Basic concepts in indian standard eq design codes
- 1. by Dr. Anand S. Arya, FNA, FNAE Padmashree awarded by the President, 2002 Professor Emeritus, Deptt. of Earthquake Engg., I.I.T. Roorkee National Seismic Advisor GoI-UNDP, New Delhi List of Indian Standards on Earthquake Engineering 1. IS 1893 (Part I), 2002: Indian Standard Criteria for Earthquake Resistant Design of Structures 2. IS 4326, 1993: Indian Standard Code of Practice for Earthquake Resistant Design & Construction of Buildings. 3. IS 13827, 1993: Indian Standard Guidelines for improving Earthquake Resistance of Earthen Buildings 4. IS 13828, 1993: Indian Standard Guidelines for Improving Earthquake Resistance of Low Strength Masonry Buildings 5. IS 13920, 1993 Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces. 6. IS 13935, 1993: Indian Standard Guidelines for Repair and Seismic Strengthening of Buildings.
- 2. Indian Standard IS: 1893 (Part 1) -2002 Criteria for Earthquake Resistant Design Seismic Zone Map of India, 2002 NOTE : Towns falling at the boundary of zones demarcation line between two zones shall be considered in High Zone. Fig 1
- 3. Basic Approach Seismic Design Philosophy 1. To ensure that structures posses at least a minimum strength to withstand minor earthquakes (<DBE)without damage. 2. Resist moderate earthquakes (DBE) without significant structural damage though some non-structural damage may occur and that 3. Structures withstand a major earthquake (MCE) without collapse. Note:- Actual forces that appear on structures during earthquakes are much greater than the design forces specified. Hence, ductility arising from inelastic material behavior and detailing and over-strength are relied upon to account for this difference in actual and design lateral loads. 6.4 Design Spectrum For the purpose of determining seismic forces, the country is classified into four seismic zones as shown in Fig. 1. The design horizontal seismic coefficient Ah for a structure shall be determined by the following expression: Ah = ZISa 2Rg Provided that for any structure with T< 0.1 s, the value of Ah will not be taken less than Z/2 whatever be the value of I/R where Z = Zone factor given in Table 2, is for the Maximum Considered Earthquake (MCE) and service life of structure. The factor 2 in the denominator of Z is used so as to reduce the Maximum Considered Earthquake (MCE) zone factor to the factor for Design Basis Earthquake (DBE ).
- 4. I = Importance factor, depending upon the functional use of the structures, characterised by hazardous consequences of its failure, post-earth quake functional needs, historical value, or economic importance ( Table 6 ). R = Response reduction factor, depending on the perceived seismic damage performance of the structure, characterised by ductile or brittle deformations. However, the ratio (I/R) shall not be greater than 1.0 (Table 7). The values of/? for buildings are given in Table 7. Sa/g = Average response acceleration coefficient for rock or soil sites as given by Fig. 2 based on appropriate natural periods and damping of the structure. These curves represent free field ground motion. NOTE — For various types of structures, the values of Importance Factor 1, Response Reduction Factor R, and damping values are given in the respective parts of this standard. The method ( empirical or otherwise ) to calculate the natural periods of the structure to be adopted for evaluating S/g is also given in the respective parts of this standard. Table 2 Zone Factor, Z (Clause 6.4.2)
- 8. 6.3 Load Combination and Increase in Permissible Stresses 6.3.1 Load Combinations When earthquake forces are considered on a structure, these shall be combined as per 6.3.1.1 and 6.3.1.2 where the terms DL, IL and EL stand for the response quantities due to dead load, imposed load and designated earthquake load respectively. 6.3.1.1 Load factors for plastic design of steel structures In the plastic design of steel structures, the following load combinations shall be accounted for: 1)1.7{DL+IL) 2)1.7(DL±EL) 3)1.3 (DL + IL±EL) 6.3.1.2 Partial safety factors for limit state design of reinforced concrete and prestressed concrete structures In the limit state design of reinforced and prestressed concrete structures, the following load combinations shall be accounted for: 1.5 (DL + IL) 1.2{DL + IL±EL) 1.5(DL±EL) 0.9DL±l.5EL.
- 9. 6.3.4 Combination for Two or Three Component Motion All possible combinations of the three components {ELx, Ely and ELz) including variations in sign {plus or minus ) shall be considered. Thus ±ELx±0.3ELy ±0.3 ELz ±ELy±0.3 ELx ±0.3 ELz ±ELz±0.3 ELx±0.3 Ely where x and y are two orthogonal directions and z is vertical direction. 7. BUILDINGS Regular and Irregular Configuration
- 11. 7.6 Fundamental Natural Period 7.6.1 The approximate fundamental natural period of vibration (Ta), in seconds, of a moment-resisting frame building without brick infil panels may be estimated by the empirical expression: Ta = 0.075 h0.75 = 0.085 h0.75 for RC frame building for steel frame building where h = Height of building, in m. This excludes the basement storeys, where basement walls are connected with the ground floor deck or fitted between the building columns. But, it includes the basement storeys, when they are not so connected. 7.6.2 The approximate fundamental natural period of vibration ( 7"a), in seconds, of all other buildings, including moment-resisting frame buildings with brick infill panels, may be estimated by the empirical expression: where h = Height of building, in m, as defined in 7.6.1; and d = Base dimension of the building at the plinth level, in m, along the considered direction of the lateral force.
- 12. 7.7 Distribution of Design Force 7.7.1 Vertical Distribution of Base Shear to Different Floor Levels The design base shear ( FB) computed in 7.5.3 shall be distributed along the height of the building as per the following expression: where Qi = Design lateral force at floor/, Wi = Seismic weight of floor;', hi = Height of floor / measured from base, and n = Number of stories in the building is the number of levels at which the masses are located. 7.7.2 Distribution of Horizontal Design Lateral Force to Different Lateral Force Resisting Elements 7.7.2.1 In case of buildings whose floors are capable of providing rigid horizontal diaphragm action, the total shear in any horizontal plane shall be distributed to the various vertical elements of lateral force resisting system, assuming the floors to be infinitely rigid in the horizontal plane.
- 13. 7.8 Dynamic Analysis 7.8.1 Dynamic analysis shall be performed to obtain die design seismic force, and its distribution to different levels along the height of the building and to the various lateral load resisting elements, for the following buildings: a. Regular buildings — Those greater than 40 m in height in Zones IV and V and those greater than 90 m in height in Zones II and III. Modelling as per 7.8.4.5 can be used b. Irregular buildings ( as defined in 7.1) — All framed buildings higher than 12 m inZones IV and V, and those greater than 40 m in height in Zones Hand III. 7.8.2 Dynamic analysis may be performed either by the Time History Method or by the Response Spectrum Method. However, in either method, the design base shear (KB) shall be compared with a base shear (FB) calculated using a fundamental period Th, where Ta is as per 7.6. Where Vg is less than FB, all the response quantities ( for example member forces, displacements, storey forces, storey shears and base reactions) shall be multiplied by FB / VB. 7.9 Torsion Provision shall be made in all buildings for increase in shear forces on the lateral force resisting elements resulting from the horizontal torsional moment arising due to eccentricity between the centre of mass and centre of rigidity. The design forces calculated as in 7.8.4.5 are to be applied at the centre of mass appropriately displaced so as to cause design eccentricity ( 7,9.2 ) between the displaced centre of mass and centre of rigidty However, negative torsional shear shall be neglected. The design eccentricity, e& lobe used at floor shall be taken as:
- 14. 7.10 Buildings with Soft Storey In case buildings with a flexible storey, such as the ground storey consisting of open spaces for parking that is Stilt buildings, special arrangement needs to be made to increase the lateral strength and stiffness of (lie soft/open storey. 7.10.2 Dynamic analysis of building is carried out including the strength and stiffness effects of infills and inelastic deformations in the members, particularly; those in the soft .storey, and the members designed accordingly. 7.10.3Alternatively, the following design criteria are to be adopted after carrying out the earthquake analysis, neglecting the effect of infill walls in other storeys: a). columns and beams of the soft storey are to be designed for2,5 times the storey shears and moments calculated under seismic loads specified in the other relevant clauses: or besides the columns designed and detailed for the calculated storey shears and moments, shear walls placed symmetrically in both, directions of the building as faraway from the centre of the building as feasible: to be designed exclusively for 1.5 times the lateral storey shear force calculated as before.
- 15. 7.11 Deformations 7.11.1 Storey Drift Limitation The storey drift in any storey due to the minimum specified design lateral force, with partial load factor of 1.0, shall not exceed 0.004 times the storey height. For the purposes of displacement requirements only {see 7,11,1, 7,11,2 and 7.11.3 only , it is permissible to use seismic force obtained from the computed fundamental period (T) of the building without the lower bound limit on design seismic force specified in 7.8.2. There shall be no drift limit for single storey building which has been designed to accommodate storey drift. 7.11.2 Deformation Compatibility of Non-Seismic For building located in seismic Zones IV and V, it shall be ensured that the structural components, that are not a part of the seismic force resisting system in the direction under consideration, do not lose their vertical load-carrying capacity under the induced moments resulting from storey deformations equal to R times the storey displacements calculated as per 7.11.1. where R is specified in Table 7. NOTE — For instance, consider a flat-slab building in which lateral load resistance is provided by shear walls. Since the lateral load resistance of the slab-column system is small, these arc often designed only for the gravity loads, while all the seismic force is resisted by the shear walls. Even though the slabs and columns arc not required to share the lateral forces, these deform with rest of the structure under seismic force. The concern js that under such deformations, the slab-column system should not lose its vertical load capacity.
- 16. 7.12 Miscellaneous 7.12.1 Foundations The use of foundations vulnerable to significant differential settlement due to ground shaking shall be avoided for structures in seismic Zones III, IV and V In seismic Zones IV and V, individual spread footings or pile caps shall be interconnected with lies, ( see 5.3.4.1 of IS 4326 ) except when individual spread footings are directly supported on rock. All ties shall be capable of carrying, in tension and in compression, an axial force equal to Ah /4 times the larger of the column or pile cap load, in addition to the otherwise computed forces Here. Ah is as per 6.4.2. 7.12.2 Cantilever Projections 7.12.2.1 Vertical projections Tower, tanks, parapets, smoke stacks ( chimneys } and other vertical cantilever projections attached to buildings and projecting above the roof, shall be designed and checked for stability for five times the design horizontal seismic coefficient Ah specified in 6.4.2. In the analysis of the building, the weight of these projecting elements will be lumped with the roof weight.
- 17. 7.12.2.2 Horizontal projection All horizontal projections like cornices and balconies shall be designed and checked for stability for five times the design vertical coefficient specified in 6.4.5 (that is =10/3Ah). 7.12.2.3 The increased design forces specified in 7.12.2.1 and 7,12.2.2 are only for designing the projecting parts and their connections with the main structures. For the design of the main structure, such increase need not be considered. 7.12.3 Compound Walls Compound walls shall be designed for the design horizontal coefficient A. with importance factor / = 1,0 specified in 6.4.2. 7.12.4 Connections Between Parts All parts of the building, except between the separation sections, shall be tied together to act as integrated single unit All connections between different parts, such as beams to columns and columns to their footings, should be made capable of transmitting a force, in all possible directions, of magnitude (Q/w,) times but not less than 0.05 times the weight of the smaller part or the total of dead and imposed load reaction. Frictional resistance shall not be relied upon for fulfilling these requirements.
- 18. IS:13920 – 1993 Ductile Detailing of Reinforced Concrete Structures subjected to Seismic Forces – Code of Practice Scope Provisions of this code shall be adopted in all RC structures located in seismic zones III, IV and V
- 19. Flexural Members 6.2 Longitudinal Reinforcement 6.2.1 a) The top as well as bottom reinforce-ment shall consist of at least two bars throughout the member length. b) The tension steel ratio on any face, at any section, shall not be less than Pmin = 0-24 v/JWA; where ck and y are in MPa. 6.2.2 The maximum steel ratio at any section, shall not exceed Pmax =0025. 6.2.3 The positive steel at a joint face must be at least equal to half the negative steel at that face. 6.2.4 The steel provided at each of the top and bottom face of the member at any section along its length shall be at least equal to one-fourth of the maximum negative moment steel provided at the face of either joint. It may be clarified that redistribution of moments permitted in IS 456 :1978 ( clause 36.1 ) will be used only for vertical load moments and not for lateral load moments 6.2.5 In an external joint, both the top and the bottom bars of the beam shall be provided with anchorage length, beyond the inner face of the column, equal to the development length in tension plus 10 times the bar diameter minus the allowance for 90 degree bend( s ) (see Fig. I ). In an internal joint, both face bars of the beam shall be taken continuously through the column
- 20. 7.2 Longitudinal Reinforcement 7.2.1 Lap splices shall be provided only in the central half of the member length. It should be proportioned as a tension splice. Hoops shall be ', provided over the entire splice length at spacing not exceeding 150 mm centre to centre. Not more than 50 percent of the bars shall be spliced at one section Capacity Design for Shear
- 21. 6d(! 65 mm) 6d(! 65 mm) 6.3.5 The spacing of hoops over a length of 2d at either end of a beam shall not exceed ( a ) d/4, and (b) 8 times the diameter of the smallest longitudinal bar; however, it need not be less than 100 mm ( see Fig. 5 ). The first hoop shall be at a distance not exceeding 50 mm from the joint face- Vertical hoops at the same spacing as above, shall also be provided over a length equal to 2d on either side of a section where flexural yielding may occur under the effect of earthquake forces. Elsewhere, the beam shall have vertical hoops at a spacing not exceeding d/2.
- 22. 7. COLUMNS AND FRAME MEMBERS SUB-JECTED TO BENDING AND AXIAL LOAD 7.1 General 7.1.1 These requirements apply to frame mem-bers which have a factored axial stress in excess of 0.1 fck under the effect of earthquake forces. 7.1.2 The minimum dimension of the member shall not be less than 200 mm. However, in frames which have beams with centre to centre span exceeding 5 m or columns of unsupported length exceeding 4 m, the shortest dimension of the column shall not be less than 300 mm. 7.1.3 The ratio of the shortest cross sectional dimension to the perpendicular dimension shall preferably not be less than 0-4. Closed Ties in Columns
- 23. Shear in Columns Shear in Columns 7.3.4 The design shear force for columns shall be the maximum of: a) calculated factored shear force as per analysis, and b) a factored shear force given by
- 24. Reinforcement Detail in Column OTHER PROVISIONS • Discontinuous Wall • Short Column Effect • Shear Walls • Shear Wall with Boundary Elements • Coupled Shear Walls
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