splices and anchorage requirements. Provisions have been included for calculation of design shear force and for detailing of transverse reinforcement in beams.
Material specifications are indicated for lateral force resisting elements of frames. The provisions are also given for detailing of reinforcement in the wall web, boundary elements, coupling beams, around openings, at construction joints, and for the development, splicing and anchorage of reinforcement.
IS 13935:2009 Seismic Evaluation, Repair and Strengthening of Masonry Buildings – Guidelines
This standard covers the selection of materials and techniques to be used for repair and seismic strengthening of damaged buildings during earthquakes. It also covers the damageability assessment and retrofitting for upgrading of seismic resistance of existing masonry buildings covered under IS 4326 and IS 13828.
The provisions of this standard are applicable for buildings in seismic Zones III to V of IS 1893 (Part-1). These are based on damaging seismic intensities VII and more on M.S.K. Intensity scales. The scheme of strengthening should satisfy the requirements stipulated for the seismic zone of IS 1893: (Part-1), building categories of IS 4326 and provisions made in this code and in IS 13828 for low strength masonry building. No special seismic resistance features are considered necessary for buildings in seismic Zones II, but the important buildings in this Zone may also be considered for upgrading their seismic resistance.
The buildings affected by earthquake may suffer both non-structural and structural damages. This standard lays down guidelines for non-structural/architectural as well as structural repairs, seismic strengthening and seismic retrofitting of existing buildings. Guidelines have been given for selection of materials for repair work such as cement, steel, epoxy resins, epoxy mortar, quick setting cement mortar and special techniques such as shotcrete, mechanical anchorage etc. Seismic strengthening techniques for the modification of roofs or floors, inserting new walls, strengthening existing walls, masonry arches, random rubble masonry walls, strengthening long walls, strengthening reinforced concrete members and strengthening of foundations have been elaborated in detail.
The following are the major important modifications made in this revision:
a) Non-shrink grouts and fibre reinforced plastics have been incorporated for repair, restoration work and strengthening.
b) Damageability assessment of existing masonry buildings under earthquake occurrences has been incorporated.
c) Assessment of retrofitting requirements and actions for retrofitting also incorporated.
d) Provision of seismic belts around door and window openings.
e) Rapid visual screening method along with RVS survey forms for masonry buildings for seismic hazards evaluation has been
ISS 6922:1973 Criteria for Safety and Design of Structures Subject to Underground Blasts
This standard deals with the safety of structures during underground blasting and is applicable to normal structures like buildings, elevated structures, bridges, retaining walls, concrete and masonry dams constructed in materials like brickwork, stone masonry and concrete.
As underground blasting operations have become almost a must for excavation purposes, this standard lays down criteria for safety of such structures from cracking and also specifies the effective accelerations for their design in certain cases.
IS 4991:1968 Criteria for Blast Resistant Design of Structures for Explosions Above Ground
This standard covers the criteria for design of structures for blast effects of explosions above ground excluding blast effects of nuclear explosions.
IS 4967:1968 Recommendations for Seismic Instrumentation for
Projects River Valley
This standard covers recommendations for instrumentation for investigation of seismicity, study of micro tremors and predominant period of a dam site and permanent installation of instruments in the dam and appurtenant structures and in surrounding areas.
These standards endeavour to provide a guideline in designing and repairing of buildings under seismic forces.