Regular and Irregular Configuration

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. A building shall be considered to be irregular for the purposes of this standard, if any one of the conditions given in Tables 4 and 5 is applicable. Limits on acceptable irregularities for seismic zones III, IV and V and special analysis requirements are laid out in Tables 4 and 5.

Table 4 Definitions of Irregular Buildings – Plan Irregularities (see Fig. 3) (Clause 7.1)

(a) Torsional Irregularity

Usually, a well proportioned building does not twist about its vertical axis, when (a) 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); and

(b) 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 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 in that direction.

Buildings in Seismic Zones III, IV and V with Torsional Irregularity shall be designed adequately to avoid torsional irregularity.

(b) Re-entrant Corners

A building is said to have a re-entrant corner, when its structural configuration in plan has a projection in a direction of size greater than 15 percent of its overall plan dimension in that direction.

Buildings in Seismic Zones III, IV and V with re-entrant corners shall be re-designed adequately to avoid plan configurations having re-entrant corners.

(c) Floor Slabs having Excessive Cut-Outs or Openings

Floor slabs having cut-outs or openings of area more than 50 percent of the full area of the floor slab, have discontinuity in their in-plane stiffness.

Buildings in Seismic Zones III, IV and V with floor slabs having excessive cut-outs or openings shall be designed adequately to avoid floor slabs with excessive cut-outs or openings.

(d) Out-of-Plane Offsets in Vertical Elements

Out-of-plane offsets in vertical elements resisting lateral loads cause discontinuities and detours in the load path.

Buildings in Seismic Zones III, IV and V with out-of-plane offsets in vertical elements shall be designed adequately to avoid out-of-plane offsets in vertical elements.

(e) Non-Parallel Lateral Force System

A building is said to have non-parallel system when the vertical elements resisting lateral force are not parallel to or symmetric about the two principal orthogonal axes.

Buildings with non-parallel lateral force resisting system shall be analysed under load combinations mentioned in 6.3.2.2.

(A) (B) (C)

A

A

A

< 0.5A









L

A

A

A

A

L

A

L

L

A

(D)

(E)

FIG. 3 DEFINITIONS OF IRREGULAR BUILDINGS – PLAN IRREGULARITIES: (A) TORSIONAL IRREGULARITY, (B) RE-ENTRANT CORNERS,

(C) FLOOR SLABS WITH EXCESSIVE CUT-OUT OR OPENING, (D) OUT-OF-PLANE OFFSETS, AND (E) NON-PARALLEL SYSTEM

Table 5 Definition of Irregular Buildings – Vertical Irregularities (see Fig. 4) (Clause 7.1)

(a) Lateral Stiffness Irregularity in two principal plan directions

Lateral stiffness of beams, columns, braces and structural walls determine the lateral stiffness of a building in each principal plan direction. Lateral storey stiffness of a building in two principal plan directions shall be such that,

(a) the first two modes of oscillation are pure translational modes, and

(b) the fundamental lateral natural periods of the building in the two principal plan directions are away from each other by 15 percent of the larger value. Buildings in Seismic Zones III, IV and V with lateral stiffness irregularity in two principal plan directions shall be designed adequately to avoid having close fundamental lateral translational natural periods in each principal plan direction.

(b) Stiffness Irregularity (Soft Storey)

A soft storey is one in which the lateral stiffness is less than that in the storey above.

Buildings in seismic zones III, IV and V with stiffness irregularity shall be designed adequately to avoid soft storeys.

(c) Mass Irregularity

Mass irregularity shall be considered to exist, when the seismic weight of any floor is more than 150 percent of that of its adjacent floors. This provision of 150 percent may be relaxed in case of roofs.

Buildings in seismic zones III, IV and V with mass irregularity shall be designed by the Dynamic Analysis Method (as per 7.7).

(d) Vertical Geometric Irregularity

Vertical geometric irregularity shall be considered to exist, when the horizontal dimension of the lateral force resisting system in any storey is more than 125 percent of that in its adjacent storey.

Buildings in seismic zones III, IV and V with vertical geometric irregularity shall be designed by the Dynamic Analysis Method (as per 7.7).

(e) In-Plane Discontinuity in Vertical Elements Resisting Lateral Force

In-plane discontinuity in vertical elements resisting lateral force shall be considered to exist, when in-plane offset of the lateral force resisting elements is greater than the plan length of those elements.

Buildings in seismic zones III, IV and V with in-plane discontinuity in vertical elements resisting lateral force shall be designed adequately to avoid in-plane discontinuity in vertical elements resisting lateral force.

(f) Discontinuity in Capacity (Weak Storey)

A weak storey is one in which the storey lateral strength is less than that in the storey above.

Buildings in seismic zones III, IV and V shall be designed adequately to avoid weak storey.

(A)

(B)

(C)

Initial

Pure Translational Mode

Pure Translational Mode

Initial Ki+2 Ki+1 Ki

K

< K

K

< K

W

< 1.5W

W

< 1.5W

(D) (E)

L

A

L

A

A

A < 0.25L

A < 0.125L

A

L

A

L

L

L

< 1.25L

L

L

Storeys 2 and 3

Storey 1

A < 0.1L

(F)

FIG. 4 DEFINITIONS OF IRREGULAR BUILDINGS – VERTICALIRREGULARITIES: (A) LATERAL STIFFNESS IRREGULARITY IN TWO PRINCIPAL HORIZONTAL

DIRECTIONS, (B) STIFFNESS IRREGULARITY (SOFT STOREY), (C) MASS IRREGULARITY, (D) VERTICAL GEOMETRIC IRREGULARITY, (E) IN-PLANE DISCONTINUITY IN VERTICAL ELEMENTS RESISTING LATERAL

FORCE, AND (F) DISCONTINUITY IN CAPACITY (WEAK STOREY) 7.2 Response Reduction Factor R

7.2.1 Response reduction factor, R, for different building systems shall be as given in Table 6. 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 built in isolation. Response reduction factor R is used to account for inherent system ductility, redundancy and overstrength normally available in the buildings, if designed and detailed as per the prevalent Indian Standards.

7.2.2 Redundancy

Redundancy means more load paths for transferring to the foundation the inertia forces induced during seismic shaking at different levels of the building. More redundancy in the structure leads to increased level of energy dissipation and more overstrength. Building shall have a high degree of redundancy for lateral load resistance. Values of R given in Table 6 for buildings are based on the assumption that buildings have sufficient level of redundancy.

Redundancy factor r can be estimated as ratio of ultimate load to first yield load; estimation of this factor requires detailed non-linear analyses. Buildings that performed well in past earthquakes are observed to have redundancy values more than 2.5. For buildings with redundancy factor r less than 2.5, (that is, in the range 1.0-2.5), design engineer shall adopt modified values Rm of response reduction factor given by the expression:

R 5 1 1 r 5 0 5 0 R_{m } _         . . . , Si+2 Si+1 Si

S

> S

S

> S

where R is the response reduction factor given in Table 6.

Table 6 Response Reduction Factor R for Building Systems (Clause 7.2.2)

SlNo.

Lateral Load Resisting System R

Moment Frame Systems

1. RC Buildings with Ordinary Moment Resisting Frame (OMRF)1 3.0 2. RC Buildings with Special Moment-Resisting Frame (SMRF) 5.0 3. Steel Buildings with Ordinary Moment Resisting Frame (OMRF)1 4.0 4. Steel Buildings with Special Moment Resisting Frame (SMRF) 5.0 Braced Frame Systems

5. Steel Buildings with Ordinary Braced Frame with Concentric Braces 4.0 6. Steel Buildings with Special Braced Frame with Concentric Braces 4.5 7. Steel Buildings with Ordinary Braced Frame with Eccentric Braces 4.5 8. Steel Buildings with Special Braced Frame with Eccentric Braces 5.0 Structural Wall Systems

9. Load Bearing Buildings with Masonry Wall

(a) Unreinforced Masonry (designed as per IS 1905) without horizontal RC Seismic Bands1

1.5 (b) Unreinforced Masonry (designed as per IS 1905) with horizontal RC Seismic Bands 2.0 (b) Unreinforced Masonry (designed as per IS 1905) with horizontal RC Seismic Bands

and vertical reinforcing bars at corners of rooms and jambs of openings (with reinforcement as per IS 4326)

2.5

(c) Reinforced Masonry 3.0

(d) Confined Masonry 3.0

10. Load Bearing Buildings with Ordinary RC Structural Walls1 3.0 11. Load Bearing Buildings with Ductile RC Structural Wall 4.0 Dual Systems

12. RC Buildings with Ordinary RC Structural Wall with RC OMRF1 3.0 13. RC Buildings with Ordinary RC Structural Wall with RC SMRF 4.0 14. RC Buildings with Ductile RC Structural Wall with RC OMRF 4.0 15. RC Buildings with Ductile RC Structural Wall with RC SMRF 5.0 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. Buildings with shear walls also include buildings having shear walls and frames, but where: (a) Frames are not designed to carry lateral loads, or

(b) Frames are designed to carry lateral loads but do not fulfill the requirements of 'Dual Systems'. 3. (a) RC OMRF and Steel OMRF are as defined in 4.15.1.

(b) RC SMRF and Steel SMRF are as defined in 4.15.2. (c) Ordinary RC Structural Wall is as defined in 4.19.1. (d) Ductile RC Structural Wall is as defined in 4.19.2.

4. Response reduction factor used in design, damping during extreme shaking, and redundancy influence the nonlinear behaviour of buildings and structures during strong earthquake shaking. Detailed study is required to understand these influences. Until such time, the values of R given in the table above shall be used with the modification given in 7.2.2.