Code of Practice for Structural Use of Unreinforced Masonry

IS 1905 – Scope: Key Specifications & Tables

The code governs masonry design, focusing on permissible stresses, slenderness, and unit properties.

1. Shape Modification Factor (kp) for Masonry Units

Clause 5.4.1.3, Table 10

2. Slenderness Ratio & Coefficients

Clause 4.5.2, Table 4.6

• Sp = Centre-to-centre spacing of piers or cross walls
• tp = Thickness of pier
• fp = Flexural strength ratio

IS 1905: Definitions - Key Formulas, Tables, and Specifications

1. Notations & Symbols (Clause 7.1)

• A = Area of section
• b = Width of bearing
• DPC = Damp proof course
• ē = Resultant eccentricity
• fb = Basic compressive stress
• f'c = Permissible compressive stress
• fd = Compressive stress due to dead loads
• fs = Permissible shear stress
• tp = Thickness of pier
• t = Thickness of wall
• H = Height between lateral supports
• P = Resultant load
• ka = Area factor
• kp = Shape modification factor (see Table below)
• L = Actual length of wall
• Sp = Spacing of piers/buttresses/cross walls
• λ = Slenderness ratio
• f'm = Compressive strength of masonry (prism test)

2. Shape Modification Factor (kp) for Masonry Units

(Clause 5.4.1.3, Table 10)

3. Slenderness Ratio & Stiffening Coefficient

(Clause 4.5.2, Table 4.6)

| Sl. No. | Slenderness Ratio (λ) | Spacing (Sp) |

IS 1905: Key Formulas, Tables & Specifications for Materials

1. Masonry Units (Clause 3.1)

• Burnt Clay Bricks: IS 1077-1986 (common), IS 2180-1985 (heavy-duty), IS 2222-1979 (perforated)
• Stones: IS 3316-1974 (granite), IS 3620-1979 (laterite)
• Sand Lime Bricks: IS 4139-1976
• Concrete Blocks: IS 2185 (Part 1 & 2), Lime based (IS 3115-1978), Burnt clay hollow blocks (IS 3952-1978), Gypsum partition blocks (IS 2849-1983), Autoclaved cellular concrete blocks (IS 2185 Part 3)

2. Basic Compressive Stresses (Table 8, Clause 5.4.1)

• Permissible compressive stress ( f_c = f_b \times k_s \times k_a \times k_p )
  • ( f_b ) = Basic compressive stress from Table 8 (N/mm²)
  • ( k_s ) = Stress reduction factor
  • ( k_a ) = Area reduction factor
  • ( k_p ) = Shape modification factor (Table 10)

3. Shape Modification Factor (k_p) (Table 10)

IS 1905: Lateral Supports and Stability (Clause 4.2)

Key Specifications & Tables

1. Stiffening Walls Thickness & Spacing (Clause 4.2.2.2, Table 2)

• Note: Spacing is center-to-center.
• For L or U-shaped walls, flange overhang width = min(12t, H/6) (t = thickness, H = wall height).

2. Minimum Basement Wall Thickness (Table 3)

• Thickness based on permanent load and soil pressure.
• For surcharge from adjoining buildings, structural analysis required.

3. Effective Overhanging Width of Flange (Fig. 10)

• For stability:
  Width = min(12 × t, H/6) for stiffening walls
  Width = min(6 × t, H/6) for other flanges

Stability Criteria (Clause 4.2.2)

• Walls/columns must have lateral supports capable of resisting lateral loads plus static reactions.
• Anchorage of slabs to walls (Fig. 4 & 5) must

Design of Masonry Walls and Columns (IS 1905)

1. Free-Standing Walls (Clause 5.5.2.1)

• Design based on permissible tensile stress or stability.
• In seismic zones I & II, use Table 11 without detailed calculations if mortar ≥ M1.
• If a horizontal damp-proof course is present, minimum thickness is the greater from:
  • Height/thickness ratio from Table 11 reduced by 25% (height from DPC level), or
  • Height/thickness ratio from Table 11 (height from lower lateral restraint).

Table 11: Height to Thickness Ratio vs Wind Pressure

• Height measured from 15 cm below ground or footing top.
• Thickness includes plaster.

2. Walls and Columns under Vertical Loads (Clause 5.5.1)

• Design based on permissible compressive stress.
• Thickness/section depends on:
  • Masonry unit strength
  • Mortar grade
  • Slenderness ratio
  • Eccentricity
  • Workmanship & supervision quality

3. Shape Modification Factor (Clause 5.4.1.3)

Adjust permissible stress using kp factor based on unit height-to-width ratio:

IS 1905: Construction Methods and Workmanship - Key Points

1. Applicable Standards for Masonry Construction (Clause 6.1.1)

2. Masonry Units Standards (Clause 3.1)

3. Compressive Strength by Prism Test (Clause B-1.1 & Table 12)

• Prism size: ≥ 40 cm height, height/thickness (h/t) ratio between 2 and 5.
• Correction factors applied based on h/t ratio:

IS 1905: Notations and Symbols (Clause 7.1 & Appendix E)

Key notations used in masonry design:

Shape Modification Factor (kp) for Units (Clause 5.4.1.3)

Additional Notes on Eccentricity (Appendix A)

• For RCC slabs with span ≤ 30 × wall thickness, load acts at center of bearing.
• For span > 30 × thickness

IS 1905: Increase in Permissible Stress (Clause 5.4.1.4 & Table 9)

Key Points on Increase in Permissible Compressive Stress:

• Eccentricity Ratio (e/t): Ratio of eccentricity to thickness of the wall.
• Permissible Stress Increase:
  • For e/t > 1/24 and ≤ 1/6:
    • 25% increase in permissible compressive stress allowed.
  • For e/t > 1/6:
    • 25% increase allowed, but tension area ignored in load capacity calculation.

Stress Reduction Factor (Table 9) for Slenderness Ratio (λ) and Eccentricity:

• Note: Linear interpolation allowed between values.
• For eccentricity between 1/3 and 1/2 thickness, factor varies linearly from 1.0 to 0.20 (for λ=6 to 20).

Summary Formula:

[ f_{perm,new} = f_{perm} \times (1 + 0.25) \quad \text{for } \frac{1}{24} < \frac{e}{t} \leq \frac{1}{6} ]

[ \text{If } \frac{e}{t} > \frac{1}{6}, \quad f_{perm,new} = f_{perm} \times 1.25

Approximate Design Guidelines for Non-Load Bearing Walls (IS 1905 - Appendix D & Clauses)

1. Panel Dimensions & Restraint Conditions (Clause 3.1)

2. Free-Standing Walls (Clause 5.5.2.1 & Table 11)

• Design based on permissible tensile stresses or stability.
• In seismic zones I & II, use Table 11 for height/thickness ratio without detailed calculations.
• Height measured from 15 cm below ground or footing top to wall top.
• Thickness includes plaster.

Interpolate linearly for intermediate pressures.

3. Additional Notes

• Provide M-15 concrete bed blocks under cantilever or concentrated loads to reduce edge stresses (Clause 3.3).
• For walls with damp-proof courses, reduce height/thickness ratio by 25% if tension cannot develop vertically near base.

flowchart TD
    A[Non-Load Bearing Wall] --> B{Lateral Restraint}
    B -->|Ends only| C[Use Clause 3.1a limits]
    B -->|Ends & Top| D[Use Clause 3.1b