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IS 1893 -2002 Code Book PDF free download | Auhippo
d) A more rational formula was used to combine modal shear forces
f) Clauses on concrete and masonry dams were modified, taking into account their dynamic behavior during earthquakes. Simplified formulae for design forces were introduced based on results of extensive studies carried out since second revision of the standard was published.
The fourth revision, brought out in 1984, was prepared to modifi some of the provisions of the standard as a result of experience gained with the use of the standard. In this revision, a number of important basic modifications with respect to load factors, field values of N, base shear and modal analysis were introduced. A new concept of performance factor depending on the structural framing system and on the ductility of construction was incorporated. Figure 2 for average acceleration spectra was also modified and a curve for zero percent damping incorporated. In the fifth revision, with a view to keep abreast with the rapid development and extensive research that has been carried out in the field of earthquake resistant design of various structures, the committee has decided to cover the provisions for different types of structures in separate parts. Hence, IS 1893 has been split into the following five parts: Part 1 General provisions and buildings Part 2 Liquid retaining tanks — Elevated and ground supported Part 3 Bridges and retaining walls Part 4 Industrial structures including stack like structures Part 5 Dams and embankments Part 1 contains provisions that are general in nature and applicable to all structures. Also, it contains provisions that are specific to buildings only. Unless stated otherwise, the provisions in Parts 2 to 5 shall be read necessarily in conjunction with the general provisions in Part 1. NOTE — Pending finalization of Parts 2 to 5 of IS 1893, provisions of Part 1 will be read along with the relevant clauses of IS 1893 : 1984 for structures other than buildings. The following are the major and important moditlcations made in the fifth revision: a) The seismic zone map is revised with only four zones, instead of five. Erstwhile Zone I has been merged to Zone 11. Hence, Zone I does not appear in the new zoning; only Zones II, 111,IV and V do. b) 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. c) Response spectra are now specified for three types of founding strata, namely rock and hard soil, medium soil and soft soil. d) Empirical expression for estimating the fundamental natural period Taof multi-storeyed buildings with regular moment resisting frames has been revised. e) This revision adopts the procedure of first calculating the actual force that maybe 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. f) A lower bound is specified for the design base shear of buildings, based on empirical estimate of the fimdarnental natural period Ta. g) The soil-foundation system factor is dropped. Instead, a clause is introduced to restrict the use of foundations vulnerable to differential settlements in severe seismic zones. h) Torsional eccentricity values have been revised upwards in view of serious darnages observed in buildings with irregular plans. J) Modal combination rule in dynamic analysis of buildings has been revised. k) Other clauses have been redrafted where necessary for more effective implementation. 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 endeavored to ensure that, as far as possible, structures are able to respond, without structural darnage to shocks of moderate intensities and without total collapse to shocks of heavy intensities. While this standard is intended for the earthquake resistant design of normal structures, it has to be emphasized that in the case of special structures, such as large and tall dams, long-span bridges, major industrial projects, etc, site-specific detailed investigation should be undertaken, unless otherwise specified in the relevant clauses. Though the basis for the design of different types of structures is covered in this standard, it is not implied that detailed dynamic analysis should be made in every case. In highly seismic areas, construction of a type which entails hea~y debris and consequent loss of life and property, such as masonry, particularly mud masonry and rubble masonry, should preferably be avoided. For guidance on precautions to be observed in the construction of buildings, reference maybe made to IS 4326, IS 13827 and IS 13828. Earthquake can cause damage not only on account of the shaking which results from them but also due to other chain effects like landslides, floods, fires and disruption to communication. It is, therefore, important to take necessary precautions in the siting, planning and design of structures so that they are safe against such secondary effects also. The Sectional Committee has appreciated that there cannot bean entirely scientific basis for zoning in view of the scanty data available. Though the magnitudes of different earthquakes which have occurred in the past are known to a reasonable degree of accuracy, the intensities of the shocks caused by these earthquakes have so far been mostly estimated by damage surveys and there is little instrumental evidence to corroborate the conclusions arrived at. Maximum intensity at different places can be fixed on a scale only on the basis of the observations made and recorded after the earthquake and thus a zoning map which is based on the maximum intensities arrived at, is likely to lead in some cases to an incorrect conclusion in view of(a) incorrectness in the assessment of intensities, (b) human error in judgment during the damage survey, and (c) variation in quality and design of structures causing variation in type and extent of damage to the structures for the same intensity of shock. The Sectional Committee has therefore, considered that a rational approach to the problem would be to arrive at a zoning map based on known magnitudes and the known epicentres ( see Annex A ) assuming all other conditions as being average and to modifi such an idealized isoseismal map in light of tectonics ( see Annex B ), lithology ( see Annex C ) and the maximum intensities as recorded from damage surveys. The Committee has also reviewed such a map in the light of the past history and future possibilities and also attempted to draw the lines demarcating the different zones so as to be clear of important towns, cities and industrial areas, after making special examination of such cases, as a little modification in the zonal demarcations may mean considerable difference to the economics of a project in that area. Maps shown in Fig. 1 and Annexes A, B and C are prepared based on information available upto 1993. In the seismic zoning map, Zone I and II of the contemporary map have been merged and assigned the level of Zone 11. 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 areas 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. The seismic hazard level with respect to ZPA at 50 percent risk level and 100 years service life goes on progressively increasing from southern peninsular portion to the Himalayan main seismic source, the revised seismic zoning map has given status of Zone III to Narmada Tectonic Domain, Mahanandi Graben and Godawari Graben. This is a logical normalization keeping in view the apprehended higher strain rates in these domains on geological consideration of higher neotectonic activity recorded in these areas. Attention is particularly drawn to the fact that the intensity of shock due to an earthquake could vary locally at anyplace due to variation in soil conditions. Earthquake response of systems would be affected by different types of foundation system in addition to variation of ground motion due to various types of soils. Considering the effects in a gross manner, the standard gives guidelines for arriving at design seismic coet%cients based on stiffness of base soil. It is important to note that the seismic coefficient, used in the design of any structure, is dependent on many variable factors and it is an extremely difficult task to determine the exact seismic coefficient in each given case. It is, therefore, necessa~ to indicate broadly the seismic coefficients that could generally be adopted in different parts or zones of the country though, of course, a rigorous analysis considering all the factors involved has to be made in the case of all important projects in order to arrive at a suitable seismic coeftlcients for design. The Sectional Committee responsible for the formulation of this standard has attempted to include a seismic zoning map (see Fig. 1 ) for this purpose. The object of this map is to classifi the area of the country into a number of zones in which one may reasonably expect earthquake shaking of more or less same maximum intensity in future. The Intensity as per Comprehensive Intensity Scale ( MSK64 ) ( see Annex D ) broadly associated with the various zones is VI ( or less ), VII, VIII and IX ( and above ) for Zones II, III, IV and V respectively. The maximum seismic ground acceleration in each zone cannot be presently predicted with accuracy either on a deterministic or on a probabilistic basis. The basic zone factors included herein are reasonable estimates of effective peak ground accelerations for the design of various structures covered in this standard.
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