Design of Masonry Structures (IS 1905) Notes

Last reviewed 16 Sept 2026 · 6 min read

Load-bearing masonry

In load-bearing construction, walls built of bricks, stone or concrete blocks in mortar carry the floor and roof loads down to the foundation. IS 1905 (Code of practice for structural use of unreinforced masonry) governs their design. Such buildings are economical for low- and medium-rise residential and institutional work.

Masonry is strong in compression and weak in tension and shear, so design ensures:

compressive stresses stay within permissible values,
slenderness is limited to avoid buckling,
eccentricity of loads is controlled to avoid tension,
walls are laterally supported by floors, roofs and cross walls.

Strength of masonry

The compressive strength of masonry depends on:

Strength of the unit (brick/block) — the most important factor.
Mortar grade — stronger mortar raises strength, but mortar much stronger than the unit gives little extra benefit and makes the wall brittle; mortar should be compatible with the unit.
Height-to-thickness ratio of units, bond pattern, thickness and filling of joints, and workmanship (frogs filled, joints fully bedded).

IS 1905 gives a basic compressive stress $f_{b}$ of masonry for combinations of unit strength and mortar type, derived from prism test data.

Effective height, length and thickness

Code ProvisionIS 1905 — effective height of walls (selected)

Support condition	Effective height $h$
Lateral as well as rotational restraint at top and bottom (e.g. RCC floor/roof slabs bearing on the wall)	$0.75 H$
Lateral restraint at top and bottom without rotational restraint (e.g. timber floor, roof truss)	$1.0 H$
Wall free at top (parapet, compound wall)	$2.0 H$

$H$ = height between centres of supports (or clear height as defined in the code).

Code ProvisionIS 1905 — effective length of walls (selected)

Condition of cross walls at ends	Effective length $L$
Continuous and supported by cross walls at both ends	$0.8 L$
Continuous at one end and supported at the other	$0.9 L$
Supported at both ends (not continuous)	$1.0 L$
Continuous at one end, free at the other	$1.5 L$
Supported at one end, free at the other	$2.0 L$

Effective thickness: for solid walls, the actual thickness; for cavity walls, two-thirds of the sum of the thicknesses of the two leaves (or the thickness of the thicker leaf, whichever is greater); for walls stiffened by piers, the thickness multiplied by a stiffening coefficient.

Slenderness ratio

S R = lesser of \frac{effective height}{effective thickness} and \frac{effective length}{effective thickness}

(For columns, effective height divided by the corresponding lateral dimension, about each axis.)

IS 1905 limits the slenderness ratio of load-bearing walls; for walls in cement or cement–lime mortar the maximum is 27, with lower limits for lime mortar and for columns. Higher slenderness reduces permissible stress.

Eccentricity

The eccentricity ratio $= e / t$ (eccentricity of the resultant load ÷ effective thickness):

$e / t$	Treatment
≤ 1/24	Considered axial
1/24 to 1/6	Small eccentricity — full section in compression with varying stress
> 1/6	Large eccentricity — tension develops; masonry tension ignored, section checked for reduced effective area

Floors bearing on only part of the wall thickness produce eccentric loads; the resultant is usually assumed at one-third of the bearing width from the face.

Permissible compressive stress

FormulaPermissible stress in masonry

f_{c} = k_{s} \times k_{a} \times k_{p} \times f_{b}

$f_{b}$ — basic compressive stress (from unit strength and mortar type).
$k_{s}$ — stress reduction factor for slenderness and eccentricity.
$k_{a}$ — area reduction factor for small sections: for a wall or column of horizontal area $A$ less than 0.2 m², $k_{a} = 0.7 + 1.5 A$ ( $A$ in m²).
$k_{p}$ — shape modification factor for the height-to-width ratio of the units (greater than 1 for tall units such as blocks).

Code ProvisionStress reduction factor

k_{s}

for axial load (e/t ≤ 1/24), selected values

SR	6	8	10	12	14	16	18	20	22	24	26	27
$k_{s}$	1.00	0.95	0.89	0.84	0.78	0.73	0.67	0.62	0.56	0.51	0.45	0.43

Factors are lower for larger eccentricity ratios.

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Design of Masonry Structures (IS 1905)