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

# Design methods and basic assumptions

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

- Asked in: 2073 Bhadra, 8 marks
- Video: [STRUCTURAL DESIGN 1: MOD 1 CLASS 2: METHODS OF STRUCTURAL DESIGNS](https://www.youtube.com/watch?v=0CvpNaniHSY) — Jitto George
- 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-methods-assumptions
- Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-2

## Question

Describe different methods of design with their basic assumptions.

## Short answer

WSM assumes elastic behavior with working loads and a single safety factor on stress. Plastic method uses elastic-perfectly plastic material, full plastic moment without buckling, and load factors. LSM applies partial factors on loads and material, checks ultimate and serviceability limit states, and idealizes steel as elastic-perfectly plastic.

## Answer

#### 1. Working Stress Method (WSM)
Assumptions:
- Steel is homogeneous, isotropic and obeys Hooke's law (linear elastic).
- Plane sections remain plane after bending.
- Stresses under working loads must not exceed permissible stresses $\sigma_{perm}=f_y/FOS$ (e.g. $0.6f_y$ in tension).
- A single factor of safety covers all uncertainties; the stress-strain curve is linear up to $f_y$.

#### 2. Plastic (ultimate load) method
Assumptions:
- Steel is ductile and behaves as elastic-perfectly plastic with a long yield plateau.
- Plane sections remain plane; the full plastic moment $M_p=f_yZ_p$ develops at the hinge.
- Local and lateral-torsional buckling are prevented, so the section reaches $M_p$ and has enough rotation capacity.
- Collapse occurs by a mechanism after enough hinges form; loads are factored (load factor, e.g. 1.7).
- Effects of shear and axial force on $M_p$ are small; deflection is neglected.

#### 3. Limit State Method (LSM)
Assumptions:
- Plane sections remain plane; the stress-strain curve is idealised as elastic-perfectly plastic.
- Characteristic loads are multiplied by partial safety factors $\gamma_f$, and material strength is divided by $\gamma_m$.
- The structure is checked for the limit state of strength (yield, buckling, rupture) and of serviceability (deflection, vibration).
- Local buckling is controlled by section classification (plastic, compact, semi-compact, slender); residual stresses and imperfections are included in the buckling curves.
- Design strength $\ge$ factored effect for each member and connection.

| Method | Load | Safety |
|---|---|---|
| WSM | Working | One factor on stress |
| Plastic | Factored (load factor) | Collapse load |
| LSM | Factored, partial factors | $\gamma_f$ and $\gamma_m$ |

## Check yourself

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

1. Which is a basic assumption of WSM?
   A. Collapse mechanism forms with plastic hinges
   B. Steel is elastic obeying Hooke's law and a single factor of safety is used
   C. Partial factors are applied to loads
   D. Loads are factored by 1.7
   Answer: B. WSM assumes linear elastic behaviour with stresses kept below permissible stresses using one factor of safety. (Key point: WSM assumption)

2. Which condition is required in plastic design for the section to reach Mp?
   A. Local and lateral-torsional buckling are prevented and rotation capacity is sufficient
   B. Section must be slender
   C. Deflection governs
   D. Stress must stay below 0.6fy
   Answer: A. Plastic design assumes buckling is prevented so that Mp = fy Zp develops with enough rotation. (Key point: Plastic method requirement)

3. How does collapse occur in plastic design?
   A. By first yield at the extreme fibre
   B. By fracture at the bolt hole
   C. By formation of enough plastic hinges to give a mechanism
   D. By exceeding the permissible stress
   Answer: C. Collapse occurs when enough plastic hinges form to produce a mechanism. (Key point: Plastic collapse)

4. What does LSM require for each member and connection?
   A. Working stress <= 0.6fy
   B. Design strength >= factored effect, checked for strength and serviceability limit states
   C. Factored load = characteristic load
   D. Only deflection check
   Answer: B. LSM applies gamma_f to loads and gamma_m to strength and checks strength and serviceability. (Key point: LSM check)

## Related in Design of Steel and Timber Structure

- [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

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