Masonry Structures SIDDHARTH SHANKAR Department of Civil(structure) Engineering Pulchowk Campus
Advantage of Masonry Structure
Strength: Strong in compression Heat Absorber: Resists temp. fluctuation Maintenance free: Most of wall not require painting . Fire resistant: Fire protection due to formation of noncombustible material. Environmental-friendly: Leading contributor to green building that has low impact on nature Great sound proofing: Blocks out noise more efficiently than more traditional building material such as timber Economic: Use of locally available materials and availability of labor.
Disadvantage
Moisture absorber: Absorbs moisture when raining. Color deterioration: Extreme weather causes masonry to degrade, materials such as wall surface decolorize due to frost damage. Strength: Masonry structure has low tensile strength. Opening: Problem in large opening. Education: Lack of education in masonry. Needs heavy foundation: Due to heavy weight, large foundation is required. Also cracking and settlement may occur.
Basic Terms in Masonry
Course: Horizontal layer of bricks or stones. Bed: Lower surface of bricks/stones in each course which is perpendicular to the line of pressure. Backing: Unexposed wall Facing: Exposed to weather Hearting: Between the facing and backing Joint: Bed and perpend. Header: Length is perpendicular to the face of the wall Stretcher: Longest side parallel to the face of the wall. Bond: Individual brick units are tied together with mortar is called as bond.
Basic Terms in Masonry
Closer: Portion of brick cut in such a manner that its one long face remains uncut. Queen closer: Length-wise cutting of brick. King closer: Cutting off the triangular piece between the center of one end and the center of the other (long) side. Bat: Brick cut across the width Soldier: Laid vertically with the long narrow side of the brick exposed. Sailor: Laid vertically with the broad face of the brick exposed. Rowlock:Laid on the long narrow side with the short end of the brick exposed.
Basic Terms in Masonry
Construction Technology
Construction Technology
Lap should be minimum 1/4th brick along the length of wall and 1/2nd brick across the thickness of wall. Bricks bats should be discouraged, except in special locations. The vertical joint in the alternate courses should be along the same perpend. All the finished masonry walls should be cured for at least 7 days. Thickness of mortar joint should be uniform and not more than 13mm in any case. Height of brick masonry in one day should not exceed 1.5m
Types of Bond
Stretcher bond Header bond English bond Flemish bond Facing bond
Brick on edge bond Raking bond Dutch bond Zigzag bond Garden wall bond Rat-trap bond
Types of Bond
Fig: Flemish Bond
Fig: English Bond
S.N.
English bond
S.N.
Flemish bond
1
Headers and stretchers are laid in alternate courses.
1
Headers and stretchers are laid alternately in each course.
2
Strongest of the types of bonds.
2
Comparatively less strong for walls more than 30cm thick.
3
Provides rough appearance.
3
Provide good appearance.
4
Absence of vertical joints in the structure.
4
Partly continuous vertical joints appear in the structure.
5
Special attention is not required for this bond.
5
Special attention is required for this bond.
6
Progress of work is more.
6
Progress of work is less.
7
Costly, no brick bats are used.
7
Economical, as brick bats are used.
8
Skilled labor is not required for its construction
8
Skilled labor required for its construction.
9
Less mortar is used.
9
More mortar is used due to use of bats.
Rat-trap bond
Shiner and rowlock are visible on the face of masonry; this gives the wall with an internal cavity bridged by the Rowlock. Economic use of brick. The cavity provides thermal comfort inside the wall. Used in load bearing as well as thick partition wall. The structural strength increased by inserting steel. Due to cavity, the weight of the building is reduced. Aesthetically pleasing wall.
Rat-trap bond
Hollow concrete blocks
Advantage of Hollow concrete blocks
Hollow block needs low maintenance. High Durable. Construction speed is high when compare to conventional construction systems. It has high tensile strength. Economical friendly and cheap rates. It reduces the building costs. The acoustic system of the room is good with hollow cement construction. Maintains the thermal balance system of any hollow block construction. It reduces the labor cost and total construction cost. High heat and water resistant. It’s no needs of plastering Low water absorption than brick. Main advantage, block voids can be filled by other filling materials.
Compressed Earth Block
Advantage of Compressed Earth Block
Shipping cost: Suitable soils are often available at or near the construction site. Uniformity: Manufactured to a predictable size and has true flat sides and 90-degree angle edges. Non-toxic: Materials are completely natural, non-toxic, chemical-free, and do not out-gas. Sound resistant: An important feature in high-density neighborhoods, residential areas adjacent to industrial zones. Fire resistant: Earthen walls do not burn. Insect resistant: Insects are discouraged since the walls are solid and very dense, and have no food value. Reduction in transportation cost due to locally available material used.
Masonry as infill walls
Masonry as infill walls
Masonry in-fills cause undesirable effects under seismic loading: short-column effect, soft-storey effect, torsion, and out-of-plane collapse. In-fills are separated from the RC frame and do not interfere with the frame under lateral deformations. All lateral force on the building is carried by the bare RC frame alone. In-fills are built integral with the RC frame as non-structural elements. The entire lateral force on the building is carried by the bare RC frame alone. Most common design practice in the developing countries. In-fills are built integral with the RC frame, and considered as structural elements. In-plane stiffness offered by the infill walls is considered in analysis. The forces from this analysis are used in the design of RC frame members and joints.
Reinforced masonry
A construction system where steel reinforcement is embedded in the mortar joints of masonry or placed in holes and after filled with concrete or grout is called Reinforced masonry. Reinforced masonry can be classified into three types: Reinforced hollow unit masonry Reinforced grouted cavity masonry Reinforced pocket type walls
Reinforced grouted cavity masonry
Reinforced pocket cavity masonry construction
Typical anchorages of reinforcing bars
Unreinforced masonry
Likely to be damaged during earthquake. Mortar holding the masonry together is generally not strong enough to resist earthquake forces. Anchorage of walls to the floor and the roof is critical. These houses are weak (brittle) and can break apart. Walls may fall away or buckle, resulting in damage.
Unreinforced masonry
Confined masonry
1. 2. 3.
The masonry walls carry the seismic loads and the concrete is used to confine the walls. Construction system where masonry structural walls are surrounded on all four sides with reinforced concrete. Confinement should be in vertical and horizontal both direction to resists lateral and gravity loads. The confining members are effective for: Enhancing the stability and integrity of masonry wall for inplane and out-of-plane earthquake loads. Enhancing the strength of masonry wall under EQ. Reducing brittleness of masonry improving EQ performance.
Confined masonry The structural component of confined masonry are: Masonry wall Confining elements (tie-beams and tie columns) Floor and roof slab Plinth band Foundation
Confined masonry
Chapter-08 Design of masonry walls for gravity loads IS1905-1987
Introduction to Codal Provisions Scope Structural design of unreinforced load bearing and non-load bearing walls constructed with solid or perforated burnt clay brick, concrete block. Do not apply to walls constructed in mud mortars.
Walls
A. B. C. D.
Most essential component of building. Enclose or divide space of building. Provide privacy, afford security and protection against heat, rain and cold. Designed wall should have Strength and stability Weather resistance and thermal insulation Durability and fire resistance Sound insulation
Walls Vertical load-bearing member, the width of which (length) exceeds 4 times the thickness. Isolated vertical load bearing member, the width of which does not exceed 4 times the thickness is called column. Types of walls Load-bearing wall- Designed to carry superimposed load and self weight. Non-load bearing wall- Designed to carry self weigh only.
Walls
Partition wall: Thin internal wall used to divide the space within building. Internal wall: Load bearing partition wall. Panel wall: External non-load bearing wall (commonly related to frame structure) Party wall: Wall separating the adjoining buildings. May or may not be load bearing. Separating wall: Wall separation different occupancies within the same building. Curtain wall: Self-supporting wall carrying no other vertical load but subject to lateral loads. E:\File\code\codes\1905.PDF Cross-walls: Load bearing walls construction in which all the loads are carried by internal walls, running at right angles to the length of building.
Load bearing Walls
Solid masonry walls: Most commonly used but may have opening. Cavity walls: Wall comprising two leaves, each leave being build of masonry units and separated by a cavity and tied together with metal ties or bonding units to ensure that two leaves act as one structural units. Space between two leaves being left continuous or filled with other non-load bearing insulating material.
Solid Wall
Cavity Wall
Load bearing Walls Faced walls: Wall in which the facing and backing are of two different materials are bonded together to ensure common action. (IS1905 Figure 3, pageno.4) E:\File\code\codes\1905.PDF Veneered walls: Wall in which facing is attached to the backing but not so bonded as to result in a common action under load.
Design Consideration General Masonry structure gain stability from the supports offered by cross walls, floors, roofs and other elements. Structure is so planned that eccentricity of loading on the members is small as possible. Mix proportions should follow Table 1 (IS1905-1987) page no. 6
Design Consideration cont… Selection of mortar Requirement of mortar for masonry structure workability, strength, water retentively and low drying shrinkage. Mortar strength in general should not be greater than that of masonry unit. Letter H, M and L indicates the high strength, medium strength and low strength
Masonry unit strength (N/mm2)
Mortar type
Below 5
M2
5 to 14.9
M1
15-24.9
H2
25 or above
H1
Design Consideration cont…. Stability requirement Lateral support: Wall can be laterally supported either at vertical intervals by floor roof transmitting horizontal forces to cross-walls and then to the foundation or at horizontal interval by cross-wall, piers transmitting horizontal forces to foundations. Lateral support has to perform two important functions as: To resist horizontal components of the forces so as to ensure the structure against over-turning. To limit the slenderness ratio of masonry elements in order to prevent failure by buckling. Adequate lateral support: If supports is capable of resisting the sum of following lateral forces than wall considered as adequate lateral support. Simple static reactions at the point of lateral support to all the lateral loads. 2.5% of the total vertical load that the wall or column is designed to carry at the point of lateral support.
Design Consideration cont…. Effective Height of wall
The effective height of a load bearing wall is assessed based on the relative stiffness of the elements of the structure connected to the wall together with the efficiency of the connection. A wall is stiffened by floors , or roofs , suitable cross walls or any similar construction element.
Refer: Effective height of wallTable 4 (IS1905-1987) page no .11 E:\File\code\c odes\1905.PDF
Design Consideration cont….
Effective length of wall Refer: Effective length of wall- Table 5 (IS1905-1987) Page no. 12 Effective thickness of wall Refer: Effective thickness of wall (Clause 4.5 of IS1905-1987) Page no: 13 Slenderness ratio (SR) The ratio of effective height or effective length to the effective thickness of wall whichever is less is the design value of slenderness ratio. The slenderness ratio for a load bearing column shall not exceed 12. Maximum SR for a load bearing wall No of Storey
Maximum Slenderness Ratio Using Portland cement in mortar
Using mortar lime
Not exceeding 2
27
20
Exceeding 2
27
13
Eccentricity: Clause 4.7 IS1905 E:\File\code\codes\1905.PDF
Refer: Table 7 (IS 1905-1987)
Structural Design General Analyzed by accepted principle of mechanics to ensure safe and proper functioning in service of its components. All components shall be capable of sustaining the most adverse combinations of loads, which the structure reasonably expected to be subjected to during and after construction. Design Loads Dead load of walls, columns, floors and roofs. Live load on floor and roof. (Calculated by using IS875 II) Wind load on walls and sloping roof. Seismic forces (In zone I and II, not necessary and zone III, IV and IV should adopted)
Structural Design cont… Load dispersion (Clause 5.3 IS1905) General: The angle of dispersion of vertical load on walls shall be taken as not more than 30° from the vertical. Arching action: The arching action of well-bonded walls supported on lintels and beams should be account.
Lintels: See detail on code (Clause 5.5.3 IS1905)
Structural Design cont….
Permissible stresses (Clause 5.4 IS1905) Permissible compressive stress (clause 5.4.1) Permissible compressive stress = Fb*Ks*Ka*Ks, where The basic compressive stress based on type and strength of the masonry units and mix of mortar, Fb (Table 8) Stress reduction factor (Ks) depends on SR and eccentricity of loading (Table 9) Area reduction factor (Ka), Ka = 0.7 +1.5A, A= area in m2 Shape reduction factor (Ks), (Table 10) Permissible tensile stress (Clause 5.4.2) Permissible shear stress (clause 5.4.3) Fa =0.1+Fd/6 ≤0.5N/mm2 , Fa =permissible shear stress, Fd =compressive stress due to dead load
Wall Opening
Effect of openings Reduce lateral strength of URM walls and should be small & centrally located. Total length of openings should be half for one-storey and onethird for two- storey Buildings.
Refer: NBC 109
