---
title: "Factors in design of masonry"
subject: "Concrete Technology and Masonry Structure"
chapter: 8
key: "semester_5/concrete_technology_and_masonry_structure:masonry-design-factors"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/masonry-design-factors
site: BCT Engineering
---

# Factors in design of masonry

Concrete Technology and Masonry Structure · Chapter 8: Design of masonry walls for gravity loads

- Asked in: 2076 Chaitra, 4 marks
- Video: [#4 Introductory | Part IV | Design of Masonry Structures](https://www.youtube.com/watch?v=0GC2lfGiqYE) — NPTEL-NOC IITM
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/masonry-design-factors
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-8

## Question

Explain various factors to be considered in design of masonry.

## Short answer

Design factors per IS 1905 include masonry unit and mortar strength, slenderness ratio (limited to 27 for walls), effective height and length depending on end restraint, eccentricity of load, section area (factor for areas below 0.2 m²), shape factor of units, type of loads (vertical, wind, seismic), openings and stiffening, workmanship and quality control, and seismic zone requirements.

## Answer

Design of masonry (IS 1905) is based on working stresses, so the following factors are to be considered:

1. **Strength of masonry unit and mortar grade:** the basic compressive stress $f_b$ depends on both (Table 8); stronger mortar and unit give higher $f_b$.
2. **Slenderness ratio:** effective height or length divided by effective thickness, whichever is less; limited to 27 (cement mortar) for walls and 12 for columns (Table 7).
3. **Effective height and length:** depend on end restraint, such as RCC slab (0.75 H), timber floor (0.85 H or 1.0 H), free top (1.5 H) (Tables 4 and 5).
4. **Eccentricity of loading:** eccentricity ratio $e/t$ reduces the permissible stress (Table 9); App. A gives guidance.
5. **Area of section:** for sections smaller than 0.2 m$^2$ the area reduction factor $k_a$ applies.
6. **Shape of masonry unit:** height-to-width ratio of units gives shape factor $k_p$ (Table 10).
7. **Type of loads:** vertical load, wind, earthquake; no tension in masonry normally; limited flexural tension (0.07 N/mm$^2$ for M1 mortar) for lateral loads.
8. **Openings and stiffening:** openings reduce the effective area; stiffening by cross walls, piers or buttresses increases effective thickness (Table 6).
9. **Workmanship and quality control:** raked joints reduce thickness; curing, bond and supervision.
10. **Seismic zone:** buildings in zones III to V need special strengthening (IS 4326 / NBC 105, 109).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the slenderness ratio limit for walls in cement mortar (Table 7)?
   A. 12
   B. 40
   C. 6
   D. 27
   Answer: D. Columns are limited to 12. (Key point: Slenderness limit)

2. What effective height factor is used with an RCC slab?
   A. 1.5 H
   B. 0.75 H
   C. 0.85 H
   D. 1.0 H always
   Answer: B. A free top gives 1.5 H. (Key point: Effective height)

3. When does k_a apply?
   A. For areas over 2 m^2
   B. Only for lime mortar
   C. For section areas smaller than 0.2 m^2
   D. Only for stone
   Answer: C. Small sections suffer more from local weakness. (Key point: Area factor)

4. What fixes the basic compressive stress f_b?
   A. Strength of unit and grade of mortar
   B. Wall colour only
   C. Roof span only
   D. Number of doors
   Answer: A. f_b is read from Table 8. (Key point: Basic stress)

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