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May, 2005 Structural designs of buildings, flyovers, tall towers, chimneys, dams, power plants or other structures are evolved after taking into account different combinations of loads such as dead loads, live loads, wind loads, earthquake forces, earth pressures, wave loads etc and the most critical load combination is chosen for design calculations. The values of these loads are worked out on the basis of past experiences and recurring intervals of natural vagaries like floods, earthquakes, cyclones and wind storms. As and when we experience a force higher in magnitude than that assumed in our designs, we tend to revise our codal instructions and newly experienced forces are taken into account. Bhuj earthquake of 2001 left the country and its designers stunned and made them put their heads together and review the seismic division of the country and codal instructions to build earthquake-resistant structures. BIS code IS 1893, laying criteria for earthquake resistant design of structures has, therefore, been revised and is in force now. All structural designers are therefore required to follow IS 1893-2002 now as the revised edition carries significant changes. Keeping in view the research carried out and study of earthquakes that occurred during 1984-2002, IS 1893-2002 has been split into five parts. Provisions of Part I are applicable to all structures. This code has therefore been released. Part II to V are still under finalisation. In the revised code, significant changes have been made as explained below : 1. Revised seismic zoning of India : Earlier the country had been divided into five zones with Zone I least prone and Zone V most prone to earthquakes. Zone I has now been merged with Zone II and now there exist only four zones i.e. Zone II to Zone V. The towns falling at the boundary of zone demarcation lines are to be considered to be in higher zone. 2. Seismic zone factors : A significant change made now is the introduction of seismic Zone Factor ‘Z’ based on Maximum Considered Earthquake and service life of structures and elimination of ‘Basic Horizontal Seismic Coefficient’ a0. For Chandigarh, falling in Zone IV, the coefficient used earlier was 0.05 while the zone factor now to be considered is 0.24. Similarly, for Bhuj, falling in Zone V, value of a0 was 0.08 while that of Z now is 0.36. The Zone Factors now incorporated in the code are more realistic. 3. Response reduction factor : Performance Factor “K” existing earlier has been dropped and Response Reduction Factor ‘R’ has been introduced. More strong is the building frame system, more is the value of ‘R’. Value of ‘R’ is least for a masonry structure and highest for steel or special RCC buildings. 4. Soil Foundation system coefficient dropped : Soil Foundation system coefficient ‘ß’ has been dropped and a special clause has been added to restrict the use of foundations vulnerable to differential settlement in severe seismic zones. 5. Torsional Eccentricity: Special attention has been paid to torsional eccentricity occurring in buildings with irregular plans as such buildings get severally damaged during earthquakes. The values of torsional eccentricity have been revised upwards. 6. Design Method : The method of design for earthquake resistance has also been modified. a0 and ah have been replaced by a new ‘Design Horizontal Seismic Coefficient’ Ah. The formula for calculation of Ah includes the Zone Factor ‘Z’ and Response Reduction Factor ‘R’ in it instead of a0. 7. Soft storey concept : Due to increased use of soft storeys in buildings such as provision of open parking space below multi-storeyed flats, design criteria to be used for soft storeys has been provided. 8.Height criteria: Earlier all buildings falling in Zone III and above 40 metres height required dynamic analysis. Now only irregular buildings of this zone require this analysis. For regular buildings, the height criteria has been revised to 90 metres. 9.Shear wall concept : Provisions of IS 13920 for Ductile detailing of RCC structures are to be adopted now for all such structures located in Zone III, IV and V. This code includes the sheer wall concept which is the best method to avoid collapse of structures during earthquakes.
As is clear from the above, the new code includes significant changes which need to be used not only to design all new structures but also for checking the existing structures for their strengthening wherever required, so as to resist any failures on occurrence of earthquakes.
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