Basic concepts in indian standard eq design codes


Published on

1 Like
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Basic concepts in indian standard eq design codes

  1. 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 Engineering1. IS 1893 (Part I), 2002: Indian Standard Criteria for Earthquake Resistant Design of Structures2. 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 Buildings4. IS 13828, 1993: Indian Standard Guidelines for Improving Earthquake Resistance of Low Strength Masonry Buildings5. 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. 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. 3. Basic ApproachSeismic Design Philosophy1. 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 that3. 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 SpectrumFor the purpose of determining seismic forces, the country isclassified into four seismic zones as shown in Fig. 1.The design horizontal seismic coefficient Ah for a structure shallbe determined by the following expression: Ah = ZISa 2RgProvided that for any structure with T< 0.1 s, the value of Ah willnot be taken less than Z/2 whatever be the value of I/RwhereZ = Zone factor given in Table 2, is for the MaximumConsidered Earthquake (MCE) and service life of structure. Thefactor 2 in the denominator of Z is used so as to reduce theMaximum Considered Earthquake (MCE) zone factor to thefactor for Design Basis Earthquake (DBE ).
  4. 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 ofImportance Factor 1, Response Reduction Factor R, anddamping values are given in the respective parts of thisstandard. The method ( empirical or otherwise ) to calculatethe natural periods of the structure to be adopted forevaluating S/g is also given in the respective parts of thisstandard. Table 2 Zone Factor, Z (Clause 6.4.2)
  5. 5. 6.3 Load Combination and Increase inPermissible Stresses6.3.1 Load CombinationsWhen earthquake forces are considered on a structure,these shall be combined as per and the terms DL, IL and EL stand for the responsequantities due to dead load, imposed load anddesignated earthquake load respectively. Load factors for plastic design of steelstructuresIn the plastic design of steel structures, the followingload combinations shall be accounted for:1)1.7{DL+IL)2)1.7(DL±EL)3)1.3 (DL + IL±EL) Partial safety factors for limit state design ofreinforced concrete and prestressed concretestructures In the limit state design of reinforced andprestressed concrete structures, the following loadcombinations shall be accounted for: 1.5 (DL + IL) 1.2{DL + IL±EL) 1.5(DL±EL) 0.9DL±l.5EL.
  6. 6. 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. BUILDINGSRegular and IrregularConfiguration
  7. 7. 7.6 Fundamental Natural Period7.6.1 The approximate fundamental natural period ofvibration (Ta), in seconds, of a moment-resisting framebuilding without brick infil panels may be estimated by theempirical expression: Ta = 0.075 h0.75 = 0.085 h0.75 for RC frame building for steelframe buildingwhere h = Height of building, in m. This excludes the basementstoreys, where basement walls are connected with theground floor deck or fitted between the buildingcolumns. But, it includes the basement storeys, when theyare not so connected.7.6.2 The approximate fundamental natural period ofvibration ( 7"a), in seconds, of all other buildings,including moment-resisting frame buildings with brickinfill panels, may be estimated by the empiricalexpression:whereh = Height of building, in m, as defined in 7.6.1; andd = Base dimension of the building at the plinth level, inm, along the considered direction of the lateral force.
  8. 8. 7.7 Distribution of Design Force7.7.1 Vertical Distribution of Base Shear to Different Floor LevelsThe design base shear ( FB) computed in 7.5.3 shall be distributed along the height of the building as per the following expression:whereQi = Design lateral force at floor/,Wi = Seismic weight of floor;,hi = Height of floor / measured from base, andn = 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 Elements7.7.2.1 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.
  9. 9. 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 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 forceson the lateral force resisting elements resulting from the horizontaltorsional moment arising due to eccentricity between the centre of massand centre of rigidity. The design forces calculated as in are tobe applied at the centre of mass appropriately displaced so as to causedesign eccentricity ( 7,9.2 ) between the displaced centre of mass andcentre of rigidty However, negative torsional shear shall be neglected.The design eccentricity, e& lobe used at floor shall be taken as:
  10. 10. 7.10 Buildings with Soft StoreyIn 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 outincluding the strength and stiffness effects of infills andinelastic deformations in the members, particularly; thosein the soft .storey, and the members designedaccordingly.7.10.3Alternatively, the following design criteria are to beadopted 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 bedesigned for2,5 times the storey shears and momentscalculated under seismic loads specified in the otherrelevant clauses: or besides the columns designedand detailed for the calculated storey shears andmoments, shear walls placed symmetrically in both,directions of the building as faraway from the centreof the building as feasible: to be designed exclusivelyfor 1.5 times the lateral storey shear force calculatedas before.
  11. 11. 7.11 Deformations7.11.1 Storey Drift LimitationThe storey drift in any storey due to the minimumspecified design lateral force, with partial load factor of1.0, 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 touse seismic force obtained from the computedfundamental period (T) of the building without the lowerbound limit on design seismic force specified in 7.8.2.There shall be no drift limit for single storey buildingwhich has been designed to accommodate storey drift.7.11.2 Deformation Compatibility of Non-SeismicFor building located in seismic Zones IV and V, it shall be ensuredthat the structural components, that are not a part of the seismicforce resisting system in the direction under consideration, do notlose their vertical load-carrying capacity under the inducedmoments resulting from storey deformations equal to R times thestorey displacements calculated as per 7.11.1. where R isspecified in Table 7.NOTE — For instance, consider a flat-slab building in whichlateral load resistance is provided by shear walls. Since the lateralload resistance of the slab-column system is small, these arcoften designed only for the gravity loads, while all the seismicforce is resisted by the shear walls. Even though the slabs andcolumns arc not required to share the lateral forces, these deformwith rest of the structure under seismic force. The concern js thatunder such deformations, the slab-column system should not loseits vertical load capacity.
  12. 12. 7.12 Miscellaneous7.12.1 FoundationsThe use of foundations vulnerable to significantdifferential settlement due to ground shaking shall beavoided for structures in seismic Zones III, IV and VIn seismic Zones IV and V, individual spreadfootings or pile caps shall be interconnected withlies, ( see of IS 4326 ) except when individualspread footings are directly supported on rock. Allties shall be capable of carrying, in tension and incompression, an axial force equal to Ah /4 times thelarger of the column or pile cap load, in addition tothe otherwise computed forces Here. Ah is as per6. Cantilever Projections7.12.2.1 Vertical projectionsTower, tanks, parapets, smoke stacks ( chimneys }and other vertical cantilever projections attached tobuildings and projecting above the roof, shall bedesigned and checked for stability for five times thedesign horizontal seismic coefficient Ah specified in6.4.2. In the analysis of the building, the weight ofthese projecting elements will be lumped with theroof weight.
  13. 13. Horizontal projectionAll horizontal projections like cornices and balconies shallbe designed and checked for stability for five times thedesign vertical coefficient specified in 6.4.5 (that is=10/3Ah). The increased design forces specified in7.12.2.1 and 7,12.2.2 are only for designing theprojecting parts and their connections with the mainstructures. For the design of the main structure, suchincrease need not be considered.7.12.3 Compound WallsCompound walls shall be designed for the designhorizontal coefficient A. with importance factor / = 1,0specified in Connections Between PartsAll parts of the building, except between the separationsections, shall be tied together to act as integratedsingle unit All connections between different parts,such as beams to columns and columns to theirfootings, should be made capable of transmitting aforce, in all possible directions, of magnitude (Q/w,)times but not less than 0.05 times the weight of thesmaller part or the total of dead and imposed loadreaction. Frictional resistance shall not be relied uponfor fulfilling these requirements.
  14. 14. IS:13920 – 1993 Ductile Detailing of ReinforcedConcrete Structures subjected to Seismic Forces – Code of Practice ScopeProvisions of this code shall be adopted in all RCstructures located in seismic zones III, IV and V
  15. 15. Flexural Members6.2 Longitudinal Reinforcement6.2.1 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 moments6.2.5 In an external joint,both the top and thebottom bars of the beamshall be provided withanchorage length, beyondthe inner face of thecolumn, equal to thedevelopment length intension plus 10 times thebar diameter minus theallowance for 90degree bend( s ) (seeFig. I ). In an internaljoint, both face barsof the beam shall be takencontinuously through thecolumn
  16. 16. 7.2 Longitudinal Reinforcement7.2.1 Lap splices shall be provided only in the central half of the memberlength. It should be proportioned as a tension splice. Hoops shall be , providedover 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
  17. 17. 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 jointface- 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 shallhave vertical hoops at a spacing not exceeding d/2.
  18. 18. 7. COLUMNS AND FRAME MEMBERS SUB-JECTED TOBENDING AND AXIAL LOAD7.1 General7.1.1 These requirements apply to frame mem-bers which have afactored axial stress in excess of 0.1 fck under the effect ofearthquake forces.7.1.2 The minimum dimension of the member shall not be lessthan 200 mm. However, in frames which have beams withcentre to centre span exceeding 5 m or columns of unsupportedlength exceeding 4 m, the shortest dimension of the column shallnot be less than 300 mm.7.1.3 The ratio of the shortest cross sectional dimension to theperpendicular dimension shall preferably not be less than 0-4. Closed Ties in Columns
  19. 19. Shear in Columns Shear in Columns7.3.4 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
  20. 20. ReinforcementDetail in Column OTHER PROVISIONS • Discontinuous Wall • Short Column Effect • Shear Walls • Shear Wall with Boundary Elements • Coupled Shear Walls
  21. 21. Thank You andwish you the best in your efforts