Design Of Brick Masonry: Procedure

Tests on bricks:

1. Water absorption:    Brick units immersed in water for 24 hours has,

i) upto 12.5 N/mm² strength and water absorption should not be greater than 20%

ii) for higher classes, water absorption should not be greater than 15%

2. Efflorescence: Leaching of water soluble salts (white coloured) under efflorescence. Test for efflorescence is done as per IS3495 (Part III) -1976. The brick is kept in a

dish with water height as 25mm and the time for water absorption and evaporation is noted. This value is compared with the same dish with 25mm water height kept for evaporation alone. Based on the code, efflorescence in brick is reported as nil, slight, moderate, heavy and serious.

Hardness: For the brick to be hard, it should create no impression by finger

3. Hardness: For the brick to be hard, it should create no impression by finger nail.

4. Soundness: When two bricks are struck, it should not break and produce a clear ringing sound.

5. Compressive test: 3.5 -40 N/mm²

6. Flexure test: Rarely done

Classification of brick based on structure and usage:

1. Solid wall

2. Cavity wall

3. Faced wall

4. Veneered wall

Based on loading, walls are classified as,

1. Axially loaded walls [Load applied at centre t/2]

2. Eccentrically loaded walls

3. Laterally loaded walls [Loading applied at sides]

Design procedure:

Slenderness (Least of l_e/t & h_e/t)_max =:27)(?

(?      =   60   for   RC   columns, nbraced?= RC45walls)&   30   for   brace

1.  Actual stress on the brick masonry wall is found based on the load from slab and self weight of wall.

The permissible compressive stress for masonry based on the type of mortar and compressive strength of brick unit is taken from Table 8, IS1905-1987. This table is valid for slenderness ratio ? ? 6 and eccentricity

3. Corrections are applied for slenderness ratio, eccentricity (if any), shape and size of brick unit. Shape modification factor and cross sectional area of masonry (area reduction factor).

4. Slenderness ratio is found as the least of l_e/t or h_e/t, where, (l_e = Effective length and h_e = effective height).

Effective length is found from Table 5, IS1905 -1987 and effective height is found from Table 4, IS1905-1987.

Table 4 -Effective height:   

Support condition                  Effective height

Fixed -Fixed      0.75H

Fixed -Hinged    0.85H

Hinged -Hinged H

Fixed -Free        1.5H

The permissible _maxvalue) for cement of mortar?(OPCis& PPC),27 given(? in Table 7, IS1905-1987.

5. Eccentricity of loading is determined (for axial loading e = 0). Eccentricities for various other cases are to be checked as per Appendix B of IS1905-1987.

6. For the permissible stress adopted, shape modification factor is found based on height to width ratio of each brick unit given in Table 10, IS1905-1987.

7. Area reduction factor is applied for elements having cross section less than 0.2m². The area modification factor, k = 0.7 + 1.5 A

8.   After applying modification factors, the actual stress is verified with a modified permissible_act< _perstress,? ?

The permissible stress (strength of the wall) depends upon the following factors: i) Compressive strength of masonry unit

ii) Compressive strength of mortar used iii) Slenderness ratio of the wall

iv) Eccentricity in loading

v) Shape and size of brick unit

vi) Cross sectional area of masonry