---
title: "Design philosophies for steel structures"
subject: "Design of Steel and Timber Structure"
chapter: 1
key: "semester_6/design_of_steel_and_timber_structure:design-philosophies-steel"
source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/design-philosophies-steel
site: BCT Engineering
---

# Design philosophies for steel structures

Design of Steel and Timber Structure · Chapter 1: Steel Structures and their Analysis and Design

- Asked in: 2078 Chaitra, 4 marks
- Video: [Design Philosophies Of Steel Structures: WSM, LSM & Ultimate Load |L-1| GATE & APPSC AEE](https://www.youtube.com/watch?v=uV9WAiNjlWc) — Civil Academy Andhra
- 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/design-philosophies-steel
- Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-1

## Question

Which design philosophies are used for the design of steel structures?

## Short answer

Three philosophies: Working Stress Method (permissible stresses under working loads), plastic design (collapse load using plastic hinges), and Limit State Method (partial safety factors for strength and serviceability, as per IS 800:2007). LSM is more rational and provides uniform safety.

## Answer

Three philosophies have been used for steel design.

1. **Working Stress Method (WSM / elastic design):** stresses under working loads are kept below permissible stresses ($\approx 0.6f_y$ in tension). Behaviour is assumed to be linearly elastic, and one single factor of safety is used. Adopted in IS 800:1984.
2. **Ultimate load / plastic design:** the structure is analysed up to the collapse load (formation of plastic hinges and a mechanism). Working loads are multiplied by a load factor (e.g. 1.7 for DL+LL) and the section is designed for the collapse moment $M_p$. Economical for continuous beams and portal frames.
3. **Limit State Method (LSM):** the structure is designed so that it does not reach any limit state of strength or serviceability, using separate partial safety factors on loads and on material strength. Adopted in IS 800:2007, the present code.

LSM is preferred because it is more rational, takes the probabilistic nature of loads and strengths into account, and gives more uniform safety.

## Check yourself

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

1. What characterises the Working Stress Method?
   A. Collapse load with a load factor
   B. Separate partial factors on loads and materials
   C. Linear elastic behaviour with stresses limited to permissible values using one factor of safety
   D. Probabilistic design with serviceability limits
   Answer: C. WSM keeps working stresses below permissible stresses (about 0.6fy in tension) using one factor of safety. (Key point: Working stress method)

2. In ultimate load/plastic design, what is the section designed for?
   A. The collapse moment Mp after multiplying working loads by a load factor
   B. The yield moment My under working loads
   C. The permissible stress 0.66fy
   D. The deflection limit only
   Answer: A. Working loads are multiplied by a load factor and the structure is designed for the collapse (plastic) moment Mp. (Key point: Plastic design)

3. Why is the Limit State Method preferred?
   A. It uses one factor of safety for simplicity
   B. It ignores serviceability
   C. It assumes only elastic behaviour
   D. It is more rational, accounts for the probabilistic nature of loads and strengths and gives more uniform safety
   Answer: D. LSM uses separate partial safety factors on a statistical basis, giving more uniform reliability. (Key point: Why LSM is preferred)

## Related in Design of Steel and Timber Structure

- [Steel versus timber structures](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Asteel-vs-timber-advantages) — Design of Steel and Timber Structure, chapter 1
- [Design methods and basis for timber structures](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Atimber-design-methods-basis) — Design of Steel and Timber Structure, chapter 9
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