CHAPTER 9 MASONRY
Definitions • Section 9.1 & 9.2 deals with the description of symbolic notations and terminologies used in the documents, such as; • Column An isolated vertical load bearing member having length to width ratio not more than 4.0. • Pier A thickened section forming integral part of a wall placed at intervals along the wall length, to increase the stiffness of the wall or to carry a vertical concentrated load. • Single-leaf walls The effective thickness of single-leaf walls of either solid or hollow units is the specified thickness of the wall. • Multileaf walls The effective thickness of multileaf walls is the specified thickness of the wall if the space between leafs is filled with mortar or grout. For walls with an open space between leafs, the effective thickness shall be determined as for cavity walls. • Types of Walls: • a) Cavity wall A wall comprising two leaves, each leaf being built of masonry units and separated by a cavity and tied together with metal ties or bonding units to ensure that the two leaves act as one structural unit, the space between the leaves being either left as continuous cavity or filled with a non-load bearing insulating and water-proofing material. • b) Faced Wall A wall in which facing and backing of two different materials are bonded together to ensure common action under load. • c) Veneered wall A wall in which the facing is attached to the backing but not so bonded as to result in a composite action under load.
9.4 - Materials • 9.4.1 – Masonry Units • Masonry units may compose of: • a) Burnt clay building bricks • b) Stones (in regular sized units) • c) Concrete blocks (solid and hollow) • d) Burnt clay hollow blocks • e) Gypsum partition blocks • The compressive strength of bricks may not be less than 1200psi for solid bricks and 800 psi for hollow blocks. • 9.4.2 – Mortar • The compressive strength of masonry mortar in seismic Zones 2, 3 and 4 shall not be less than 4.10 MPa (595 psi) and not greater than 75% of the compressive strength of the masonry unit in the direction normal to the bed face. Can not be used in Zone 2, 3 and 4.
9.6 Working Stress Design of Masonry • The design of masonry structures is recommended to be carried out under the working stress method, for all loads such as: • Self Weight • Live Loads • Seismic and Wind Lateral Loads • Snow & Vibration loads • Vertical load dispersion at not more than 30◦ angle is not allowed. Whereas, arching action is permitted at lintel and beam joints. • Bearing length of lintel should not be less than 100mm or one-tenth of the span. Span Bearing length of lintel 100 mm or 1/10 of span
188.8.131.52 –Permissible Compressive Stresses • The basic compressive stress is further to be multiplied by stress reduction factor, area reduction factor and shape modification factor as to convert to permissible stress values.
184.108.40.206. – Reduction Factors 220.127.116.11.2 – Area Reduction Factor: This factor takes into consideration smallness of the sectional area of the element and is applicable when sectional area of the element is less than 0.2 m2 (2.15 ft2). The factor ka = 0.7 + 1.5A, A being the area of section in m2. 18.104.22.168.3 – Shape Modification Factor: This factor is applicable for units for cursing strength up to 2200 psi.
22.214.171.124 Permissible Tensile stresses • In general tensile stresses are not permitted in masonry structures. But when considering out of plane bending, under lateral loads, the following stresses may be permitted: • Grade M1 or better mortar: • 10 psi for bending in the vertical direction where tension developed is normal to bed joints. • 20 psi for bending longitudinal direction where tension developed is parallel to bed joints provided crushing strength of masonry units is not less than 1450 psi. • Grade M2 mortar: • 7.25 psi for bending in the vertical direction where tension developed is normal to bed joints. • 14.5 psi for bending in the longitudinal direction where tension developed is parallel to bed joints provided crushing strength of masonry unit is not less than 1100 psi. • NOTES: • 1. No tensile stress is permitted in masonry in case of water retaining structures in view of water in contact with masonry. Also no tensile stress is permitted in earth-retaining structures, in view of the possibility of presence of water at the back of such walls. • 2. Allowable tensile stress in bending in the vertical direction may be increased to 14.5psi for M1 mortar and 10psi for M2 mortar in case of boundary walls/compound at the desecration of the designer.
Permissible Shear Stress & Reinforcement • 126.96.36.199 – Permissible Shear Stresses: • Shear stresses shall be given by the following formula, not exceeding a maximum of 75 psi; • fs = 0.1 + fd/6 • fd = Compressive stress due to dead loads in MPa. • If there is tension in any part of a section of masonry the area under tension shall be ignored while working out shear stress on the section. • 188.8.131.52 – Permissible stresses in Reinforcement • Tensile Stresses: • Deformed bars: Fs = 0.5 fy , 24 Ksi max • Wire Reinforcement: Fs = 0.5 fy , 30 Ksi max • Ties, anchors, smooth bars: Fs = 0.4 fy , 20 Ksi max • Compressive Stresses: • Deformed bars in columns: Fs = 0.4 fy , 24 Ksi max • Deformed bars in flexural members: Fs = 0.4 fy , 24 Ksi max • Deformed bars in shear walls, confined by ties: Fs = 0.4 fy , 24 Ksi max
9.6.7 – Design TH. / Cross Section • Wall and columns bearing vertical loads shall be designed on the basis or permissible compressive stress. • Design thickness of solid walls should be as per layout, in consideration with mortar joints and layout. • Thickness of each leaf of cavity walls should not be less than 75mm. • The leaf should be supported at most at a height of 10m. • In faced walls, the permissible load per length of wall shall be taken as the product of the total thickness of the wall and the permissible stress in the weaker of the two materials. • Veneered walls are not to be accounted in calculations for load carrying purposes. • Free standing walls, subjected to wind pressure or seismic forces shall be designed on the basis of permissible tensile stress in masonry or stability. • Normally masonry of retaining walls shall be designed on the basis of zero-tension and permissible compressive stress. • Walls subjected to in-plane bending and vertical loads that is shear walls shall be designed on the basis of no tension with permissible shear stress and permissible compressive stress.
9.7 – Empirical Design Provisions • Storey height of masonry buildings shall not be more than 10ft. • 184.108.40.206 – Load Bearing Walls • Masonry materials to be used in the construction of load-bearing walls shall be natural stone, solid burnt clay brick or concrete blocks, hollow bricks or concrete block. • Natural stone load-bearing walls shall be used only in the basement and ground stories of masonry buildings. The use of undressed stones is not allowed. Stones shall at least be semi-dressed.
Load Bearing Walls (Min TH.) The ratio of the total length of masonry load-bearing walls in each of the orthogonal directions in plan, to gross floor area shall not be less 0.85 ft/ft2. Unsupported length of a load-bearing wall between cross walls in the perpendicular direction shall not exceed 18 ft in Seismic zones 3 & 4 and 23 ft in other seismic areas. Otherwise provide intermediate columns at a spacing of 13 ft up to a maximum of 50ft length. • The following rules shall be followed to provide openings in load-bearing walls: • a) Plan length of the load-bearing wall segment between the corner of the building and the nearest window or door opening shall not be less than ¼ of the height of opening but not less than 24 in. • b) The total length of openings shall not exceed 50% of length of wall between consecutive cross walls in single storey construction, 42% in two storey construction and 33% in three storey buildings. • c) The vertical distance from opening to opening directly above it shall not be less than 24 in. nor less than ½ the width of smaller opening. • d) Plan length of each window or door opening shall not be more than 10 ft.
9.7.2 – Seismic Strengthening • The masonry buildings shall be strengthened using seismic bands and vertical bars or by utilizing provisions for confined masonry. • 220.127.116.11.1 – Lintel Bands • Bearing of window and door lintels on the walls shall not be less than 15% of lintel clear span nor less than 8 in. • 18.104.22.168.2 – Horizontal Bands • Reinforced concrete bands shall be provided continuously through all load-bearing longitudinal and transverse walls at plinth, lintel and roof-eave levels and also at the top of gables.
22.214.171.124.3 – Vertical Reinforcement in Walls • Steel bars shall be installed at the critical sections (i.e. the comers of walls, junctions of walls, and jambs of doors) and well anchored into the foundation concrete. • The vertical steel at openings may be stopped by embedding it into the lintel band, but the vertical steel at the comers and junctions of walls must be taken into either the floor and roof slabs or the roof band.
126.96.36.199 – Confined Masonry • The horizontal and vertical confining elements shall be bonded together and anchored to the elements of the main structural system. • Masonry walls shall be tied to the vertical confining elements by providing horizontal reinforcement. 2 - ¼ in. diameter bars spaced at a maximum spacing of 24 in. • The cross-sectional dimensions of both horizontal and vertical confining elements shall not be less than 6 in. • Vertical confining elements should be placed at the free edges of a structural wall, within the wall (not exceeding a spacing of 15ft) , at the intersections of structural walls and at both sides of any wall opening. • Horizontal confining elements shall be placed in the plane of the wall at every floor level and in any case with a vertical spacing of not more than 10 ft. • The longitudinal reinforcement of confining elements shall not have a cross-sectional area less than 0.4 in2 , nor more than 1% of the cross-sectional area of the confining element. • Stirrups not less than ¼ in. in diameter and spaced not more than 6 in. shall be provided around the longitudinal reinforcement. • Lap splices shall not be less than 60 bar diameters in length.
Slabs, Roofs & Non Bearing Walls • Slabs and Roofs shall be used as RCC slabs or joist floors, or as timber or steel trussed roofs, with the cantilever balconies being the extension of the floor slab. • 9.7.4 – Non Bearing Walls • The thickness of non-bearing partition walls should be at least ½ brick size or 4 in. • Should be constructed with load bearing walls at both ends. • Height of parapet walls should not be more than 24 in. • Height of garden walls shall not be more than 3 ft from pavement level.