---
title: "Basic assumptions of working stress method"
subject: "Design of Steel and Timber Structure"
chapter: 2
key: "semester_6/design_of_steel_and_timber_structure:wsm-basic-assumptions"
source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/wsm-basic-assumptions
site: BCT Engineering
---

# Basic assumptions of working stress method

Design of Steel and Timber Structure · Chapter 2: Working Stress Design Method

- Asked in: 2072 Asoj, 4 marks
- Video: [assumptions in working stress method,rcc design part 2](https://www.youtube.com/watch?v=_PO3M9Qle3M) — Civil wale here
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/wsm-basic-assumptions
- Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-2

## Question

What are the basic assumptions of Working Stress Design method?

## Short answer

WSM assumes steel is homogeneous, isotropic, and linearly elastic; plane sections remain plane. Working loads are used, stresses must not exceed permissible values (e.g., 0.6f_y tension). A single factor of safety covers all uncertainties, and residual stresses are ignored.

## Answer

The Working Stress Method (IS 800:1984) is based on the theory of elasticity. Basic assumptions:

1. Steel is homogeneous and isotropic.
2. Stress is proportional to strain up to the permissible stress (Hooke's law); $E$ is constant ($2\times10^5$ N/mm²).
3. Plane sections before bending remain plane after bending (linear strain distribution).
4. Loads used are working (service) loads without load factors.
5. Stresses in members and connections under working loads must not exceed the permissible stresses, which are the yield stress divided by a factor of safety (about $0.6f_y$ for tension, $0.66f_y$ for bending).
6. A single overall factor of safety accounts for variations in load, material, workmanship and analysis.
7. Effects such as residual stress, post-yield strength and redistribution are ignored.
8. Deformations are small and the principle of superposition applies.

## Check yourself

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

1. Which loads are used in the Working Stress Method?
   A. Factored loads
   B. Collapse loads
   C. Working (service) loads without load factors
   D. Ultimate loads
   Answer: C. WSM uses unfactored working loads and checks stresses against permissible values. (Key point: Loads in WSM)

2. What are the permissible stresses in WSM for tension and bending?
   A. 0.6fy and 0.66fy
   B. 0.66fy and 0.6fy
   C. 0.45fy and 0.75fy
   D. fy and fy
   Answer: A. Permissible stress is about 0.6fy in tension and 0.66fy in bending. (Key point: Permissible stress)

3. What does WSM ignore?
   A. Hooke's law
   B. Plane sections remaining plane
   C. Working loads
   D. Residual stress, post-yield strength and redistribution
   Answer: D. WSM is purely elastic, so post-yield strength and redistribution are not used. (Key point: Ignored effects)

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