---
title: "Permissible compressive stress of masonry as per IS 1905"
subject: "Design of Timber and Masonry Structures"
chapter: 5
key: "semester_5/design_of_timber_and_masonry_structures:masonry-permissible-stress-factors-code"
source: https://www.bctengineer.com/q/semester_5/design_of_timber_and_masonry_structures/masonry-permissible-stress-factors-code
site: BCT Engineering
---

# Permissible compressive stress of masonry as per IS 1905

Design of Timber and Masonry Structures · Chapter 5: Design of Masonry Walls for Gravity Loads

- Asked in: year not given, 6 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/design_of_timber_and_masonry_structures/masonry-permissible-stress-factors-code
- Whole chapter: https://www.bctengineer.com/md/semester_5/design_of_timber_and_masonry_structures/chapter-5

## Question

Explain how the permissible compressive stress of a masonry wall or column is obtained according to IS 1905. Define the basic compressive stress and each modification factor, and state the increases allowed for wind and earthquake.

## Short answer

Permissible compressive stress = f_b × k_s × k_a × k_p. f_b is basic compressive stress from tables based on masonry unit strength and mortar grade. k_s accounts for slenderness and eccentricity; k_a = 0.7+1.5A if A<0.2 m², else 1.0; k_p is shape factor (1.0 for bricks). Wind/earthquake loads allow a 33% increase in permissible stresses.

## Answer

IS 1905 (Code of practice for structural use of unreinforced masonry) uses the working stress method. The permissible stress is the **basic compressive stress** multiplied by factors for slenderness, area and shape:

$$
f_c = f_b \times k_s \times k_a \times k_p
$$

#### Basic compressive stress $f_b$
It is the compressive stress that a short, concentrically loaded masonry element can carry safely. It is given in a table against the **compressive strength of the masonry unit** and the **grade of mortar** (cement–sand, cement–lime–sand or lime mortars). Higher unit strength and stronger mortar give higher $f_b$. For ordinary brick masonry it is of the order of 0.5 to 2 N/mm².

#### Modification factors
1. **Stress reduction factor $k_s$**: accounts for slenderness ($SR$) and eccentricity of load. It is 1.0 for low slenderness (about 6 or less) and reduces as $SR$ rises. Typical values for axial load:

| Slenderness ratio | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 | 22 | 24 | 26 | 27 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| $k_s$ (axial load) | 1.00 | 0.95 | 0.89 | 0.84 | 0.78 | 0.73 | 0.67 | 0.62 | 0.57 | 0.51 | 0.46 | 0.44 |

2. **Area factor $k_a$**: small sections have a higher chance of defects. For a loaded area $A < 0.2$ m², $k_a = 0.7 + 1.5A$ (with $A$ in m²); $k_a = 1.0$ for $A \ge 0.2$ m².
3. **Shape modification factor $k_p$**: applies to units whose height-to-width ratio differs from that of the standard brick; for bricks $k_p = 1.0$. Squat blocks give higher values, which are taken from tables.

#### Increase for wind and earthquake
When wind or earthquake loads act together with gravity loads, the permissible stresses may be increased, commonly by one-third (33 percent) as in working stress design (some editions of the code quote 25 percent, so check the edition in use). The section must also be safe for the gravity load alone.

#### Other points
- The permissible stress is checked against the actual stress $P/A$ (plus bending effects if the load is eccentric).
- The slenderness ratio of a load-bearing wall should not exceed 27.
- Permissible tensile and shear stresses are given separately and are small; masonry is not designed to take tension.

## Check yourself

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

1. According to IS 1905, the permissible compressive stress is:
   A. f_c = f_b + k_s + k_a + k_p
   B. f_c = f_b x k_s x k_a x k_p
   C. f_c = f_b / (k_s k_a k_p)
   D. f_c = f_b x k_s only
   Answer: B. The basic compressive stress is multiplied by the slenderness (k_s), area (k_a) and shape (k_p) factors. (Key point: Permissible stress expression)

2. What does the basic compressive stress f_b depend on?
   A. Wall height only
   B. Compressive strength of the masonry unit and grade of mortar
   C. Load eccentricity
   D. Type of roof
   Answer: B. f_b is tabulated against unit strength and mortar grade; stronger units and mortar give higher f_b. (Key point: Basic compressive stress)

3. How does k_s vary?
   A. It is 1.0 at SR of about 6 or less and falls as slenderness (and eccentricity) increases
   B. It increases as SR rises
   C. It is always 1.0
   D. It depends only on the area
   Answer: A. k_s accounts for slenderness and eccentricity: 1.0 at SR of about 6 or less, falling to 0.44 at SR 27 for axial load. (Key point: Stress reduction factor)

4. For a loaded area below 0.2 m^2, k_a = 0.7 + 1.5A. What increase is allowed for wind or earthquake?
   A. Stresses may be doubled
   B. No increase is allowed
   C. Stresses may be increased by one-third, provided it is also safe for gravity load alone
   D. Stresses may be increased by 10 percent
   Answer: C. Small areas get a reduced k_a (more defects); with wind or earthquake, permissible stresses may rise by one-third provided gravity load alone is safe. (Key point: Area factor and wind/earthquake increase)

## Related in Design of Timber and Masonry Structures

- [Safe load on a brick masonry column](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Abrick-masonry-column-safe-load) — Design of Timber and Masonry Structures, chapter 5
- [Out-of-plane bending of a masonry wall](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aout-of-plane-wall-flexure-check) — Design of Timber and Masonry Structures, chapter 6
- [Check of a solid brick wall under gravity load](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Asolid-masonry-wall-gravity-check) — Design of Timber and Masonry Structures, chapter 5
- [Masonry pier under axial load and in-plane moment](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Amasonry-pier-axial-moment-check) — Design of Timber and Masonry Structures, chapter 7
- [Multi-storey wall: stresses at each level](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Athree-storey-wall-base-stress-check) — Design of Timber and Masonry Structures, chapter 5
- [Safe axial load on a timber column](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aaxial-capacity-timber-column) — Design of Timber and Masonry Structures, chapter 3

## Related in other subjects

- [Reduction factors in permissible stress of masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Areduction-factors-permissible-stress) — Concrete Technology and Masonry Structure, chapter 8, asked 2×
- [Permissible stresses as per IS 800](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Apermissible-stresses-is800) — Design of Steel and Timber Structure, chapter 2
- [Compressive and diagonal shear tests of masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acompressive-diagonal-shear-tests) — Concrete Technology and Masonry Structure, chapter 10, asked 2×
- [Compressive strength test of brick wall](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acompressive-test-masonry-wall) — Concrete Technology and Masonry Structure, chapter 10, asked 4×