Construction of the following types of load bearing and non-load bearing masonry walls shall be carried out in accordance with good practice [6-4(4)]:
a) Brickwork,
b) Stone masonry,
c) Hollow concrete block masonry,
d) Gypsum partition blocks,
e) Autoclaved cellular concrete block masonry, and
f) Lightweight concrete block masonry.
6.1.2 Construction of Buildings in Seismic Zones
Special features of construction for earthquake resistant masonry buildings in Seismic Zones II, III, IV and V shall be applicable according to 8.
6.2 Minimum Thickness of Walls from Consideration other than Structural
Thickness of walls determined from consideration of strength and stability may not always be adequate in respect of other requirements, such as resistance to fire, thermal insulation, sound insulation and resistance to damp penetration for which reference may be made to the appropriate Parts/Sections of the Code, and thickness suitably increased, where found necessary.
6.3Workmanship
6.3.1 General
Workmanship has considerable effect on strength of
masonry and bad workmanship may reduce the strength of brick masonry to as low as half the intended strength. The basic compressive stress values for masonry as given in Table 9 holds good for commercially obtainable standards of workmanship with reasonable degree of supervision.
6.3.2 Bedding of Masonry Units
Masonry units shall be laid on a full bed of mortar with frog, if any, upward such that cross-joints and wall joints are completely filled with mortar. Masonry units which are moved after initial placement shall be re-laid in fresh mortar, discarding the disturbed mortar.
6.3.3 Bond
Cross-joints in any course of one brick thick masonry wall shall be not less than one-fourth of a masonry unit in horizontal direction from the cross-joints in the course below. In masonry walls more than one brick in thickness, bonding through the thickness of wall shall be provided by either header units or by other equivalent means in accordance with good practice [6-4(5)].
6.3.4 Verticality and Alignment
All masonry shall be built true and plumb within the tolerances prescribed below; care shall be taken to keep the perpends properly aligned:
a) Deviation from vertical within a storey shall not exceed 6 mm per 3 m height.
b) Deviation in verticality in total height of any wall of a building more than one storey in height shall not exceed 12.5 mm.
c) Deviation from position shown on plan of any brickwork shall not exceed 12.5 mm.
d) Relative displacement between load bearing walls in adjacent storeys intended to be in vertical alignment shall not exceed 6 mm.
e) Deviation of bed-joint from horizontal in a length of 12 m shall not exceed 6 mm subject to a maximum deviation of 12 mm.
f) Deviation from the specified thickness of bed joints, cross-joints and perends shall not exceed one-fifth of the specified thickness.
NOTE — These tolerances have been specified from the point of view of their effect on the strength of masonry. The permissible stress recommended in 5.3 may be considered applicable only, if these tolerances are adhered to.
6.4 Joints to Control Deformation and Cracking
Special provision shall be made to control or isolate thermal and other movements so that damage to the fabric of the building is avoided and its structural sufficiency preserved. Design and installation of joints shall be done according to the appropriate
recommendations in accordance with good practice [6-4(6)].
6.6Corbelling
6.6.1 Where corbelling is required for the support of some structural element, maximum projection of masonry unit should not exceed one-half of the height of the unit or one-half of the built-in part of the unit and the maximum horizontal projection of the corbel should not exceed one-third of the wall thickness.
6.6.2 The load per unit length on a corbel shall not be greater than half of the load per unit length on the wall above the corbel. The load on the wall above the corbel, together with four times the load on the corbel, shall not cause the average stress in the supporting wall or leaf to exceed the permissible stresses given in 5.4.
6.6.3 It is preferable to adopt header courses in the corbelled portion of masonry from considerations of economy and stability.
7 REINFORCED BRICK AND REINFORCED BRICK CONCRETE FLOORS AND ROOFS
The construction and design of reinforced brick and reinforced brick concrete floors and roof shall be in accordance with good practices [6-4(7)].
8 SPECIAL CONSIDERATION FROM EARTHQUAKE POINT OF VIEW
8.0 Special features of design and construction for earthquake resistant masonry buildings are given in 8.2 to 8.8.2. However, special features of design and construction for improving earthquake resistance of low strength masonry buildings is covered in 9.
8.1 For the purpose of 8 and 9, the definitions given below shall apply in addition to those given in 2.
8.1.1 Separation Section — A gap of specified width
between adjacent buildings or parts of the same building either left uncovered or covered suitably to permit movement in order to avoid pounding due to earthquake.
8.1.1.1 Crumple section — The separation gap filled with appropriate material that crumples or factures in the event of an earthquake.
8.1.2 Centre of Rigidity — The point in a structure where application of lateral force produces equal deflections of its components at any level in any particular direction.
8.1.3 Shear Wall — A wall designed to resist lateral force in the own plane. Braced frames, subjected primarily to axial stresses, shall be considered as shear walls for the purpose of this definition.
8.1.4 Space Frame — A three-dimensional structural system comprised of interconnected members, without shear or bearing walls, so that to function as a complete self-contained unit with or without the aid of horizontal diaphragms or floor bracing systems.
8.1.4.1 Vertical load carrying frame — A space frame designed to carry all the vertical loads, the horizontal loads being resisted by shear walls.
8.1.4.2 Moment resistant frame — A space frame capable of carrying all vertical and horizontal loads, by developing bending moments in the members and at joints.
8.1.4.3 Moment resistant frame with shear walls — A space frame with moment resistant joints and strengthened by shear walls to assist in carrying horizontal loads.
8.1.5 Box System — A bearing wall structure without a space frame, the horizontal forces being resisted by the walls that act as shear walls.
8.1.6 Band — A reinforced concrete or reinforced brick runner provided in the walls to tie them together and to impart horizontal bending strength in them.
NOTE — For low strength masonry buildings, wooden runners may also be used as runner.
8.1.7 Seismic Zone and Seismic Coefficient — The Seismic Zones II to IV as classified in 5 of Part 6 'Structural Design, Section 1 Loads, Forces and Effects' of the Code and corresponding basic seismic coefficient αo as specified in 3.4 of good practice [6-4(8)].
8.1.8 Design Horizontal Seismic Coefficient — The value of horizontal seismic coefficient Ah computed taking into account the soil-foundation system and the importance factor as specified in 6.4 of good practice [6-4(9)].
8.1.9 Concrete Grades — 28 day compressive strength
of concrete cubes of 150mm size, in MPa; for example, for Grade M20 as per good practice[6-4(10)], the characteristic compressive strength equals to 20MPa.
8.1.10 Zone Factor (Z) — It is a factor to obtain the design spectrum depending on the perceived maximum seismic risk characterized by maximum considered earthquake (MCE) in the zone in which the structure is located.
8.2General Principles
The general principles given in 8.2.1 to 8.2.9 shall be observed in construction of earthquake resistant buildings.
8.2.1 Lightness
Since the earthquake force is a function of mass, the weight of the building shall be as minimum as possible, consistent with structural safety and functional requirements. Roofs and upper storeys of buildings, in particular, should be designed as light as possible.
8.2.2 Continuity of Construction
8.2.2.1 As far as possible, the parts of the building should be tied together in such a manner that the building acts as one unit.
8.2.2.2 For parts of buildings between separation or crumple sections to expansion joints, floor slabs shall be continuous throughout as far as possible. Concrete slabs shall be rigidly connected or integrally cast with the support beams.
8.2.2.3 Additions to the structures shall be accompanied by the provision of separation or crumple sections between the new and the existing structures as far as possible, unless positive measures are taken to establish continuity between the existing and the new construction.
8.2.2.4 Alteration to the building structure shall be done by maintaining its structural stability by ensuring proper load path.
8.2.3 Projecting and Suspended Parts
8.2.3.1 Projecting parts shall be avoided as far as possible. If the projecting parts cannot be avoided, they shall be properly reinforced and firmly tied to the main structure, and their design shall be in accordance with good practice [6-4(8)].
8.2.3.2 Ceiling plaster shall preferably be avoided. When it is unavoidable, the plaster shall be as thin as possible.
8.2.3.3 Suspended ceiling shall be avoided as far as possible. Where provided they shall be light, adequately framed and secured to which electrical fixtures shall be fully secured.
8.2.4 Building Configuration
8.2.4.1 In order to minimise torsion and stress concentration, provisions given in 8.2.4.2 to 8.2.4.4 should be complied with as relevant.
8.2.4.2 The building should have a simple rectangular plan and be symmetrical both with respect to mass and rigidity so that the centre of mass and rigidity of the building coincide with each other in which case no separation sections other than expansion joints are necessary. For provision of expansion joints reference may be made to good practice [6-4(6)].
8.2.4.3 If symmetry of the structure is not possible in plan, elevation or mass, provision shall be made for torsional and other effects due to earthquake forces in the structural design or the parts of different rigidities may be separated through crumple sections. The length of such building between separation sections shall not preferably exceed three times the width.
NOTE — As an alternative to separation section to reduce torsional moments, the centre of rigidity of the building may be brought close or coincident to the centre of mass by adjusting the locations and/or sizes of columns and walls.
8.2.4.4 Buildings having plans with shapes like L, T, E and Y shall preferably be separated into rectangular parts by providing separation sections at appropriate places. Typical examples are shown in Fig. 15.
NOTES
1 The buildings with small lengths of projections forming L, T, E or Y shapes need not be provided with separation section. In each cases the length of the projection may not exceed 15 to 20 percent of the total dimension of the building in the direction of the projection (see Fig. 16).
2 For buildings with minor asymmetry in plan and elevation, separation sections may be omitted.
8.2.5 Strength in Various Directions
The structure shall be designed to have adequate strength against earthquake effects along both the horizontal axes. The design shall also be safe considering the reversible nature of earthquake forces.
8.2.6 Foundations
The structure shall not be founded on such soils which shall subside or liquefy during an earthquake, resulting in large differential settlements (see also 8.3.3.3).
8.2.7 Ductility
The main structural elements and their connection shall be designed to have a ductile failure. This shall enable the structure to absorb energy during earthquakes to avoid sudden collapse of the structure. Providing reinforcing steel in masonry at critical sections, as specified in this Section shall not only increase strength and stability but also ductility. The details for achieving ductility in reinforced concrete structures is given in good practice [6-4(11)].
L - SHAPE
T-SHAPE
Y-SHAPE
Y-SHAPE
E-SHAPE
FIG. 15 TYPICAL SHAPES OF BUILDING WITH SEPARATION SECTIONS
16B Vertical Irregularities
FIG. 16 PLAN AND VERTICAL IRREGULARITIES
PART 6 STRUCTURAL DESIGN — SECTION 4 MASONRY
8.2.8 Damage to Non-Structural Parts
Suitable details shall be worked out to connect the non-structural parts with the structural framing so that the deformation of the structural frame leads to minimum damage of the non-structural elements.
8.2.9 Fire Safety
Fire frequently follows an earthquake and therefore, buildings shall be constructed to make them fire resistant in accordance with the provisions of Part 4 ‘Fire and Life Safety’ of the Code for fire safety.
8.3Special Construction Features
8.3.1 Separation of Adjoining Structures
8.3.1.1 Separation of adjoining structures or parts of the same structure is required for structures having different total heights or storey heights and different dynamic characteristics. This is to avoid collision during an earthquake.
8.3.1.2 Minimum width of separation gaps as mentioned in 8.3.1.1, shall be as specified in Table 13. The design seismic coefficient to be used shall be in accordance with 5 of Part 6 'Structural Design, Section 1 Loads, Forces and Effects' of the Code.
8.3.1.3 Where separation is necessary, a complete separation of the parts shall be made except below the plinth level. The plinth beams, foundation beams and footings may be continuous. Where separation sections
17A BRICK WALL
17C METAL SIDING INDUSTRIAL WORK
Table 13Gap Width for Adjoining Structures (Clause 8.3.1.2)
Sl.No.
Buildings Categories for Earthquake Resisting Features
Gap Width per Storey for Adjoining Masonry Building
mm
(1)
(2)
(3)
i)
Frames with shear walls
15
ii)
Category B1)
10
iii)
Category C1)
10
iv)
Category D1)
15
v)
Category E1)
20
1) See Table 14.
NOTE — Minimum total gap shall be 25 mm. For any other value of Ah the gap width shall be determined proportionately.
are provided in a long building, they shall take care of movement owing to temperature changes also.
8.3.2 Separation or Crumple Section
8.3.2.1 In case of framed construction, members shall be duplicated on either side of the separation or crumple section. As an alternative, in certain cases, such duplication may not be provided, if the portions on either side can act as cantilevers to take the weight of the building and other relevant loads.
8.3.2.2 Typical details of separation and crumple sections are shown in Fig. 17. For other types of joint details, reference may be made to good practice [6-4(6)].
17B CONCRETE WALLS
17D WOOD SHEATHING INDUSTRIAL WORK
FIG. 17 TYPICAL DETAILS OF SEPARATION OR CRUMPLE SECTION (CONTINUED)
17E RCC SLAB ON ROOF SURFACE
17F SEPARATION JOINT DETAILS AT ROOF
NOTE — Fragile cover may consist of asbestos cement sheet, particle board and like.
FIG. 17 TYPICAL DETAILS OF SEPARATION OR CRUMPLE SECTION
PART 6 STRUCTURAL DESIGN — SECTION 4 MASONRY
8.3.3 Foundations
8.3.3.1 For the design of foundations, the provisions of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ and Section 2 ‘Soils and Foundation’ of the Code shall generally be followed.
8.3.3.2 The subgrade below the entire area of the building shall preferably be of the same type of the soil. Wherever this is not possible, a suitably located separation or crumple section shall be provided.
8.3.3.3 Loose fine sand, soft silt and expansive clays should be avoided. If unavoidable, the building shall rest either on a rigid raft foundation or on piles taken to a firm stratum. However, for light constructions the following measures may be taken to improve the soil on which the foundation of the building may rest:
a) Sand piling, and
b) Soil stabilization.
8.3.3.4 Isolated footings for columns
All the individual footings or pile caps where used in Type III Soft soils {Table 1 of good practice [6-4(9)]}, shall be connected by reinforced concrete ties at least in two directions approximately at right angles to each other. For buildings with no basement, the ties may be placed at or below the plinth level and for buildings with basement they may be placed at the level of basement floor. They shall need to be designed to carry the load of the panel walls also.
NOTE — The ties may not be necessary where structural floor connects the columns at or below the plinth level.
8.3.3.4.1 Where ties are used, their sections shall be designed to carry in tension as well as in compression, an axial load not less than the earthquake force in the direction the tie acting on the heavier of the columns connected, but the sections shall not be less than 200mm×200mm with M20 concrete reinforced with 4 bars of 12 mm diameter plain mild steel bars or 10 mm diameter high strength deformed bars, one at each corner, bound by 8 mm diameter stirrups not more than 150 mm apart.
NOTE — In working out the buckling strength of ties, the lateral support provided by the soil may be taken into account. Calculations show that for such buried ties, lateral buckling is not a problem and the full section of the tie may be taken effective as a short column.
8.3.3.4.2 In the case of reinforced concrete slab, the thickness shall not be less than 1/50th of the clear distance between the footings, but not less than 100 mm in any case. It shall be reinforced with not less than 0.15 percent mild steel bars or 0.12 percent high strength deformed bars in each direction placed symmetrically at top and bottom.
8.3.4 Roofs and Floors
8.3.4.1 Flat roof or floor shall not preferably be made of tiles of ordinary bricks supported on steel, timber or reinforced concrete joists, nor they shall be of a type which in the event of an earthquake is likely to be loosened and parts or all of which may fall. If this type of construction cannot be avoided, the joists should be blocked at ends and bridged at intervals such that their spacing is not altered during an earthquake.
8.3.4.1.1 For pitched roofs, corrugated iron or asbestos sheets shall be used in preference to country, Allahabad or Mangalore tiles or other loose roofing units. All roofing materials shall be properly tied to the supporting members. Heavy roofing materials shall generally be avoided.
8.3.4.2 Pent roofs
8.3.4.2.1 All roof trusses shall be supported on reinforced concrete or reinforced brick band (see 8.6.4.3). The holding down bolts shall have adequate size and length as required for earthquake forces in accordance with 5 of Part 6 'Structural Design, Section 1 Loads, Forces and Effects' of the Code.
Where a trussed roof adjoins a masonry gable, the ends of the purlins shall be carried on and secured to a plate or bearer which shall be adequately bolted to reinforced concrete or reinforced brick band at the top of gable end masonry (see 8.6.4.4).
NOTE — Hipped roof in general have shown better structural behaviour during earthquakes than gable ended roofs.
8.3.4.2.2 At tie level all the trusses and the gable end shall be provided with diagonal braces in plan so as to transmit the lateral shear due to earthquake force to the gable walls acting as shear walls at the ends as specified in 8.6.4.
8.3.4.3 Jack arches
Jack arched roofs or floors, where used shall be provided with mild steel ties in all spans along with diagonal braces in plan to ensure diaphragms actions.
8.3.5 Staircases
8.3.5.1 The inter-connection of the stairs with the adjacent floors should be appropriately treated by providing sliding joints at the stairs to eliminate their bracing effect on the floors. Large stair halls shall preferably be separated from the rest of the building by means of separation or crumple sections.
8.3.5.2 Three types of stair construction may be adopted as described below:
a) Separated staircases — One end of the staircase rests on a wall and the other end is carried by columns and beams which have no
connection with the floors. The gap at the vertical joints between the floor and the staircase may be covered either with a tread plate attached to one side of the joint and sliding on the other side, or covered with some appropriate material which could crumple or fracture during an earthquake without causing structural damage. The supporting members, columns or walls, are isolated from the surrounding floors by means of separation or crumple sections. A typical example is shown in Fig. 18.
as mentioned in respect of separated staircases, shall not be necessary.
The two walls mentioned above, enclosing the staircase, shall extend through the entire height of the stairs and to the building foundations.
c) Staircases with sliding joints — In case it is not possible to provide rigid walls around stair openings for built-in staircase or to adopt the separated staircases, the staircases shall have sliding joints so that they shall not act as diagonal bracing.
b) Built-in staircase — When stairs are built monolithically with floors, they can be protected against damage by providing rigid walls at the stair opening. An arrangement, in which the staircase is enclosed by two walls, is given in Fig. 19. In such cases, the joints,
