Code of Practice for Structural Use of Unreinforced Masonry
1987 Edition

Overview of Structural Masonry Design: This section establishes the code’s jurisdiction over design criteria including allowable stresses, slenderness limits, and properties of masonry units.

1.1 Shape Modification Factor (kp) for Masonry Elements

Refer Clause 5.4.1.3, Table 10

| Height-to-Width Ratio | Crushing Strength (N/mm²) 17 5.0 | 7.5 | 10.0 | 15.0 | |----------------------|------------------------------|-----|-----|------|------| | Up to 0.75 | | 1.0 | 1.0 | 1.0 | 1.0 | | 1.0 | | 1.2 | 1.1 | 1.1 | 1.0 | | 1.5 | | 1.5 | 1.3 | 1.2 | 1.1 | | 2.0 to 4.0 | | 1.8 | 1.5 | 1.3 | 1.2 |

1.2 Slenderness Ratio and Coefficients

Refer Clause 4.5.2, Table 4.6

• Sp represents the center-to-center spacing between piers or cross walls.
• tp denotes pier thickness.
• fp refers to the flexural strength ratio.

Key Terms and Symbols in IS 1905

2.1 Common Notations (Clause 7.1)

• A: Sectional area
• b: Bearing width
• DPC: Damp proof course
• ē: Resultant eccentricity
• fb: Basic compressive stress
• f'c: Permissible compressive stress
• fd: Compressive stress owing to dead load
• fs: Permissible shear stress
• tp: Pier thickness
• t: Wall thickness
• H: Distance between lateral supports
• P: Resultant load
• ka: Area factor
• kp: Shape modification factor (refer to tables)
• L: Actual wall length
• Sp: Spacing of piers or cross walls
• λ: Slenderness ratio
• f'm: Masonry compressive strength from prism tests

2.2 Shape Modification Factor Table (Clause 5.4.1.3)

2.3 Slenderness and Stiffening Coefficients (Clause 4.5.2)

Refer to slenderness ratio and associated coefficients for design application.

Overview of Materials According to IS 1905

3.1 Masonry Units (Clause 3.1)

• Burnt Clay Bricks: Covered under IS 1077-1986 (Common), IS 2180-1985 (Heavy-duty), IS 2222-1979 (Perforated)
• Stone Units: IS 3316-1974 (Granite), IS 3620-1979 (Laterite)
• Sand Lime Bricks: IS 4139-1976
• Concrete Blocks: IS 2185 Parts 1 & 2; Lime-based blocks per IS 3115-1978; Burnt clay hollow blocks IS 3952-1978; Gypsum partition blocks IS 2849-1983; Autoclaved cellular concrete blocks IS 2185 Part 3

3.2 Basic Compressive Stress (Clause 5.4.1, Table 8)

• Permissible compressive stress is computed as:
  ( f_c = f_b imes k_s imes k_a imes k_p )
  where ( f_b ) is basic compressive stress from Table 8, ( k_s ) is stress reduction factor, ( k_a ) is area factor, and ( k_p ) is shape modification factor.

3.3 Shape Modification Factor (kp) Table

Refer to previous tables under Clause 5.4.1.3 for detailed kp values.

Lateral Support Requirements and Stability Criteria (Clause 4.2)

Important Specifications & Tables

4.1 Thickness and Spacing of Stiffening Walls (Clause 4.2.2.2, Table 2)

• Note: Spacing is measured center-to-center.
• For L or U-shaped walls, flange overhang width is the lesser of 12 times the thickness or one-sixth of wall height.

4.2 Minimum Basement Wall Thickness (Table 3)

• Thickness depends on dead load and soil pressure.
• For surcharge from adjacent structures, structural analysis is necessary.

4.3 Effective Overhang Width of Flanges

• For stiffening walls: Width = minimum of (12 × thickness, H/6)
• For other flanges: Width = minimum of (6 × thickness, H/6)

Stability Requirements (Clause 4.2.2)

• Walls and columns must be laterally supported to resist lateral forces plus static load reactions.
• Anchorage details for slabs to walls are provided in figures 4 and 5 of the standard.

Guidelines for Designing Masonry Walls and Columns (IS 1905)

5.1 Free-Standing Walls (Clause 5.5.2.1)

• Design is based either on permissible tensile stresses or stability considerations.
• For seismic zones I and II, Table 11 can be used to determine height/thickness ratios without detailed calculations if mortar grade is M1 or higher.
• When a horizontal damp-proof course is included, the minimum thickness corresponds to the greater of:
  • Height/thickness ratio from Table 11 reduced by 25% (height measured from DPC level), or
  • Height/thickness ratio from Table 11 (height measured from the lower lateral support).

Table 11: Height-to-Thickness Ratios for Various Wind Pressures

• Height is measured from 15 cm below ground or footing top.
• Wall thickness includes plaster.

5.2 Walls and Columns Under Vertical Loads (Clause 5.5.1)

• Design is based on allowable compressive stresses.
• Thickness and cross-section depend on:
  • Masonry unit strength
  • Mortar grade
  • Slenderness ratio
  • Eccentricity of loads
  • Quality of workmanship and supervision

5.3 Application of Shape Modification Factor (Clause 5.4.1.3)

• Permissible stresses are adjusted using the kp factor based on unit height-to-width ratio.

Best Practices for Construction and Craftsmanship (IS 1905)

6.1 Applicable Codes for Masonry Construction (Clause 6.1.1)

6.2 Masonry Unit Standards (Clause 3.1)

6.3 Compressive Strength Testing by Prism (Clause B-1.1 & Table 12)

• Prism height should be at least 40 cm with height-to-thickness ratio between 2 and 5.
• Correction factors vary according to h/t ratio:

Key Symbols and Their Definitions (Clause 7.1 & Appendix E)

Shape Modification Factor (kp) Table

Refer to Clause 5.4.1.3 for the kp values based on unit height-to-width ratios.

Guidelines for Increasing Permissible Compressive Stress (Clause 5.4.1.4 & Table 9)

Important Points on Stress Increase:

• The ratio of eccentricity to wall thickness (e/t) governs permissible stress adjustments.
• For eccentricity ratios between 1/24 and 1/6, a 25% increase in allowable compressive stress is permitted.
• For e/t greater than 1/6, a 25% increase is allowed but the tension zone is disregarded in load capacity computations.

Stress Reduction Factor Table (Table 9)

• Linear interpolation can be used between tabulated values.
• For eccentricity between 1/3 and 1/2 thickness, factors decrease linearly from 1.0 to 0.20 (for λ=6 to 20).

Formula Summary:

[ f_{perm,new} = f_{perm} \times 1.25 \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 \quad \text{but tension area neglected} ]

Approximate Design Recommendations for Non-Load Bearing Masonry Panels (Appendix D)

D.1 Panel Dimensions and Restraint Conditions (Clause 3.1)

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

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

Linear interpolation is allowed for intermediate wind pressures.

D.3 Additional Recommendations

• Use M-15 concrete bed blocks under cantilever or concentrated loads to reduce stress concentrations (Clause 3.3).
• For walls with damp proof courses, reduce the height/thickness ratio by 25% if vertical tensile stresses near the base cannot develop.

flowchart TD
    A[Non-Load Bearing Wall] --> B{Check Lateral Restraint}
    B -->|Ends only| C[Apply Clause 3.1a Limits]
    B -->|Ends and Top| D[Apply Clause 3.1b Limits]
    B -->|Top only| E[Apply Clause 3.1c Limits]