---
title: "Working stress vs limit state method"
subject: "Design of Steel and Timber Structure"
chapter: 3
key: "semester_6/design_of_steel_and_timber_structure:wsm-vs-lsm-difference"
source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/wsm-vs-lsm-difference
site: BCT Engineering
---

# Working stress vs limit state method

Design of Steel and Timber Structure · Chapter 3: Limit State Design Method

- Asked in: 2080 Chaitra, 4 marks; 2076 Baisakh, 6 marks; 2070 Magh, 4 marks (asked 3 times)
- Video: [Working Stress Method V/S Limit State Method | ReactionTest](https://www.youtube.com/watch?v=X00-ri4GmqQ) — Reaction Test
- 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-vs-lsm-difference
- Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-3

## Question

Differentiate between the working stress method of design and the limit state method of design (with sketches).

## Short answer

WSM is based on elasticity with working loads and a single safety factor on stress. LSM considers ultimate and serviceability limit states, uses factored loads and partial factors on loads and material, idealizing steel as elastic-perfectly plastic, achieving more uniform reliability and economy.

## Answer

| Point | Working Stress Method | Limit State Method |
|---|---|---|
| Basis | Elastic theory | Behaviour up to failure and at service |
| Loads | Working (characteristic) loads | Factored loads $\gamma_f\times$ characteristic |
| Safety | One factor of safety on material stress | Partial factors $\gamma_f$ and $\gamma_m$ |
| Design check | Stress $\le$ permissible stress | Factored action $\le$ design strength |
| Stress-strain | Linear elastic | Idealised (elastic-plastic) |
| Failure | Failure not considered | Limit states of strength and serviceability |
| Uniform safety | Not uniform | Nearly uniform reliability |
| Economy | Generally heavier | More economical |
| Code | IS 800:1984 | IS 800:2007 |

```text
 WSM                         LSM
 stress                      stress
  fy |      /                 fy |  ______ (plastic)
 0.6fy|----/  perm.           fd |-/
     |   /                       |/ design
     |__/____ strain             |________ strain
```
In WSM the structure is designed to keep stress below $0.6f_y$ under working load; in LSM it is designed so that the factored load does not exceed the design strength $f_y/\gamma_{m0}$ and deflection stays within limits.

## Check yourself

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

1. Which design check belongs to LSM?
   A. Stress <= permissible stress
   B. Factored action <= design strength
   C. Working load <= collapse load
   D. Stress <= 0.6fy
   Answer: B. LSM compares factored action effect with design strength. (Key point: Design check)

2. Which codes correspond to WSM and LSM?
   A. IS 800:2007 for WSM and IS 800:1984 for LSM
   B. IS 456 for WSM and IS 800 for LSM
   C. IS 800:1984 for WSM and IS 800:2007 for LSM
   D. Both IS 800:1984
   Answer: C. WSM is in IS 800:1984 and LSM in IS 800:2007. (Key point: Code editions)

3. How is failure treated in WSM compared with LSM?
   A. WSM considers limit states of strength and serviceability
   B. WSM does not consider failure, LSM considers limit states of strength and serviceability
   C. Both consider failure equally
   D. LSM ignores failure
   Answer: B. WSM only keeps stress below permissible; LSM checks limit states. (Key point: Failure consideration)

## Related in Design of Steel and Timber Structure

- [Merits of limit state over working stress](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Awhy-lsm-better-wsm) — Design of Steel and Timber Structure, chapter 3, asked 3×
- [Basic assumptions of working stress method](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Awsm-basic-assumptions) — Design of Steel and Timber Structure, chapter 2
- [Different limit states in steel design](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Atypes-of-limit-states) — Design of Steel and Timber Structure, chapter 3

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