Chapter 5Seismic Force

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 481

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This clause primarily deals with earthquake hazard assessment and earthquake-resistant design of buildings.

All structures, like parking structures, security cabins and ancillary structures need to be designed for

Chart

10A VALUES OF THE CROSS WIND FORCE SPECTRUM COEFFICIENT FOR SQUARE SECTION BUILDINGS

Chart

10B VALUES OF THE CROSS WIND FORCE SPECTRUM COEFFICIENT FOR 2:1 AND 1:2 RECTANGULAR SECTION BUILDINGS

FIG. 10 WIND FORCE SPECTRUM COEFFICIENT

appropriate earthquake effects as per this clause. Also, temporary elements, such as scaffolding and temporary excavations, need to be designed as per this clause. This clause does not deal with construction features relating to earthquake-resistant buildings and other

structures. For guidance on earthquake-resistant construction of buildings, reference may be made to the latest revisions of the following good practice [6 1(6)].

The provisions of this clause are applicable even to critical and special structures, like nuclear power plants and petroleum refinery plants. For such structures, additional requirements may be imposed based on special studies, such as site-specific hazard assessment. In such cases, the earthquake effects specified by this clause shall be taken as at least the minimum.

For the purpose of this clause the symbols given at Annex G are applicable.

5.1Terminology

For the purpose of this clause, definitions given below shall apply to all structures, in general. For definition of terms pertaining to soil mechanics and soil dynamics, reference may be made to Part 6 ‘Structural Design, Section 2 Soils and Foundations’ of the Code; and for definition of terms pertaining to ‘Loads’, 3.2 and 4.3 may be referred.

5.1.1 Closely-Spaced Modes — Closely-spaced modes of a structure are those of the natural modes of oscillation of a structure, whose natural frequencies differ from each other by 10 percent or less of the lower frequency.

5.1.2 Critical Damping — The damping beyond which the free vibration motion will not be oscillatory.

5.1.3 Damping — The effect of internal friction, inelasticity of materials, slipping, sliding, etc, in reducing the amplitude of oscillation; it is expressed as a fraction of critical damping (see 5.1.2).

5.1.4 Design Acceleration Spectrum — Design acceleration spectrum refers to an average smoothened graph of maximum acceleration as a function of natural frequency or natural period of oscillation for a specified damping ratio for the expected earthquake excitations at the base of a single degree of freedom system.

5.1.5 Design Horizontal Acceleration Coefficient $(A_{\mathrm{h}})$ — It is a horizontal acceleration coefficient that shall be used for design of structures.

5.1.6 Design Horizontal Force — It is the horizontal seismic force prescribed by this Section that shall be used to design a structure.

5.1.7 Ductility — It is the capacity of a structure (or its members) to undergo large inelastic deformations without significant loss of strength or stiffness.

5.1.8 Epicentre — It is the geographical point on the

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

surface of earth vertically above the point of origin of the earthquake.

5.1.9 Floor Response Spectrum — It is the response spectrum (for a chosen material damping value) of the time history of the shaking generated at a floor of a structure, when the structure is subjected to a given earthquake ground motion at its base.

5.1.10 Importance Factor (I) — It is a factor used to estimate design seismic force depending on the functional use of the structure, characterized by hazardous consequences of its failure, post-earthquake functional needs, historical value, or economic importance.

5.1.11 Intensity of Earthquake — It is the measure of the strength of ground shaking manifested at a place during the earthquake, and is indicated by a roman capital numeral on the MSK scale of seismic intensity (see Annex H).

5.1.12 Liquefaction — It is a state primarily in saturated cohesionless soils wherein the effective shear strength is reduced to negligible value for all engineering purposes, when the pore pressure approaches the total confining pressure during earthquake shaking. In this condition, the soil tends to behave like a fluid mass (see Annex J).

5.1.13 Lithological Features — Features that reflect the nature of the geological formation of the earth's crust above bed rock characterized on the basis of structure, mineralogical composition and grain size.

5.1.14 Modal Mass ( $M_{k}$ ) in Mode (k) of a Structure — It is a part of the total seismic mass of the structure that is effective in natural mode k of oscillation during horizontal or vertical ground motion.

5.1.15 Modal Participation Factor ( $P_{k}$ ) in Mode (k) of a Structure — The amount by which natural mode k contributes to overall oscillation of the structure during horizontal or vertical earthquake ground motion. Since the amplitudes of mode shapes can be scaled arbitrarily, the value of this factor depends on the scaling used for defining mode shapes.

5.1.16 Modes of Oscillation — See 5.1.19.

5.1.17 Mode Shape Coefficient ( $\phi_{ik}$ ) — It is the spatial deformation pattern of oscillation along degree of freedom i, when the structure is oscillating in its natural mode k. A structure with N degrees of freedom possesses N natural periods and N associated natural mode shapes. These natural mode shapes are together presented in the form of a mode shape matrix [ $\phi$ ], in which each column represents one natural mode shape. The element $\phi_{ik}$ is called the mode shape coefficient

associated with degree of freedom $i$ , when the structure is oscillating in mode $k$ .

5.1.18 Natural Period ( $T_{k}$ ) in Mode (k) of Oscillation — The time taken (in second) by the structure to complete one cycle of oscillation in its natural mode k of oscillation.

5.1.18.1 Fundamental lateral translational natural period $(T_{1})$ — It is the longest time taken (in second) by the structure to complete one cycle of oscillation in its lateral translational mode of oscillation in the considered direction of earthquake shaking. This mode of oscillation is called the fundamental lateral translational natural mode of oscillation. A three-dimensional model of a structure will have one such fundamental lateral translational mode of oscillation along each of the two orthogonal plan directions.

5.1.19 Normal Mode of Oscillation — The mode of oscillation in which there are special undamped free oscillations in which all points on the structure oscillate harmonically at the same frequency (or period), such that all these points reach their individual maximum responses simultaneously.

5.1.20 Peak Ground Acceleration — It is the maximum acceleration of the ground in a given direction of ground shaking. Here, the acceleration refers to that of the horizontal motion, unless specified otherwise.

5.1.21 Response Reduction Factor(R) — It is the factor by which the base shear induced in a structure, if it were to remain elastic, is reduced to obtain the design base shear. It depends on the perceived seismic damage performance of the structure, characterized by ductile or brittle deformations, redundancy in the structure, or overstrength inherent in the design process.

5.1.22 Response Spectrum — It is the representation of maximum responses of a spectrum of idealized single degree freedom systems of different natural periods but having the same damping, under the action of the same earthquake ground motion at their bases. The response referred to here can be maximum absolute acceleration, maximum relative velocity, or maximum relative displacement.

5.1.23 Response Acceleration Coefficient of a Structure $(S_{\mathrm{a}}/\mathrm{g})$ — It is a factor denoting the normalized design acceleration spectrum value to be considered for the design of structures subjected to earthquake ground shaking; this value depends on the natural period of oscillation of the structure and damping to be considered in the design of the structure.

5.1.24 Seismic Mass of a Floor — It is the seismic

weight of the floor divided by acceleration due to gravity.

5.1.25 Seismic Mass of a Structure — It is the seismic weight of a structure divided by acceleration due to gravity.

5.1.26 Seismic Weight of a Floor (W) — It is the sum of dead load of the floor, appropriate contributions of weights of columns, walls and any other permanent elements from the storeys above and below, finishes and services, and appropriate amounts of specified imposed load on the floor.

5.1.27 Seismic Weight of a Structure $(W)$ — It is the sum of seismic weights of all floors.

5.1.28 Seismic Zone Factor (Z) — It is the value of peak ground acceleration considered by this Section for the design of structures located in each seismic zone.

5.1.29 Time History Analysis — It is an analysis of the dynamic response of the structure at each instant of time, when its base is subjected to a specific ground motion time history.

5.2Special Terminology for Buildings

5.2.1 The definitions given below shall apply for the purpose of earthquake resistant design of buildings, as enumerated in this Section.

5.2.2 Base — It is the level at which inertia forces generated in the building are considered to be transferred to the ground through the foundation. For buildings with basements, it is considered at the bottommost basement level. For buildings resting on,

a) pile foundations, it is considered to be at the top of pile cap;

b) raft, it is considered to be at the top of raft; and

c) footings, it is considered to be at the top of the footing.

For buildings with combined types of foundation, the base is considered as the bottom-most level of the bases of the constituent individual foundations as per definitions above.

5.2.3 Base Dimension (d) — It is the dimension (in metre) of the base of the building along a direction of shaking.

5.2.4 Centre of Mass (CM) — The point in the floor of a building through which the resultant of the inertia force of the floor is considered to act during earthquake shaking. Unless otherwise stated, the inertia force considered is that associated with the horizontal shaking of the building.

5.2.5 Centre of Resistance (CR)

5.2.5.1 For single storey buildings — It is the point on

the roof of a building through which when the resultant internal resistance acts, the building undergoes,

a) pure translation in the horizontal direction; and

b) no twist about vertical axis passing through the $CR$ .

5.2.5.2 For multi-storey buildings — It is the set of points on the horizontal floors of a multi-storey building through which, when the resultant incremental internal resistances across those floors act, all floors of the building undergo,

a) pure translation in the horizontal direction; and

b) no twist about vertical axis passing through the $CR$ .

5.2.6 Eccentricity

5.2.6.1 Design eccentricity ( $e_{di}$ ) — It is the value of eccentricity to be used for floor i in calculations of design torsion effects.

5.2.6.2 Static eccentricity $(e_{\mathrm{si}})$ — It is the distance between centre of mass (CM) and centre of resistance (CR) of floor $i$ .

5.2.7 Design Seismic Base Shear ( $V_{B}$ ) — It is the horizontal lateral force in the considered direction of earthquake shaking that the structure shall be designed for.

5.2.8 Diaphragm — It is a horizontal or nearly horizontal structural system (for example, reinforced concrete floors and horizontal bracing systems), which transmits lateral forces to vertical elements connected to it.

5.2.9 Height of Floor ( $h_{i}$ ) — It is the difference in vertical elevations (in metre) of the base of the building and top of floor i of the building.

5.2.10 Height of Building (h) — It is the height of building (in metre) from its base to top of roof level,

a) excluding the height of basement storeys, if basement walls are connected with the ground floor slab or basement walls are fitted between the building columns, but

b) including the height of basement storeys, if basement walls are not connected with the ground floor slab and basement walls are not fitted between the building columns.

In step-back buildings, it shall be taken as the average of heights of all steps from the base, weighted with their corresponding floor areas. And, in buildings founded on hill slopes, it shall be taken as the height of the roof from the top of the highest footing level or pile cap level.

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

5.2.11 Horizontal Bracing System — It is a horizontal truss system that serves the same function as a diaphragm.

5.2.12 Joints — These are portions of columns that are common to beams/braces and columns, which frame into columns.

5.2.13 Lateral Force Resisting System — It is part of the structural system, and consists of all structural members that resist lateral inertia forces induced in the building during earthquake shaking.

5.2.14 Moment-Resisting Frame — It is an assembly of beams and columns that resist induced and externally applied forces primarily by flexure.

5.2.14.1 Ordinary moment-resisting frame (OMRF)—It is a moment-resisting frame designed and detailed as per relevant good practices [6-1(7)], but not meeting special detailing requirements for ductile behaviour as per good practices [6-1(8)] or [6-1(9)], as applicable.

5.2.14.2 Special moment-resisting frame (SMRF) — It is a moment-resisting frame designed and detailed as per relevant good practices [6-1(7)], and meeting special detailing requirements for ductile behaviour as per good practices [6-1(8)] or [6-1(9)], as applicable.

5.2.15 Number of Storeys $(n)$ — It is the number of levels of a building above the base at which mass is present in substantive amounts. This,

a) excludes the basement storeys, where basement walls are connected with the ground floor deck or fitted between the building columns; and

b) includes the basement storeys, when they are not so connected.

5.2.16 Core Structural Walls, Perimeter Columns, Outriggers and Belt Truss System — It is a structural system comprising a core of structural walls and perimeter columns, resisting the vertical and lateral loads, with

a) the core structural walls connected to select perimeter column element(s) (often termed outrigged columns) by deep beam elements, known as outriggers, at discrete locations along the height of the building; and

b) the outrigged columns connected by deep beam elements (often known as belt truss), typically at the same level as the outrigger elements.

A structure with this structural system has enhanced lateral stiffness, wherein core structural walls and perimeter columns are mobilized to act with each other

through the outriggers, and the perimeter columns themselves through the belt truss. The global lateral stiffness is sensitive to: flexural stiffness of the core element, the flexural stiffness of the outrigger element(s), the axial stiffness of the outrigged column(s), and the flexural stiffness of the outrigger elements connecting the core structural walls to the perimeter columns.

5.2.17 Principal Plan Axes — These are two mutually perpendicular horizontal directions in plan of a building along which the geometry of the building is oriented.

5.2.18 P- $\Delta$ Effect — It is the secondary effect on shear forces and bending moments of lateral force resisting elements generated under the action of the vertical loads, interacting with the lateral displacement of building resulting from seismic effects.

5.2.19 RC Structural Wall — It is a wall designed to resist lateral forces acting in its own plane.

5.2.19.1 Ordinary RC structural wall — It is a reinforced concrete (RC) structural wall designed and detailed as per good practice [6-1(10)], but not meeting special detailing requirements for ductile behaviour as per good practice [6-1(8)].

5.2.19.2 Special RC structural wall — It is a RC structural wall designed and detailed in accordance with and meeting special detailing requirements for ductile behaviour, as per good practice [6-1(8)].

5.2.20 Storey — It is the space between two adjacent floors.

5.2.20.1 Soft storey — It is one in which the lateral stiffness is less than that in the storey above. The storey lateral stiffness is the total stiffness of all seismic force resisting elements resisting lateral earthquake shaking effects in the considered direction.

5.2.20.2 Weak storey — It is one in which the storey lateral strength [cumulative design shear strength of all structural members other than that of unreinforced masonry (URM) infills] is less than that in the storey above. The storey lateral strength is the total strength of all seismic force resisting elements sharing the lateral storey shear in the considered direction.

5.2.21 Storey Drift — It is the relative displacement between the floors above and/or below the storey under consideration.

5.2.22 Storey Shear ( $V_{i}$ ) — It is the sum of design lateral forces at all levels above the storey i under consideration.

5.2.23 Storey Lateral Shear Strength $(S_{\mathrm{i}})$ — It is the total lateral strength of all lateral force resisting

elements in the storey considered in a principal plan direction of the building.

5.2.24 Storey Lateral Translational Stiffness ( $K_{i}$ ) — It is the total lateral translational stiffness of all lateral force resisting elements in the storey considered in a principal plan direction of the building.

5.2.25 RC Structural Wall Plan Density ( $\rho_{sw}$ ) — It is the ratio of the cross-sectional area at the plinth level of RC structural walls resisting the lateral load and the plinth of the building, expressed as a percentage.

5.3General Principles and Design Criteria

5.3.1 General Principles

5.3.1.1 Ground motion

The characteristics (intensity, duration, frequency content, etc) of seismic ground vibrations expected at any site depend on magnitude of earthquake, its focal depth, epicentral distance, characteristics of the path through which the seismic waves travel, and soil strata on which the structure is founded. The random earthquake ground motions, which cause the structure to oscillate, can be resolved in any three mutually perpendicular directions. The predominant direction of ground vibration is usually horizontal.

Effects of earthquake-induced vertical shaking can be significant for overall stability analysis of structures, especially in structures (a) with large spans, and (b) those in which stability is a criterion for design. Reduction in gravity force due to vertical ground motions can be detrimental particularly in prestressed horizontal members, cantilevered members and gravity structures. Hence, special attention shall be paid to effects of vertical ground motion on prestressed or cantilevered beams, girders and slabs.

5.3.1.2 The response of a structure to ground vibrations depends on (a) type of foundation; (b) materials, form, size and mode of construction of structures; and (c) duration and characteristics of ground motion. This Section specifies design forces for structures founded on rocks or soils, which do not settle, liquefy or slide due to loss of strength during earthquake ground vibrations.

5.3.1.3 Actual forces that appear on structures during earthquakes are much higher than the design forces specified in the Section. Ductility arising from inelastic material behaviour with appropriate design and detailing, and overstrength resulting from the additional reserve strength in structures over and above the design strength are relied upon for the deficit in actual and design lateral loads. In other words,

earthquake resistant design as per this Section relies on inelastic behaviour of structures. But, the maximum ductility that can be realized in structures is limited. Therefore, structures shall be designed for at least the minimum design lateral force specified in this Section.

5.3.1.4 Members and connections of reinforced and prestressed concrete structures shall be designed (as per good practices [6-1(11)] such that premature failure does not occur due to shear or bond. Some provisions for appropriate ductile detailing of RC members are given in good practice [6-1(8)]. Members and their connections of steel structures should be so proportioned that high ductility is obtained in the structure, avoiding premature failure due to elastic or inelastic buckling of any type. Some provisions for appropriate ductile detailing of steel members are given in good practice [6-1(9)].

5.3.1.5 The soil-structure interaction refers to effects of the flexibility of supporting soil-foundation system on the response of structure. Soil-structure interaction may not be considered in the seismic analysis of structures supported on rock or rock-like material at shallow depth.

5.3.1.6 Equipment and other systems, which are supported at various floor levels of a structure, will be subjected to different motions at their support points. In such cases, it may be necessary to obtain floor response spectra for design of equipment and its supports. For details, reference may be made to good practice [6-1(12)].

5.3.1.7 Additions to existing structures

Additions shall be made to existing structures only as follows:

a) An addition that is structurally independent from an existing structure shall be designed and constructed in accordance with the seismic requirements for new structures.

b) An addition that is structurally connected to an existing structure shall be designed and constructed such that the entire structure conforms to the seismic force resistance requirements for new structures, unless the following three conditions are complied with:

Addition shall comply with the requirements for new structures,
Addition shall not increase the seismic forces in any structural element of the existing structures by more than 5 percent, unless the capacity of the element subject to the increased force is

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

still in compliance with this Section, and

Addition shall not decrease the seismic resistance of any structural element of the existing structure unless reduced resistance is equal to or greater than that required for new structures.

5.3.1.8 Change in occupancy

When a change of occupancy results in a structure being reclassified to a higher importance factor (I), the structure shall conform to seismic requirements laid down for new structures with the higher importance factor.

5.3.3Load Combinations and Increase in Permissible Stresses

5.3.3.1 Load combinations

The load combinations shall be considered as specified in respective standards due to all load effects mentioned therein. In addition, those specified in this standard shall be applicable, which include earthquake effects.

5.3.3.1.1 Even when load combinations that do not contain earthquake effects, indicate larger demands than combinations including them, the provisions shall be adopted related to design, ductile detailing and construction relevant for earthquake conditions, which are given in this Section, good practice [6-1(8)] and good practice [6-1(9)].

5.3.3.2 Design horizontal earthquake load

5.3.3.2.1 When lateral load resisting elements are oriented along two mutually orthogonal horizontal directions, structure shall be designed for effects due to full design earthquake load in one horizontal direction at a time, and not in both directions simultaneously.

5.3.3.2.2 When lateral load resisting elements are not oriented along mutually orthogonal horizontal directions [as per 5.4.1 and Sl No. (v) of Table 44], structure shall be designed for the simultaneous effects due to full design earthquake load in one horizontal direction plus 30 percent of design earthquake load along the other horizontal direction. Thus, structure should be designed for the following sets of combinations of earthquake effects:

\begin{array}{l l} \text {a)} & \pm E L _ {\mathrm{X}} \pm 0. 3 E L _ {\mathrm{Y}}, \text {and} \\ \text {b)} & \pm 0. 3 E L _ {\mathrm{X}} \pm E L _ {\mathrm{Y}}, \end{array}

where X and Y are two orthogonal horizontal plan directions. Thus, EL in the load combinations shall be replaced by $(EL_{X} \pm 0.3\ EL_{Y})$ or $(EL_{Y} \pm 0.3\ EL_{X})$ . Hence, the sets of load combinations to be considered shall be as given below:

$1.2\left[DL + IL \pm (EL_{\mathrm{X}} \pm 0.3EL_{\mathrm{Y}})\right]$ and

1. 2 \left[ D L + I L \pm (E L _ {\mathrm{Y}} \pm 0. 3 E L _ {\mathrm{X}}) \right];

$1.5[DL\pm (EL_{\mathrm{X}}\pm 0.3EL_{\mathrm{Y}})]$ and

1. 5 \left[ D L \pm (E L _ {\mathrm{Y}} \pm 0. 3 E L _ {\mathrm{X}}) \right]; \text {and}

$0.9DL\pm 1.5(EL_{\mathrm{X}}\pm 0.3EL_{\mathrm{Y}})$ and

0. 9 D L \pm 1. 5 (E L _ {\mathrm{Y}} \pm 0. 3 E L _ {\mathrm{X}}).

5.3.3.3 Design vertical earthquake effects

5.3.3.3.1 Effects due to vertical earthquake shaking shall be considered when any of the following conditions apply:

a) Structure is located in Seismic Zone IV or V;

b) Structure has vertical or plan irregularities;

c) Structure is rested on soft soil;

d) Bridges;

e) Structure has long spans; or

f) Structure has large horizontal overhangs of structural members or sub-systems.

5.3.3.3.2 When effects due to vertical earthquake shaking are to be considered, the design vertical force shall be calculated for vertical ground motion as detailed in 5.3.4.6.

5.3.3.3.3 Where both horizontal and vertical seismic

forces are taken into account, load combination specified in 5.3.3.4 shall be considered.

5.3.3.4 Combinations to account for three directional earthquake ground shaking

5.3.3.4.1 When responses from the three earthquake components are to be considered, the responses due to each component may be combined using the assumption that when the maximum response from one component occurs, the responses from the other two components are 30 percent each of their maximum. All possible combinations of three components ( $EL_{X}$ , $EL_{Y}$ and $EL_{Z}$ ) including variations in sign (plus or minus) shall be considered. Thus, the structure should be designed for the following sets of combinations of earthquake load effects:

\begin{array}{l l} \text {a)} & \pm E L _ {\mathrm{X}} \pm 0. 3 E L _ {\mathrm{Y}} \pm 0. 3 E L _ {\mathrm{Z}}, \\ \text {b)} & \pm E L _ {\mathrm{Y}} \pm 0. 3 E L _ {\mathrm{Z}} \pm 0. 3 E L _ {\mathrm{X}}, \text {and} \\ \text {c)} & \pm E L _ {\mathrm{Z}} \pm 0. 3 E L _ {\mathrm{X}} \pm 0. 3 E L _ {\mathrm{Y}}, \end{array}

where $X$ and $Y$ are orthogonal plan directions and $Z$ vertical direction. Thus, $EL$ in the above referred load combinations shall be replaced by $(EL_{\mathrm{X}} \pm 0.3 EL_{\mathrm{Y}} \pm 0.3 EL_{\mathrm{Z}})$ , $(EL_{\mathrm{Y}} \pm 0.3 EL_{\mathrm{Z}} \pm 0.3 EL_{\mathrm{X}})$ or $(EL_{\mathrm{Z}} \pm 0.3 EL_{\mathrm{X}} \pm 0.3 EL_{\mathrm{Y}})$ . This implies that the sets of load combinations involving earthquake effects to be considered shall be as given below:

$1.2[\mathrm{DL} + \mathrm{IL}\pm (EL_{\mathrm{x}}\pm 0.3EL_{\mathrm{y}}\pm 0.3EL_{\mathrm{z}})]$ and

1. 2 \left[ \mathrm{DL} + \mathrm{IL} \pm (E L _ {\mathrm{Y}} \pm 0. 3 E L _ {\mathrm{X}} \pm 0. 3 E L _ {\mathrm{Z}}) \right];

$1.5\left[\mathrm{DL}\pm (EL_{\mathrm{X}}\pm 0.3EL_{\mathrm{Y}}\pm 0.3EL_{Z})\right]$ and

1. 5 \left[ \mathrm{DL} \pm (E L _ {\mathrm{Y}} \pm 0. 3 E L _ {\mathrm{X}} \pm 0. 3 E L _ {\mathrm{Z}}) \right];

\begin{array}{r l} 3) & 0. 9 \mathrm{DL} \pm 1. 5 (E L _ {\mathrm{X}} \pm 0. 3 E L _ {\mathrm{Y}} \pm 0. 3 E L _ {\mathrm{Z}}) \text {and} \\ & 0. 9 \mathrm{DL} \pm 1. 5 (E L _ {\mathrm{Y}} \pm 0. 3 E L _ {\mathrm{X}} \pm 0. 3 E L _ {\mathrm{Z}}). \end{array}

5.3.3.4.2 As an alternative to the procedure in 5.3.3.4.1, the net response (EL) due to the combined effect of the three components can be obtained by:

E L = \sqrt {\left(E L _ {\mathrm{X}}\right) ^ {2} + \left(E L _ {\mathrm{Y}}\right) ^ {2} + \left(E L _ {\mathrm{Z}}\right) ^ {2}}

Caution may be exercised on loss of sign especially of the axial force, shear force and bending moment quantities, when this procedure is used; it can lead to grossly uneconomical design of structures.

5.3.3.4.3 Procedure for combining shaking effects given by 5.3.3.4.1 and 5.3.3.4.2 apply to the same response quantity (say, bending moment in a column about its major axis, or storey shear force in a frame) due to different components of the ground motion.

5.3.3.4.4 When components corresponding to only two ground motion components (say one horizontal and one vertical, or only two horizontal) are combined, the equations in 5.3.3.4.1 and 5.3.3.4.2 should be modified by deleting the term representing the response due to the component of motion not being considered.

5.3.3.5 Increase in net pressure on soils in design of foundations

5.3.3.5.1 In the design of foundations, unfactored loads shall be combined in line with good practices [6-1(14)], while assessing the bearing pressure in soils.

5.3.3.5.2 When earthquake forces are included, net bearing pressure in soils can be increased as per Table 40, depending on type of foundation and type of soil. For determining the type of soil for this purpose, soils shall be classified in four types as given in Table 41. In soft soils, no increase shall be applied in bearing pressure, because settlements cannot be restricted by increasing bearing pressure.

5.3.3.5.3 In soil deposits consisting of submerged loose sands and soils falling under classification SP with corrected standard penetration test values N, less than 15 in Seismic Zones III, IV and V, and less than 10 in Seismic Zone II, the shaking caused by earthquake ground motion may cause liquefaction or excessive total and differential settlements. Such sites should be avoided preferably for locating new structures, and should be avoided for locating structures of important projects. Otherwise, settlements need to be investigated, and appropriate methods adopted of compaction or stabilization to achieve N values indicated in Note 4 of Table 40. Alternatively, deep pile foundations may be adopted and anchored at depths well below the underlying soil layers, which are likely to liquefy or undergo excessive settlements.

Also, marine clay layers and other sensitive clay layers are known to liquefy, undergo excessive settlements or even collapse, owing to low shear strength of the said soil; such soils will need special treatment according to site condition (see Table 41).

A simplified method is given in Annex J, for evaluation of liquefaction potential.

5.3.4Design Acceleration Spectrum

5.3.4.1 For the purpose of determining design seismic force, the country is classified into four seismic zones as shown in Fig. 11.

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Table 40Percentage Increase in Net Bearing Pressure and Skin Friction of Soils (Clause 5.3.3.5.2)

Sl No.	Soil Type	Percentage Increase Allowable
(1)	(2)	(3)
i)	Type A: Rock or hard soils	50
ii)	Type B: Medium or stiff soils	25
iii)	Type C: Soft soils	0

NOTES

1 The net bearing pressure shall be determined in accordance with good practice [6-1(15)] or accepted standards [6-1(16)]

2 Only corrected values of N shall be used.

3 If any increase in net bearing pressure has already been permitted for forces other than seismic forces, the total increase in allowable bearing pressure, when seismic force is also included, shall not exceed the limits specified above.

4 The desirable minimum corrected field values of $N$ shall be as specified below:

Seismic Zone	Depth (m) belowGround Level	N Values	Remarks
III, IV and V	$\leq 5$ $\geq 10$	1525	For values of depths between 5 m and 10 m, linear interpolation is recommended
II	$\leq 5$ $\geq 10$	1020

If soils of lower N values are encountered than those specified in the table above, then suitable ground improvement techniques shall be adopted to achieve these values. Alternately, deep pile foundations should be used, which are anchored in stronger strata, underlying the soil layers that do not meet the requirement.

5 Piles should be designed for lateral loads neglecting lateral resistance of those soil layers (if any), which are liable to liquefy.

6 Accepted standard [6-1(17)] may be referred for soil notation, and corrected N values shall be determined by applying correction

factor $C_N$ for effective overburden pressure $\sigma'_{\text{vo}}$ using relation $N = C_N N_1$ , where $C_N = \sqrt{P_\text{s} / \sigma'_{\text{vo}}} \leq 1.7$ , $P_\text{s}$ is the atmospheric pressure, and $N_1$ is the uncorrected SPT value for soil.

7 While using this table, the value of $N$ to be considered shall be determined as below:

a) Isolated footings — Weighted average of N of soil layers from depth of founding, to depth of founding plus twice the breadth of footing;

b) Raft foundations — Weighted average of N of soil layers from deptb of founding, to depth of founding plus twice the breadth of raft;

c) Pile foundation — Weighted average of N of soil layers from depth of bottom tip of pile, to depth of bottom tip of pile plus twice the diameter of pile;

d) Group pile foundation — Weighted average of N of soil layers from depth of bottom tip of pile group, to depth of bottom tip of pile group plus twice the width of pile group; and

e) Well foundation — Weighted average of N of soil layers from depth of bottom tip of well, to depth of bottom tip of well plus twice the width of well.

Table 41Classification of Types of Soils for Determining Percentage Increase in Net Bearing Pressure and Skin Friction (Clause 5.3.3.5.2)

Sl No.(1)	Soil Type(2)	Remarks(3)
i)	Type ARock or hard soils	Well graded gravel (GW) or well graded sand (SW) both with less than 5 percent passing 75 μm sieve (Fines)Well graded gravel - sand mixtures with or without fines (GW-SW)Poorly-graded sand (SP) or Clayey sand (SC), all having N above 30 stiff to hard clays having N above 30, where N is corrected standard penetration test value
ii)	Type BMedium or stiff soils	Poorly graded sands or poorly graded sands with gravel (SP) with little or no fines having N between 10 and 30Stiff to medium stiff fine-grained soils, like silts of low compressibility (ML) or clays of low compressibility (CL) baving N between 10 and 30
iii)	Type CSoft soils	All soft soils other than SP with N<10. The various possible soils are:a) Silts of intermediate compressibility (MI);b) Silts of high compressibility (MH);c) Clays of intermediate compressibility (CI);d) Clays of high compressibility (CH);e) Silts and clays of intermediate to high compressibility (MI-MH or CI-CH);f) Silt with clay of intermediate compressibility (MI-CI); andg) Silt with clay of high compressibility (MH-CH).
iv)	Type DUnstable, collapsible, liquefiable soils	Requires site-specific study and special treatment according to site condition (see 5.3.3.5.3)

Based upon Survey of India Political map printed in 2002. The territorial waters of India extend into the sea to a distance of twelve nautical miles measured from the appropriate baseline. The interstate boundaries between Arunachal Pradesh, Assam and Meghalaya shown on this map are as interpreted from the North-Eastern Areas (Reorganization) Act, 1971, but have yet to be verified. The state boundaries between Uttarakhand & Uttar Pradesh, Bihar & Jharkhand, and Chhattisgarh & Madhya Pradesh have not been verified by the Governments concerned. The administrative headquarters of Chandigarh, Haryana and Punjab are at Chandigarh. The external boundaries and coastlines of India agree with the Record/Master Copy certified by Survey of India. The responsibility for the correctness of internal details rests with the publisher. NOTE – Towns falling at the boundary of zones demarcation line between two zones shall be considered in higher zone.

FIG. 11 SEISMIC ZONES OF INDIA

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5.3.4.2 The design horizontal seismic coefficient $A_{\mathrm{h}}$ for a structure shall be determined by:

A _ {\mathrm{h}} = \frac {\left(\frac {Z}{2}\right) \left(\frac {S _ {\mathrm{a}}}{\mathrm{g}}\right)}{\left(\frac {R}{I}\right)}

where

$Z =$ seismic zone factor given in Table 42. Zone factor for some important towns are given at Annex K;

$I =$ importance factor given in good practice [6-1(18)] for the corresponding structures; when not specified, the minimum values of $I$ shall be,

a) 1.5 for critical and lifeline structures;

b) 1.2 for business continuity structures; and

c) 1.0 for the rest.

$R =$ response reduction factor given in good practice [6-1(18)] for the corresponding structures; and

$\left(\frac{S_{\mathrm{a}}}{\mathrm{g}}\right)=$ design acceleration coefficient for different soil types, normalized with peak ground acceleration,

corresponding to natural period $T$ of structure (considering soil-structure interaction, if required). It shall be as given in good practice [6-1(18)] for the corresponding structures; when not specified, it shall be taken as that corresponding to 5 percent damping, given by expressions below:

a) For use in equivalent static method (see Fig. 12A):

\frac {S _ {\mathrm{a}}}{g} = \left\{ \begin{array}{l l} \text {For rocky or hard soil sites} & \left\{ \begin{array}{l l} 2. 5 & 0 < T < 0. 4 0 \mathrm{s} \\ \frac {1}{T} & 0. 4 0 \mathrm{s} < T < 4. 0 0 \mathrm{s} \\ 0. 2 5 & T > 0. 4 0 0 \mathrm{s} \end{array} \right. \\ \text {For medium stiff soil sites} & \left\{ \begin{array}{l l} 2. 5 & 0 < T < 0. 5 5 \mathrm{s} \\ \frac {1 . 3 6}{T} & 0. 5 5 \mathrm{s} < T < 4. 0 0 \mathrm{s} \\ 0. 3 4 & T > 0. 4 0 0 \mathrm{s} \end{array} \right. \\ \text {For soft soil sites} & \left\{ \begin{array}{l l} 2. 5 & 0 < T < 0. 6 7 \mathrm{s} \\ \frac {1 . 6 7}{T} & 0. 6 7 \mathrm{s} < T < 4. 0 0 \mathrm{s} \\ 0. 4 2 & T > 0. 4 0 0 \mathrm{s} \end{array} \right. \end{array} \right.

b) For use in response spectrum method (see Fig. 12B):

\left\{ \begin{array}{l l} 1 + 1 5 T & T < 0. 1 0 \mathrm{s} \\ 2. 5 & 0. 1 0 \mathrm{s} < T < 0. 4 0 \mathrm{s} \\ \frac {1}{T} & 0. 4 0 \mathrm{s} < T < 4. 0 0 \mathrm{s} \\ 0. 2 5 & T > 4. 0 0 \mathrm{S} \end{array} \right.

\left\{ \begin{array}{l l} 1 + 1 5 T & T < 0. 1 0 \mathrm{s} \\ 2. 5 & 0. 1 0 \mathrm{s} < T < 0. 5 5 \mathrm{s} \\ \frac {1 . 3 6}{T} & 0. 5 5 \mathrm{s} < T < 4. 0 0 \mathrm{s} \\ 0. 3 4 & T > 4. 0 0 \mathrm{s} \end{array} \right.

\left\{ \begin{array}{l l} 2. 5 & 0. 1 0 \mathrm{s} < T < 0. 6 7 \mathrm{s} \\ \frac {1 . 6 7}{T} & 0. 6 7 \mathrm{s} < T < 4. 0 0 \mathrm{s} \\ 0. 2 5 & T > 0. 4 0 0 \mathrm{s} \end{array} \right.

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Chart

12A SPECTRA FOR EQUIVALENT STATIC METHOD

Chart

12B SPECTRA FOR RESPONSE SPECTRUM METHOD

FIG. 12 DESIGN ACCELERATION COEFFICIENT $(S_{\mathrm{a}} / \mathrm{g})$ (CORRESPONDING TO 5 PERCENT DAMPING)

Table 42Seismic Zone Factor Z (Clause 5.3.4.2)

Seismic Zone Factor(1)	II(2)	III(3)	IV(4)	V(5)
Z	0.10	0.16	0.24	0.36

5.3.4.2.1 For determining the correct spectrum to be used in the estimate of $S_{\mathrm{a}} / g$ , the type of soil on which the structure is placed shall be identified by the classification given in Table 43, as:

a) Soil type I — Rock or hard soils;

b) Soil type II — Medium or stiff soils; and

c) Soil type III — Soft soils.

In Table 43, the value of $N$ to be used shall be the weighted average of $N$ of soil layers from the existing ground level, to $30\mathrm{m}$ below the existing ground level; here, the $N$ values of individual layers shall be the corrected values.

5.3.4.3 Effects of design earthquake loads applied on structures can be considered in two ways, namely:

a) Equivalent static method, and

b) Dynamic analysis method.

In turn, dynamic analysis can be performed in three ways, namely:

Response spectrum method,
Modal time history method, and
Time history method.

In this Section, equivalent static method, response spectrum method and time history method are adopted. Equivalent static method may be used for analysis of regular structures with approximate natural period $T_{a}$ less than 0.4 s.

5.3.4.3.1 For structural analysis, the moment of inertia shall be taken as:

a) In RC and masonry structures: 70 percent of $I_{\mathrm{gross}}$ of columns, and 35 percent of $I_{\mathrm{gross}}$ of beams; and

b) In steel structures: $I_{\mathrm{gross}}$ of both beams and columns.

5.3.4.4 Where a number of modes are to be considered in response spectrum method, $A_{\mathrm{h}}$ as defined in 5.3.4.2 for each mode $k$ shall be determined using natural period $T_{\mathrm{k}}$ of oscillation of that mode.

Table 43Classification of Types of Soils for Determining the Spectrum to be Used to Estimate Design Earthquake Force (Clause 5.3.4.2.1)

Sl No.	Soil Type	Remarks
(1)	(2)	(3)
i)	IRock or Hard soils	a) Well graded gravel (GW) or well graded sand (SW) both with less than 5 percent passing 75 μm sieve (Fines)b) Well graded gravel-sand mixtures with or without fines (GW-SW)c) Poorly graded sand (SP) or clayey sand (SC), all having N above 30d) Stiff to hard clays having N above 30, where N is standard penetration test value
ii)	IIMedium or Stiff soils	a) Poorly graded sands or poorly graded sands with gravel (SP) with little or no fines having N between 10 and 30b) Stiff to medium stiff fine-grained soils, like silts of low compressibility (ML) or clays of low compressibility (CL) having N between 10 and 30
iii)	IIISoft soils	All soft soils other than SP with N<10.The various possible soils are:a) Silts of intermediate compressibility (MI)b) Silts of high compressibility (MH);c) Clays of intermediate compressibility (CI)d) Clays of high compressibility (CH);e) Silts and clays of intermediate to high compressibility (MI-MH or CI-CH)d) Silt with clay of intermediate compressibility (MI-CI)g) Silt with clay of high compressibility (MH-CH).

5.3.4.5 For underground structures and buildings whose base is located at depths of $30\mathrm{m}$ or more, $A_{\mathrm{h}}$ at the base shall be taken as half the value obtained from 5.3.4.2. This reduced value shall be used only for estimating inertia effects due to masses at the corresponding levels below the ground; the inertia effects for the above ground portion of the building shall be estimated based on the unreduced value of $A_{\mathrm{h}}$ . For estimating inertia effects due to masses of structures and foundations placed between the ground level and $30\mathrm{m}$ depth, the design horizontal acceleration spectrum value shall be linearly interpolated between $A_{\mathrm{h}}$ and $0.5A_{\mathrm{h}}$ where $A_{\mathrm{h}}$ is as specified in 5.3.4.2.

5.3.4.6 The design seismic acceleration spectral value $A_{\mathrm{v}}$ for vertical motions shall be taken as:

A _ {v} = \left\{ \begin{array}{l l} \frac {\left(\frac {2}{3} \times \frac {Z}{2}\right) (2 . 5)}{\left(\frac {R}{I}\right)} & \text {For buildings governed by IS 1893 (Part 1)} \\ \frac {\left(\frac {2}{3} \times \frac {Z}{2}\right) (2 . 5)}{\left(\frac {R}{I}\right)} & \text {For liquid retaining tanks governed by IS 1893 (Part 2)} \\ \frac {\left(\frac {2}{3} \times \frac {Z}{2}\right) \left(\frac {S _ {\mathrm{a}}}{\mathrm{g}}\right)}{\left(\frac {R}{I}\right)} & \text {For bridges governed by IS 1893 (Part 3)} \\ \frac {\left(\frac {2}{3} \times \frac {Z}{2}\right) \left(\frac {S _ {\mathrm{a}}}{\mathrm{g}}\right)}{\left(\frac {R}{I}\right)} & \text {For industrial structures governed by IS 1893 (Part 4)} \end{array} \right.

The value of $S_{\mathrm{a}} / g$ shall be based on natural period $T$ corresponding to the first vertical mode of oscillation, using 5.3.4.2.

5.3.4.7 When design acceleration spectrum is developed specific to a project site, the same may be used for design of structures of the project. In such cases, effects of the site-specific spectrum shall not be less than those arising out of the design spectrum specified in this Section.

5.4Buildings

The four main desirable attributes of an earthquake resistant building are:

a) Robust structural configuration,

b) At least a minimum elastic lateral stiffness,

c) At least a minimum lateral strength, and

d) Adequate ductility.

5.4.1 Regular and Irregular Configurations

Buildings with simple regular geometry and uniformly distributed mass and stiffness in plan and in elevation, suffer much less damage, than buildings with irregular configurations. All efforts shall be made to eliminate irregularities by modifying architectural planning and structural configurations. A building shall be considered to be irregular for the purpose of this Section, even if any one of the conditions given in Tables 44 and 45 is applicable. Limits on irregularities for Seismic Zones III, IV and V and special requirements are laid out in Tables 44 and 45.

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Table 44 Definitions of Irregular Buildings —Plan Irregularities (see Fig. 13)(Clause 5.4.1)
SI No.(1)	Type of Plan Irregularity(2)
i) Torsional IrregularityUsually, a well-proportioned building does not twist about its vertical axis, whena) the stiffness distribution of the vertical elements resisting lateral loads is balanced in plan according to the distribution of mass in plan (at each storey level); andb) the floor slabs are stiff in their own plane (this happens when its plan aspect ratio is less than 3)A building is said to be torsionally irregular, when,1) the maximum horizontal displacement of any floor in the direction of the lateral force at one end of the floor is more than 1.5 times its minimum horizontal displacement at the far end of the same floor in that direction; and2) the natural period corresponding to the fundamental torsional mode of oscillation is more than those of the first two translational modes of oscillation along each principal plan directionsIn torsionally irregular buildings, when the ratio of maximum horizontal displacement at one end and the minimum horizontal displacement at the other end is,i) in the range 1.5 - 2.0, (a) the building configuration shall be revised to ensure that the natural period of the fundamental torsional mode of oscillation shall be smaller than those of the first two translational modes along each of the principal plan directions, and then (b) three dimensional dynamic analysis method shall be adopted; andii) more than 2.0, the building configuration shall be revised
ii) Re-entrant CornersA building is said to have a re-entrant corner in any plan direction, when its structural configuration in plan has a projection of size greater than 15 percent of its overall plan dimension in that directionIn buildings with re-entrant corners, three-dimensional dynamic analysis method shall be adoptediii) Floor Slabs having Excessive Cut-Outs or OpeningsOpenings in slabs result in flexible diaphragm behaviour, and hence the lateral shear force is not shared by the frames and/or vertical members in proportion to their lateral translational stiffness. The problem is particularly accentuated when the opening is close to the edge of the slab. A building is said to have discontinuity in their in-plane stiffness, when floor slabs have cut-outs or openings of area more than 50 percent of the full area of the floor slabIn buildings with discontinuity in their in-plane stiffness, if the area of the geometric cut-out is,a) less than or equal to 50 percent, the floor slab shall be taken as rigid or flexible depending on the location of and size of openings; andb) more than 50 percent, the floor slab shall be taken as flexibleiv) Out-of-Plane Offsets in Vertical ElementsOut-of-plane offsets in vertical elements resisting lateral loads cause discontinuities and detours in the load path, which is
Table 44 — (Concluded)known to be detrimental to the earthquake safety of the building. A building is said to have out-of-plane offset in vertical elements, when structural walls or frames are moved out of plane in any storey along the height of the buildingIn a building with out-of-plane offsets in vertical elements,a) specialist literature shall be referred for design of such a building, if the building is located in Seismic Zone II; andb) the following two conditions shall be satisfied, if the building is located in Seismic Zones III, IV and V:1) Lateral drift shall be less than 0.2 percent in the storey having the offset and in the storeys below; and2) Specialist literature shall be referred for removing the irregularity arising due to out-of-plane offsets in vertical elements.v) Non-Parallel Lateral Force SystemBuildings undergo complex earthquake behaviour and hence damage, when they do not have lateral force resisting systems oriented along two plan directions that are orthogonal to each other. A building is said to have non-parallel system when the vertically oriented structural systems resisting lateral forces are not oriented along the two principal orthogonal axes in planBuildings with non-parallel lateral force resisting system shall be analysed for load combinations mentioned in 5.3.3.2.2 or 5.3.3.4.1.
Table 45 Definition of Irregular Buildings —Vertical Irregularities (see Fig. 14)(Clause 5.4.1)
SI No.(1)	Type of Plan Irregularity(2)
i) Stiffness Irregularity (Soft Storey)A soft storey is a storey whose lateral stiffness is less than that of the storey above.The structural plan density (SPD) shall be estimated when unreinforced masonry infills are used. When SPD of masonry infills exceeds 20 percent, the effect of URM infills shall be considered by explicitly modelling the same in structural analysis (as per 5.4.9). The design forces for RC members shall be larger of that obtained from analysis of:a) Bare frame, andb) Frames with URM infills,using 3D modelling of the structure. In buildings designed considering URM infills, the inter-storey drift shall be limited to 0.2 percent in the storey with stiffening and also in all storeys below.ii) Mass IrregularityMass irregularity shall be considered to exist, when the seismic weight (as per 5.4.7) of any floor is more than 150 percent of that of the floors below.In buildings with mass irregularity and located in Seismic Zones III, IV and V, the earthquake effects shall be estimated by Dynamic Analysis Method (as per 5.4.7).iii) Vertical Geometric IrregularityVertical geometric irregularity shall be considered to exist, when the horizontal dimension of the lateral force resisting

Table 45 — (Continued)		Table 45 — (Concluded)
(1)	(2)	(1)	(2)
	system in any storey is more than 125 percent of the storey below.In buildings with vertical geometric irregularity and located in Seismic Zones III, IV and V, the earthquake effects shall be estimated by Dynamic Analysis Method (as per 5.4.7).	This feature is undesirable, and hence should be prohibited, if it is part of or supporting the primary lateral load resisting system.vii) Irregular Modes of Oscillation in Two Principal Plan DirectionsStiffnesses of beams, columns, braces and structural walls determine the lateral stiffness of a building in each principal plan direction. A building is said to have lateral storey irregularity in a principal plan direction, ifa) the first three modes contribute less than 65 percent mass participation factor in each principal plan direction, andb) the fundamental lateral natural periods of the building in the two principal plan directions are closer to each other by 10 percent of the larger value.In buildings located in Seismic Zones II and III, it shall be ensured that the first three modes together contribute at least 65 percent mass participation factor in each principal plan direction. And, in buildings located in Seismic Zones IV and V, it shall be ensured that,1) the first three modes together contribute at least 65 percent mass participation factor in each principal plan direction, and2) the fundamental lateral natural periods of the building in the two principal plan directions are away from each other by at least 10 percent of the larger value.
iv)	In-Plane Discontinuity in Vertical Elements Resisting Lateral ForceIn-plane discontinuity in vertical elements which are resisting lateral force shall be considered to exist, when in-plane offset of the lateral force resisting elements is greater than 20 percent of the plan length of those elements.In buildings with in-plane discontinuity and located in Seismic Zones II, the lateral drift of the building under the design lateral force shall be limited to 0.2 percent of the building height; in Seismic Zones III, IV and V, buildings with in-plane discontinuity shall not be permitted.
v)	Strength Irregularity (Weak Storey)A weak storey is a storey whose lateral strength is less than that of the storey above.In such a case, buildings in Seismic Zones III, IV and V shall be designed such that safety of the building is not jeopardized; also, provisions of 5.4.10 shall be followed.
vi)	Floating or Stub ColumnsSuch columns are likely to cause concentrated damage in the structure.

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Chart

OPENING LOCATED ANYWHERE IN THE SLAB

PLAN

Chart

OPENING LOCATED ALONG ANY EDGE OF THE SLAB

PLAN

13C FLOOR SLABS HAVING EXCESSIVE CUT-OUT AND OPENINGS

13D OUT-OF-PLANE OFFSETS IN VERTICAL ELEMENTS

13E NON-PARALLEL LATERAL FORCE SYSTEM: (i) MOMENT FRAME BUILDING, and (ii) MOMENT FRAME BUILDING WITH STRUCTURAL WALLS

FIG. 13 DEFINITIONS OF IRREGULAR BUILDINGS — PLAN IRREGULARITIES

Chart

14A STIFFNESS IRREGULARITY (SOFT STOREY)

ELEVATION

14B MASS IRREGULARITY

FIG. 14 — (Continued)

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Chart

14D IN-PLANE DISCONTINUITY IN VERTICAL ELEMENTS RESISTING LATERAL FORCE

Chart

14E STRENGTH IRREGULARITY (WEAK STOREY)

FIG. 14 DEFINITIONS OF IRREGULAR BUILDINGS — VERTICAL IRREGULARITIES

5.4.2Lateral Force

5.4.2.1 Design lateral force

Buildings shall be designed for the design lateral force $V_{\mathrm{B}}$ given by:

V _ {\mathrm{B}} = A _ {\mathrm{h}} W

where $A_{\mathrm{h}}$ shall be estimated as per 5.3.4.2, and $W$ as per 5.4.4.

5.4.2.2 Minimum design lateral force

Buildings and portions there of shall be designed and constructed to resist at least the effects of design lateral force specified in 5.4.2.1. But, regardless of design earthquake forces arrived at as per 5.4.3.1, buildings shall have lateral load resisting systems capable of resisting a horizontal force not less than $(V_{\mathrm{B}})_{\mathrm{min}}$ as given in Table 46.

Table 46Minimum Design Earthquake Horizontal Lateral Force for Buildings (Clause 5.4.2.2)

SlNo.(1)	Seismic Zone(2)	ρpercent(3)
i)	II	0.7
ii)	III	1.1
iii)	IV	1.6
iv)	V	2.4

5.4.2.3 Importance factor (I)

In estimating design lateral force $V_{\mathrm{B}}$ of buildings as per 5.4.2.1, the importance factor $I$ of buildings shall be taken as per Table 47.

Table 47Importance Factor (I) (Clause 5.4.2.3)

Sl No.	Structure	I
(1)	(2)	(3)
i)	Important service and community buildings or structures (for example critical governance buildings, schools), signature buildings, monument buildings, lifeline and emergency buildings (for example hospital buildings, telephone exchange buildings, television station buildings, radio station buildings, bus station buildings, metro rail buildings and metro rail station buildings), railway stations, airports, food storage buildings (such as warehouses), fuel station buildings, electric power station buildings, and fire station buildings), and large community hall buildings (for example cinema halls, shopping malls, assembly halls and subway stations) and power station buildings	1.5
ii)	Residential or commercial buildings [other than those listed in Sl No. (i)] with occupancy more than 200 persons	1.2
iii)	All other buildings	1.0
NOTES
1 Owners and design engineers of buildings or structures may choose values of importance factor I more than those mentioned above.
2 Buildings or structures covered under Sl No. (iii) may be designed for higher value of importance factor I, depending on economy and strategy.
3 In Sl No. (ii), when a building is composed of more than one structurally independent unit, the occupancy size shall be for each of the structurally independent unit of the building.
4 In buildings with mixed occupancies, wherein different I factors applicable for the respective occupancies, larger of the importance factor I values shall be used for estimating the design earthquake force of the building.

5.4.2.4 Damping ratio

The value of damping shall be taken as 5 percent of critical damping for the purposes of estimating $A_{h}$ in the design lateral force $V_{B}$ of a building as per 5.4.2.1, irrespective of the material of construction (namely steel, reinforced concrete, masonry, or a combination thereof of these three basic materials) of its lateral load resisting system, considering that buildings experience inelastic deformations under design level earthquake effects, resulting in much higher energy dissipation than that due to initial structural damping in buildings. This value of damping shall be used, irrespective of the method of the structural analysis employed, namely Equivalent Static Method (as per 5.4.6) or Dynamic Analysis Method (as per 5.4.7).

5.4.2.5 Design acceleration spectrum

Design acceleration coefficient $S_{\mathrm{a}} / \mathrm{g}$ corresponding to 5 percent damping for different soil types, normalized to peak ground acceleration, corresponding to natural period $T$ of structure considering soil-structure interaction,

irrespective of the material of construction of the structure. $S_{\mathrm{a}} / \mathrm{g}$ shall be as given by expressions in 5.3.4.2.

5.4.2.6 Response reduction factor (R)

Response reduction factor, along with damping during extreme shaking and redundancy: (a) influences the nonlinear behaviour of buildings during strong earthquake shaking, and (b) accounts for inherent system ductility, redundancy and overstrength normally available in buildings, if designed and detailed as per this Section and the associated Indian Standards.

For the purpose of design as per this Section, response reduction factor $R$ for different building systems shall be as given in Table 48. The values of $R$ shall be used for design of buildings with lateral load resisting elements, and NOT for just the lateral load resisting elements, which are built in isolation.

5.4.2.7 Dual system

Buildings with dual system consist of moment resisting frames and structural walls (or of moment resisting frames and bracings) such that both of the following conditions are valid:

a) Two systems are designed to resist total design lateral force in proportion to their lateral stiffness, considering interaction of two systems at all floor levels; and

b) Moment resisting frames are designed to resist independently at least 25 percent of the design base shear.

5.4.3 Design Imposed Loads for Earthquake Force Calculation

5.4.3.1 For various loading classes specified in 3, design seismic force shall be estimated using full dead load plus percentage of imposed load as given in Table 49. The same shall be used in the three-dimensional dynamic analysis of buildings also.

5.4.3.2 For calculation of design seismic forces of buildings, imposed load on roof need not be considered. But, weights of equipment and other permanently fixed facilities should be considered; in such a case, the reductions of imposed loads mentioned in Table 49 are not applicable to that part of the load.

5.4.3.3 Imposed load values indicated in Table 49 for calculating design earthquake lateral forces are applicable to normal conditions. When loads during earthquakes are more accurately assessed, designers may alter imposed load values indicated or even replace the entire imposed load given in Table 49 with actual assessed load values, subject to the values given in Table 46 as the minimum values. Where imposed load is not assessed as per 5.4.3.1 and 5.4.3.2,

a) only that part of imposed load, which possesses mass, shall be considered; and

b) lateral earthquake design force shall not be calculated on contribution of impact effects from imposed loads.

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Table 48Response Reduction Factor R for Building Systems (Clause 5.4.2.6)

Sl No.(1)	Lateral Load Resisting System(2)	R(3)
i)	Moment Frame Systems
	a) RC buildings with ordinary moment resisting frame (OMRF) (see Note 1)	3.0
	b) RC buildings with special moment resisting frame (SMRF)	5.0
	c) Steel buildings with ordinary moment resisting frame (OMRF) (see Note 1)	3.0
	d) Steel buildings with special moment resisting frame (SMRF)	5.0
ii)	Braced Frame Systems (see Note 2)
	a) Buildings with ordinary braced frame (OBF) having concentric braces	4.0
	b) Buildings with special braced frame (SBF) having concentric braces	4.5
	c) Buildings with special braced frame (SBF) having eccentric braces	5.0
iii)	Structural Wall Systems (see Note 3)
	a) Load bearing masonry buildings
	1) Unreinforced masonry {designed as per good practice [6-1(19)]} without horizontal RC seismic bands (see Note 1)	1.5
	2) Unreinforced masonry {designed as per good practice [6-1(19)]} with horizontal RC seismic bands	2.0
	3) Unreinforced masonry {designed as per good practice [6-1(19)]} with horizontal RC seismic bands and vertical reinforcing bars at corners of rooms and jambs of openings {with reinforcement as per good practice [6-1(20)]}	2.5
	4) Reinforced masonry [see Part 6 'Structural Design, Section 4 Masonry' of the Code]	3.0
	5) Confined masonry	3.0
	b) Buildings with ordinary RC structural walls (see Note 1)	3.0
	c) Buildings with ductile RC structural walls	4.0
iv)	Dual Systems (see Note 3)
	a) Buildings with ordinary RC structural walls and RC OMRFs (see Note 1)	3.0
	b) Buildings with ordinary RC structural walls and RC SMRFs (see Note 1)	4.0
	c) Buildings with ductile RC structural walls with RC OMRFs (see Note 1)	4.0
	d) Buildings with ductile RC structural walls with RC SMRFs	5.0
v)	Flat Slab - Structural Wall Systems (see Note 4)
	RC building with the three features given below:	3.0
	a) Ductile RC structural walls (which are designed to resist 100 percent of the design lateral force),b) Perimeter RC SMRFs (which are designed to independently resist 25 percent of the design lateral force), and preferablyc) An outrigger and belt truss system connecting the core ductile RC structural walls and the perimeter RC SMRFs (see Note 1).
NOTES
1 RC and steel structures in Seismic Zones III, IV and V shall be designed to be ductile. Hence, this system is not allowed in these seismic zones.2 Eccentric braces shall be used only with SBFs.3 Buildings with structural walls also include buildings having structural walls and moment frames, but where,a) frames are not designed to carry design lateral loads, orb) frames are designed to carry design lateral loads, but do not fulfill the requirements of 'dual systems'.4 In these buildings, (a) punching shear failure shall be avoided, and (b) lateral drift at the roof under design lateral force shall not exceed 0.1 percent.

Table 49Percentage of Imposed Load to be Considered in Calculation of Seismic Weight (Clauses 5.4.3.1)

Sl No.	Imposed Uniformity Distributed Floor Loads kN/m2	Percentage of Imposed Load
(1)	(2)	(3)
i)	Up to and including 3.0	25
ii)	Above 3.0	50

5.4.3.4 Loads other than those given above (for example snow and permanent equipment) shall be considered appropriately.

5.4.3.5 In regions of severe snow loads and sand storms exceeding intensity of $1.5 \, kN/m^{2}$ , 20 percent of uniform design snow load or sand load respectively, shall be included in the estimation of seismic weight. In case the minimum values of seismic weights corresponding to these load effects given in 2, 3, 4, 6 and 7 are higher, the higher values shall be used.

5.4.3.6 In buildings that have interior partitions, the weight of these partitions on floors shall be included in the estimation of seismic weight; this value shall not

be less than $0.5 \, kN/m^{2}$ . In case the minimum values of seismic weights corresponding to partitions given in 2, 3, 4, 6 and 7 are higher, the higher values shall be used. It shall be ensured that the weights of these partitions shall be considered only in estimating inertial effects of the building.

5.4.4 Seismic Weight

5.4.4.1 Seismic weight of floors

Seismic weight of each floor is its full dead load plus appropriate amount of imposed load, as specified in 5.4.3. While computing the seismic weight of each floor, the weight of columns and walls in any storey shall be appropriately apportioned to the floors above and below the storey.

5.4.4.2 Any weight supported in between storeys shall be distributed to floors above and below in inverse proportion to its distance from the floors.

5.4.6Equivalent Static Method

As per this method, first, the design base shear $V_{B}$ shall be computed for the building as a whole. Then, this $V_{B}$ shall be distributed to the various floor levels at the corresponding centre of mass. And, finally, this design seismic force at each floor level shall be distributed to individual lateral load resisting elements through structural analysis considering the floor diaphragm action. This method shall be applicable for regular buildings with height less than 15 m in Seismic Zone II.

5.4.6.1 The design base shear $V_{\mathrm{B}}$ along any principal direction of a building shall be determined by:

V _ {\mathrm{B}} = A _ {\mathrm{h}} W

where

$A_{h}$ = design horizontal acceleration coefficient value as per 5.3.4.2, using approximate fundamental natural period $T_{a}$ as per 5.4.6.2 along the considered direction of shaking; and

$W =$ seismic weight of the building as per 5.4.4.2.

5.4.6.2 The approximate fundamental translational natural period $T_{\mathrm{a}}$ of oscillation, in second, shall be estimated by the following expressions:

a) Bare MRF buildings (without any masonry infills):

T _ {\mathrm{a}} = \left\{ \begin{array}{l l} 0. 0 7 5 h ^ {0. 7 5} & (\text {for RC MRF building}) \\ 0. 0 8 0 h ^ {0. 7 5} & (\text {for RC - Steel Composite MRF building}) \\ 0. 0 8 5 h ^ {0. 7 5} & (\text {for steel MRF building}) \end{array} \right.

where

h = height (in m) of building (see Fig. 15). This excludes the basement storeys, where basement storey, walls are connected with the ground floor deck or fitted between the building columns, but includes the basement storeys, when they are not so connected.

b) Buildings with RC structural walls:

T _ {\mathrm{a}} = \frac {0 . 0 7 5 h ^ {0 . 7 5}}{\sqrt {A _ {\mathrm{w}}}} \geq \frac {0 . 0 9 h}{\sqrt {d}}

where $A_{\mathrm{w}}$ is total effective area $(\mathrm{m}^2)$ of walls in the first storey of the building given by:

A _ {\mathrm{w}} = \sum_ {i = 1} ^ {N _ {\mathrm{w}}} \left[ A _ {\mathrm{wi}} \left\{0. 2 + \left(\frac {L _ {\mathrm{wi}}}{h}\right) ^ {2} \right\} \right] \tag {1}

where

$h =$ height of building as defined in 5.4.6.2(a), in m;

$A_{\mathrm{wi}} =$ effective cross-sectional area of wall $i$ in first storey of building, in $\mathrm{m}^2$ ;

$L_{wi}$ = length of structural wall i in first storey in the considered direction of lateral forces, in m;

d = base dimension of the building at the plinth level along the considered direction of earthquake shaking, in m; and

$N_{w}$ = number of walls in the considered direction of earthquake shaking.

The value of $L_{\mathrm{wi}} / h$ to be used in this equation shall not exceed 0.9.

c) All other buildings:

T _ {\mathrm{a}} = \frac {0 . 0 9 h}{\sqrt {d}}

where

$h =$ height of building, as defined in 5.4.6.2(a), in m; and

$d =$ base dimension of the building at the plinth level along the considered direction of earthquake shaking, in m.

5.4.6.3 The design base shear $(V_{\mathrm{B}})$ computed in 5.4.6.1 shall be distributed along the height of the building and in plan at each floor level as below:

a) Vertical distribution of base shear to different floor levels — The design base shear $V_{B}$ computed in 5.4.6.1 shall be distributed along the height of the building as per the following expression:

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

FIG. 15 DEFINITIONS OF HEIGHT AND BASE WIDTH OF BUILDINGS

Q _ {\mathrm{i}} = \left(\frac {W _ {\mathrm{i}} h _ {\mathrm{i}} ^ {2}}{\sum_ {j = 1} ^ {n} W _ {\mathrm{j}} h _ {\mathrm{j}} ^ {2}}\right) V _ {\mathrm{B}},

where

$Q_{\mathrm{i}} =$ design lateral force at floor $i$ ;

$W_{\mathrm{i}} =$ seismic weight of floor $i$

$h_{i} =$ height of floor $i$ measured from base; and

$n =$ number of storeys in building, that is, number of levels at which masses are located.

b) In-plan distribution of design lateral force at floor i to different lateral force resisting elements — The design storey shear in any storey shall be calculated by summing the design lateral forces at all floor above that storey. In buildings whose floors are capable of providing rigid horizontal diaphragm action in their own plane, the design storey shear shall be distributed to the various vertical elements of lateral force resisting system in proportion to the lateral stiffness of these vertical elements.

5.4.6.4 Diaphragm

In buildings whose floor diaphragms cannot provide rigid horizontal diaphragm action in their own plane, design storey shear shall be distributed to the various vertical elements of lateral force resisting system considering the in-plane flexibility of the diaphragms.

A floor diaphragm shall be considered to be flexible, if it deforms such that the maximum lateral displacement measured from the chord of the deformed shape at any point of the diaphragm is more than 1.2 times the average displacement of the entire diaphragm (see Fig. 16).

Usually, reinforced concrete monolithic slab-beam floors or those consisting of prefabricated or precast elements with reasonable reinforced screed concrete (at least a minimum of 50 mm on floors and of 75 mm on roof, with at least a minimum reinforcement of 6 mm bars spaced at 150 mm centres) as topping, and of plan aspect ratio less than 3, can be considered to be providing rigid diaphragm action.

5.4.7Dynamic Analysis Method

5.4.7.1 Linear dynamic analysis shall be performed to obtain the design lateral force (design seismic base shear, and its distribution to different levels along the

FIG. 16 DEFINITION OF FLEXIBLE FLOOR DIAPHRAGM

height of the building, and to various lateral load resisting elements) for all buildings, other than regular buildings lower than $15\mathrm{m}$ in Seismic Zone II.

5.4.7.2 The analytical model for dynamic analysis of buildings with unusual configuration should be such that it adequately represents irregularities present in the building configuration.

5.4.7.3 Dynamic analysis may be performed by either the Time History Method or the Response Spectrum Method. When either of the methods is used, the design base shear $V_{\mathrm{B}}$ estimated shall not be less than the design base shear $\overline{V}_{\mathrm{B}}$ calculated using a fundamental period $T_{\mathrm{a}}$ , where $T_{\mathrm{a}}$ is as per 5.4.6.2.

When $V_{\mathrm{B}}$ is less than $\overline{V}_{\mathrm{B}}$ , the force response quantities (for example member stress resultants, storey shear forces, and base reactions) shall be multiplied by $\overline{V}_{\mathrm{B}} / V_{\mathrm{B}}$ .

For earthquake shaking considered along,

a) the two mutually perpendicular plan directions X and Y, separate multiplying factors shall be calculated, namely $\overline{V}_{\mathrm{BX}} / V_{\mathrm{BX}}$ and $\overline{V}_{\mathrm{BY}} / V_{\mathrm{BY}}$ , respectively; and

b) the vertical Z direction, the multiplying factor shall be taken as

$\mathrm{Max}\left[\bar{V}_{\mathrm{BX}} / V_{\mathrm{BX}}; \bar{V}_{\mathrm{BY}} / V_{\mathrm{BY}}\right]$

5.4.7.4 Time history method

Time history method shall be based on an appropriate ground motion (preferably compatible with the design acceleration spectrum in the desired range of natural periods) and shall be performed using accepted principles of earthquake structural dynamics.

5.4.7.5 Response spectrum method

Response spectrum method may be performed for any building using the design acceleration spectrum

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

specified in 5.3.4.2, or by a site-specific design acceleration spectrum mentioned in 5.3.4.7.

5.4.7.5.1 Natural modes of oscillation

Undamped free vibration analysis of the entire building shall be performed as per established methods of structural dynamics using appropriate mass and elastic stiffness of the structural system, to obtain natural periods $T_{\mathrm{k}}$ and mode shapes $\{\phi\}_{\mathrm{k}}$ of those of its $N_{\mathrm{m}}$ modes of oscillation $[\mathrm{k} \in [1, N_{\mathrm{m}}]]$ that need to be considered as per 5.4.7.5.2.

5.4.7.5.2 Number of modes to be considered

The number of modes $N_{m}$ to be used in the analysis for earthquake shaking along a considered direction, should be such that the sum total of modal masses of these modes considered is at least 90 percent of the total seismic mass.

If modes with natural frequencies beyond 33 Hz are to be considered, the modal combination shall be carried out only for modes with natural frequency less than 33 Hz; the effect of modes with natural frequencies more than 33 Hz shall be included by the missing mass correction procedure following established principles of structural dynamics. If justified by rigorous analysis, designers may use a cut off frequency other than 33 Hz.

5.4.7.5.3 Combination of modes

The responses of different modes considered shall be combined by one of the two methods given below:

a) Peak response quantities (for example member forces, displacements, storey forces, storey shears, and base reactions) may be combined as per Complete Quadratic Combination (CQC) method, as given below:

\lambda = \sqrt {\sum_ {i = 1} ^ {N _ {\mathrm{m}}} \sum_ {j = 1} ^ {N _ {\mathrm{m}}} \lambda_ {\mathrm{i}} \rho_ {\mathrm{ij}} \lambda_ {\mathrm{j}}}

where

$\lambda =$ estimate of peak response quantity;

$\lambda_{i} =$ response quantity in mode $i$ (with sign);

$\lambda_{\mathrm{ij}} =$ response quantity in mode $j$ (with sign);

$\rho_{ij} =$ cross-modal correlation co-efficient

= \frac {8 \zeta^ {2} (1 + \beta) \beta^ {1 . 5}}{\left(1 - \beta^ {2}\right) ^ {2} + 4 \zeta^ {2} \beta (1 + \beta) ^ {2}};

$N_{m}$ = number of modes considered;

$\zeta =$ modal damping coefficient ratio which shall be taken as 0.05;

$\beta =$ natural frequency ratio $= \frac{\omega_{\mathrm{j}}}{\omega_{\mathrm{i}}}$

$\omega_{j} =$ circular natural frequency in mode $j$ ; and

$\omega_{i} =$ circular natural frequency in mode $i$ .

b) Alternatively, the peak response quantities may be combined as follows:

If building does not have closely-spaced modes, then net peak response quantity $\lambda$ due to all modes considered shall be estimated as:

\lambda = \sqrt {\sum_ {k = 1} ^ {N _ {\mathrm{m}}} \left(\lambda_ {\mathrm{k}}\right) ^ {2}}

where

$\lambda_{\mathrm{k}} =$ peak response quantity in mode $k$ , and

$N_{m}=$ number of modes considered.

If building has a few closely-spaced modes, then net peak response quantity $\lambda^{*}$ due to these closely space modes alone shall be obtained as:

\left| \lambda^ {*} = \sum_ {c} \left| \lambda_ {\mathrm{c}} \right| \right.

where

$\lambda_{c} =$ peak response quantity in closely spaced mode $c$ . The summation is for closely spaced modes only. Then, this peak response quantity $\lambda^{*}$ due to closely spaced modes is combined with those of remaining well-separated modes by method described above.

5.4.7.5.4 Simplified method of dynamic analysis of buildings

Regular buildings may be analysed as a system of masses lumped at the floor levels with each mass having one degree of freedom, that of lateral displacement in the direction under consideration. In such a case, the following expressions shall hold in the computation of the various quantities:

a) Modal mass — Modal mass $M_{\mathrm{k}}$ of mode $k$ is given by:

M _ {\mathrm{k}} = \frac {\left[ \sum_ {i = 1} ^ {n} W _ {\mathrm{i}} \phi_ {\mathrm{ik}} \right] ^ {2}}{\mathrm{g} \sum_ {i = 1} ^ {n} W _ {\mathrm{i}} \left(\phi_ {\mathrm{ik}}\right) ^ {2}}

where

$g =$ acceleration due to gravity,

$\phi_{ik} =$ mode shape coefficient at floor $i$ in mode k,

$W_{\mathrm{i}} =$ seismic weight of floor $i$ of the structure, and

$n =$ number of floors of the structure.

b) Mode participation factor — Mode participation factor $P_{\mathrm{k}}$ of mode $k$ is given by:

P _ {\mathrm{k}} = \frac {\sum_ {i = 1} ^ {n} W _ {\mathrm{i}} \phi_ {\mathrm{ik}}}{\sum_ {i = 1} ^ {n} W _ {\mathrm{i}} \left(\phi_ {\mathrm{ik}}\right) ^ {2}}

c) Design lateral force at each floor in each mode — Peak lateral force $Q_{ik}$ at floor i in mode k is given by:

Q _ {\mathrm{ik}} = A _ {\mathrm{k}} \phi_ {\mathrm{ik}} P _ {\mathrm{k}} W _ {\mathrm{i}}

where

$A_{k}$ = design horizontal acceleration spectrum value as per 5.3.4.2 using natural period of oscillation $T_{k}$ of mode k obtained from dynamic analysis.

d) Storey shear forces in each mode — Peak shear force $V_{ik}$ acting in storey i in mode k is given by:

V _ {\mathrm{ik}} = \sum_ {j = i + 1} ^ {n} Q _ {\mathrm{ik}}

e) Storey shear force due to all modes considered — Peak storey shear force $V_{i}$ in storey i due to all modes considered, shall be obtained by combining those due to each mode in accordance with 5.4.7.5.3.

f) Lateral forces at each storey due to all modes considered — Design lateral forces $F_{roof}$ at roof level and $F_{i}$ at level of floor i shall be obtained as:

F _ {\mathrm{roof}} = V _ {\mathrm{roof}}, \text {and}

F _ {\mathrm{i}} = V _ {\mathrm{i}} - V _ {\mathrm{i+1}}.

5.4.8Torsion

5.4.8.1 Provision shall be made in all buildings for increase in shear forces on the lateral force resisting elements resulting from twisting about the vertical axis of the building, arising due to eccentricity between the centre of mass and centre of resistance at the floor levels. The design forces calculated as in 5.4.6 and 5.4.7.5 shall

be applied at the displaced centre of mass so as to cause design eccentricity (as given by 5.4.8.2) between the displaced centre of mass and centre of resistance.

5.4.8.2 Design eccentricity

While performing structural analysis by the Seismic Coefficient Method or the Response Spectrum Method,

the design eccentricity $e_{di}$ to be used at floor i shall be taken as:

e _ {\mathrm{di}} = \left\{ \begin{array}{l} 1. 5 e _ {\mathrm{si}} + 0. 0 5 b _ {\mathrm{i}} \\ e _ {\mathrm{si}} - 0. 0 5 b _ {\mathrm{i}} \end{array} \right.

whichever gives the more severe effect on lateral force resisting elements;

where

$e_{\mathrm{si}} =$ static eccentricity at floor $i$

= distance between centre of mass and centre of stiffness, and

$b_{\mathrm{i}} =$ floor plan dimension of floor $i$ , perpendicular to the direction of force.

The factor 1.5 represents dynamic amplification factor, and $0.05b_{i}$ represents the extent of accidental eccentricity. The above amplification of 1.5 need not be used, when performing structural analysis by the Time History Method.

5.4.9 RC Frame Buildings with Unreinforced Masonry Infill Walls

5.4.9.1 In RC buildings with moment resisting frames and unreinforced masonry (URM) infill walls, variation of storey stiffness and storey strength shall be examined along the height of the building considering in-plane stiffness and strength of URM infill walls. If storey stiffness and strength variations along the height of the building render it to be irregular as per Table 45, the irregularity shall be corrected especially in Seismic Zones III, IV and V.

5.4.9.2 The estimation of in-plane stiffness and strength of URM infill walls shall be based on provisions given hereunder.

5.4.9.2.1 The modulus of elasticity $E_{\mathrm{m}}$ (in MPa) of masonry infill wall shall be taken as:

E _ {\mathrm{m}} = 5 5 0 f _ {\mathrm{m}}

where $f_{\mathrm{m}}$ is the compressive strength of masonry prism (in MPa) obtained as per good practice [6-1(19)] or given by expression:

f _ {\mathrm{m}} = 0. 4 3 3 f _ {\mathrm{b}} ^ {0. 6 4} f _ {\mathrm{mo}} ^ {0. 3 6}

where

$f_{\mathrm{b}} =$ compressive strength of brick, in MPa and

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

$f_{\mathrm{mo}} =$ compressive strength of mortar, in MPa.

5.4.9.2.2 URM infill walls shall be modeled by using equivalent diagonal struts as below:

a) Ends of diagonal struts shall be considered to be pin-jointed to RC frame;

b) For URM infill walls without any opening, width $w_{ds}$ of equivalent diagonal strut (see Fig. 17) shall be taken as:

w _ {\mathrm{ds}} = 0. 1 7 5 \alpha_ {\mathrm{h}} ^ {- 0. 4} L _ {\mathrm{ds}}

where

\alpha_ {\mathrm{h}} = h \left(\sqrt [ 4 ]{\frac {E _ {\mathrm{m}} t \sin 2 \theta}{4 E _ {\mathrm{f}} I _ {\mathrm{c}} h}}\right)

where $E_{m}$ and $E_{f}$ are the moduli of elasticity of the materials of the URM infill and RC MRF, $I_{c}$ the moment of inertia of the adjoining column, t the thickness of the infill wall, and $\theta$ the angle of the diagonal strut with the horizontal;

c) For URM infill walls with openings, no reduction in strut width is required; and

d) Thickness of the equivalent diagonal strut shall be taken as thickness $t$ of original URM infill wall, provided $h / t < 12$ and $l / t < 12$ , where $h$ is clear height of URM infill wall between the top beam and bottom floor slab, and $l$ clear length of the URM infill wall between the vertical RC elements (columns, walls or a combination thereof) between which it spans.

5.4.10RC Frame Buildings with Open Storeys

5.4.10.1 RC moment resisting frame buildings, which have open storey(s) at any level, such as due to discontinuation of unreinforced masonry (URM) infill walls or of structural walls, are known to have flexible and weak storeys as per Table 45. In such buildings, suitable measures shall be adopted, which increase both stiffness and strength to the required level in the open storey and the storeys below. These measures shall be taken along both plan directions as per requirements laid down under 5.4.10.2 to 5.4.10.4. The said increase may be achieved by providing measures, like:

a) RC structural walls, or

b) Braced frames, in select bays of the building. 5.4.10.2 When the RC structural walls are provided, they shall be,

a) founded on properly designed foundations;

b) continuous preferably over the full height of the building; and

c) connected preferably to the moment resisting frame of the building.

5.4.10.3 When the RC structural walls are provided, they shall be designed such that the building does NOT have the following:

a) Additional torsional irregularity in plan than that already present in the building. In assessing this, lateral stiffness shall be included of all elements that resist lateral actions at all levels of the building;

b) Lateral stiffness in the open storey(s) is less

FIG. 17 EQUIVALENT DIAGONAL STRUT OF URM INFILL WALL

than 80 percent of that in the storey above; and

c) Lateral strength in the open storey(s) is less than 90 percent of that in the storey above.

5.4.10.4 When the RC structural walls are provided, the RC structural wall plan density $\rho_{sw}$ of the building shall be at least 2 percent along each principal direction in Seismic Zones III, IV and V. These walls shall be well distributed in the plan of the building along each plan direction. RC structural walls of this measure can be adopted even in regular buildings that do not have open storey(s).

5.4.10.5 RC structural walls in buildings located in Seismic Zones III, IV and V shall be designed and detailed to comply with all requirements of good practice [6-1(8)].

5.4.11Deformation

Deformation of RC buildings shall be obtained from structural analysis using a structural model based on section properties given in 5.3.4.3.

5.4.11.1 Storey drift limitation

5.4.11.1.1 Storey drift in any storey shall not exceed 0.004 times the storey height, under the action of design base of shear $V_{B}$ with no load factors mentioned in 5.3.3, that is, with partial safety factor for all loads taken as 1.0.

5.4.11.1.2 Displacement estimates obtained from dynamic analysis methods shall not be scaled as given in 5.4.7.3.

5.4.11.2 Deformation capability of non-seismic members

For buildings located in Seismic Zones III, IV and V, it shall be ensured that structural components, that are not a part of seismic force resisting system in considered direction of ground motion but are monolithically connected, do not lose their vertical load-carrying capacity under induced net stress resultants, including additional bending moments and shear forces resulting from storey deformations equal to R times storey displacements calculated as per 5.4.11.1, where R is specified in Table 48.

5.4.11.3 Separation between adjacent units

Two adjacent buildings, or two adjacent units of the same building with separation joint between them, shall be separated by a distance equal to $R$ times sum of storey displacements $\Delta_1$ and $\Delta_2$ calculated as per 5.4.11.1 of the two buildings or two units of the same building, to avoid pounding as the two buildings

or two units of the same building oscillate towards each other.

When floor levels of the adjacent units of a building or buildings are at the same level, the separation distance shall be calculated as $(R_{1}\Delta_{1} + R_{2}\Delta_{2})/2$ , where $R_{1}$ and $\Delta_{1}$ correspond to building 1, and $R_{2}$ and $\Delta_{2}$ to building 2.