Design of Brick Masonry Walls - LCETED

Brick walls are subjected to compression, and IS 1905 — 1987, "Code of Practice for Structural Use of Unreinforced Masonry Walls", and SP 20 deal with their design. In this article, we will examine the fundamental aspects of design. Details of the design can be obtained from the above references.

 

EFFECTIVE HEIGHT, EFFECTIVE LENGTHS OF WALLS AND SLENDERNESS RATIO

Designs of all compression members such as columns and walls are based on the slenderness ratio of the member, which is defined as the effective length divided by the thickness of the walls. Unlike in a column (which is defined as a member with a breadth less than four times its thickness), for calculating the effective length of a wall, we have to consider not only its dimension along with the height but also the dimension along its length between supports such as cross walls.

Effective height

= 0.75 H to 0.8 H, for walls restrained at both the ends

= 1.5 H, for walls restrained only at one end such as in compound walls and parapets

 

Effective Length

= L, for walls restrained at both the ends

= 1.5 L, for walls restrained at one end

= 2 L, for walls with no restraints

 

Where,

H = The height of the wall between the restraints

L = The length of the wall between the cross walls, pilasters and other constraints

 

The larger of the above values with respect to H and L is taken for calculation of the slenderness ratio (for details refer to IS 1905).

  

Slenderness ratio	Larger the values of effective length and effective height of the wall
	The thickness of the wall

Brick walls with a slenderness ratio of up to 6.0 are considered short walls which fail under compression. Walls with a slenderness ratio greater than 24 are unstable and are prohibited. To find the reduced load that can be carried on walls with a slenderness ratio greater than 6.0, we use a reduction factor explained in the Section above

 

FAILURE LOAD ON SHORT WALLS

The crushing strength of common bricks varies from 5 N/mm2 to 35 N/mm2 depending on the different regions in India. It is recommended that the mortar to be used in brickwork should have a strength very nearly equal to the strength of brick If such a short-brick pillar is loaded, the failure load of the pillar will be much less than that of the individual brick.

This is because when we load the pillar, each brick with its mortar bed behaves as a beam on an elastic foundation and the failure is in bending by tension corresponding to its modulus of rupture. Hence, the maximum allowable strength A_max of a short column is as follows:

 

p_max = 1/10 x compressive strength of the bricks

 

REDUCTION FACTOR FOR SAFE STRESS IN BRICK WALLS

The reduction factors given in Table A.1 are recommended in IS 1905, 1987 and SP20 for central loading (e = 0). It will be further reduced for eccentricity of load as shown in the table below

Stress reduction factors for slenderness ratio and eccentricity for brick walls (SF)

 

 

AN EXAMPLE OF THE APPROXIMATE DESIGN OF BRICK WALLS

Problem. A brick wall of a two-storey building with each floor 3 m high has a thickness of 200 mm. Let the distance between cross walls in the room be 3 m. Assume that the load from each floor is 10 kN per metre length of the wall. What should be the minimum strength of bricks that can be used for this building? Assume that there is no eccentricity of load.

 

Solution:

1. Find the total load on the base.

We have

Total weight of two-storey high wall @ 20 kN/m³ for brickwork for 0.2 m thick wall = 0.2 x 20(2 x 3) = 24 kN/m (DL)

Live load from the floors = 2 x 10 = 20 kN/m (for two floors)

Total load = 44 kN/m

 

2. Find effective height, length and slenderness ratio.

Effective height = 0.75 x H = 0.75 x 3 = 2.25 m

Effective length = L = 3 m (larger value = 3 m)

Larger slenderness ratio = 3/0.2 =15

 

3. Find stress factor SF for eccentricity, e = 0.

From Above Table for slenderness ratio = 15, S_F = 0.625 (by interpolation)

4. Find the necessary strength of brickwork at the base of brickwork (σ).

We have Load/Area = Stress = S_F x strength of masonry

or

(44 x1000)/(200x1000) = S_F σ N/mm²

or

σ = 44/(200 x 0.625) = 0.4 N/mm² (approx)

 

5. Find brick strength for the brickwork of strength 0.4 N/mm².

We have strength of brick = 10 x strength of the brickwork

= 10 x 0.4 = 4 N/mm²

 

Note: Ordinary well-burnt country bricks with a minimum strength of 5 N/mm2 will be suitable for this building. Further details of the design can be obtained from IS 1905.

 

Conclusion: The design of masonry walls is important when we build buildings more than one storey in height. The slenderness of the walls and the required strength of the masonry units are important items to be checked in these constructions.

Must read: How to calculate dead load of brick wall or block wall?

Must read: Method of Laying Brickwork in Masonry Construction 

FAQ

 

What are the reference codes used for brick wall design?

IS: 4326, IS 1905 — 1987, "Code of Practice for Structural Use of Unreinforced Masonry Walls", and SP 20 deal with their design