Masonry Design & Specifications

INTRODUCTION:

Masonry is defined as any structural assemblage of masonry units like stones, bricks, blocks etc. with a binding material which is known as mortar. The walls of the masonry buildings are designed to be stable,strong and durable enough to withstand the most severe combination of loads also called as design load. Masonry construction is flexible in building layout ,quite economical, provides thermal and acoustic insulation and as well as it is fire and weather resistant. Masonry units that have been previously used should not be reused in brickwork or block work construction unless they are thoroughly cleaned and conform to relevent code. The minimum compressive strength of masonry units to be used in reinforced masonry shall be 7 N/mm2. But the strength of bricks in India varies from place to place depending on the nature of local soil, technique adopted in moulding and burning of bricks.

Note: At present, in India, the IS CODE for masonry construction is IS 1905:1988 " Structural Use of Unreinforced Masonry". SP 20 is a hand book for masonry design and construction. The IS code for Structural Use of Reinforced Masonry is under making process. However IS 4326 provides some guidelines for construction of reinforced masonry. And IS 13828:1998 provides some guideline for earthquake resistant low strength masonry buildings.

Important Terminologies:

Bed Block : A block that is bedded on a wall, a column or a pier inorder to distribute a concentrated load on a masonry element.

Curtain wall: It is a wall which is subjected to lateral loads only. It is a non load bearing wall. It may be supported by horizontal and vertical structural members laterally.

Cavity wall: Cavity walls are such type of walls which consists of two leaves with each leaf separated by a cavity and combined together with metal ties or bonding units in order to ensure that leaves act as a single unit. The space between the leaves is filled with either an insulating material or a water proofing material. It may be also be left vacent as a continuous cavity.

Effective height: It refers to that height or portion of wall which is used to compute the slenderness ratio.

Free Standing wall: It is a type of compound or parapet wall. These type of walls are acted upon by lateral forces such as wind which try to overturn it but the overturning is resited by gravity force that is self weight of the wall, and flrxural moment which accounts for tensile strength of the masonry.

Grout: It is the mixture of sand, cement and water with a certain consistency so that it fills the voids in any structure.

Grouted Cavity reinforced masonry: In this masonry two single leaf walls spaced at least 50 mm apart and effectively tied together with wall ties. The cavity consits of steel reinforcement and is filled with concrete so that it performs commonly with the masonry under load.

Grouted hollow masonry unit: In this type of grouted masonry construction, certain designated cells of hollow units are continuously filled with grout.

Grouted multi wythe unit: In this form of grouted masonry construction certain space between the wythes is solidly or periodically filled with grout.

Hollow Concrete Block or HCB: It is a masonry unit in which net cross sectional area in any plane parallel to the bearing surface is less than 75% of its gross cross sectional area which is measured in the same plane.

Leaf: It is component of cavity wall. The inner or outer component of cavity wall is called as leaf.

Load Bearing wall: It is the wall that is designed for various load combinations. It is designed to resist imposed vertical load, dead load or self wieght of the structure and also the lateral loads if any.\

Masonry unit: It is a combination of individual units like brick, blocks which are bonded together to form a masonry element like walls, columns etc.

Partition wall: It is a type of interior non load bearing wall. It may be one or half storey in height.

Panel wall: It is an exterior non load bearing wall. It is generally preffered in framed construction and is placed as the exterior walls supported at each storey. These walls are subjected to lateral loads in outplane direction like wind and earthquake loads.

Shear wall and Cross wall: It is the wall designed to carry the lateral forces acting in its plane with or without vertical imposed loads. The walls normal to shear walls are referred to as cross walls.

SMB: Stabilized Mud Block

SCB: Solid Concrete Block

TMB: Table Moulded Brick

URM: Unreinforced Masonry

WCB: Wire Cut Brick

Wythe: It is the continuous vertical tie having thickness equal to one masonry.

Wall Tie: It is a metal fastener which connects wythes of masonry to each other and also to other materials.

Design loads:

Loads to be considered in the masonry Design are:

1. Dead load of walls, floors, columns and roofs. It is to be designed as per IS 875(Part-I). 

2. Live load or Imposed loads on floors and roofs. It is to be designed as per IS 875 Part-II

3. Wind loads on walls and sloping roofs. It is to be designed as per IS 875 Part-III. 

4. Seismic loads as per IS 1893 Part-I and Snow loads as per IS 875 Part-IV. 

Load Combinations:

In structural design of masonry structures, the most common design philosophy is the allowable design method. As per this design method, the combination of loads are:

1. DL+IL

2. DL+IL+WL OR EQL

3. DL+WL

4. 0.9DL+EQL

Where, DL=Dead load, IL= Imposed load, 

WL= Wind load, EQL= Earthquake load. 

Permissible stresses and loads :

In the design load combination, if wind load or earthquake load is being considered then permissible stresses may be increased by 33.33%.

Alternatively, instead of increasing permissible stresses, we can use 25% reduced load for load combinations which accounts wind load or earthquake load and also take into account the full permissible stresses. Thus the modified design combinations for loads are:

1. 0.75(DL+IL+WL or EQL) 

2. 0.75(DL+WL)

3. 0.75(0.9DL+EQL).

THANKS FOR READING PEACEFULLY.