---
title: "Define plastic hinge"
subject: "Theory of Structures II"
chapter: 5
key: "semester_5/theory_of_structures_ii:define-plastic-hinge"
source: https://www.bctengineer.com/q/semester_5/theory_of_structures_ii/define-plastic-hinge
site: BCT Engineering
---

# Define plastic hinge

Theory of Structures II · Chapter 5: Introduction to plastic analysis

- Asked in: 2070 Chaitra (old course), 2 marks; 2069 Asar, 1 marks (asked 2 times)
- Video: [Plastic Analysis of Structures: Collapse Mechanisms Explained| IStructE Structural Behaviour Course](https://www.youtube.com/watch?v=c-X_eIbg-rU) — Dr Jawed Qureshi
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/theory_of_structures_ii/define-plastic-hinge
- Whole chapter: https://www.bctengineer.com/md/semester_5/theory_of_structures_ii/chapter-5

## Question

Define plastic hinge.

## Short answer

A plastic hinge is a fully yielded section where the moment reaches M_p, allowing rotation under constant moment. It forms at maximum moment locations, and when enough hinges develop a mechanism, collapse occurs.

## Answer

A **plastic hinge** is a section of a flexural member where the bending moment has reached the plastic moment $M_p$, so the whole depth has yielded and the section can rotate under a constant moment $M_p$, like a hinge that carries a moment. The yielding actually spreads over a finite length of the member, called the plastic hinge length.

Plastic hinges form at points of maximum moment: fixed supports, joints, and under concentrated loads or at the section of zero shear under a UDL. When enough hinges form to turn the structure into a mechanism, it collapses.

## Check yourself

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

1. A plastic hinge is a section that:
   A. has zero moment
   B. has reached M_p, is fully yielded and rotates at constant M_p
   C. has cracked in tension
   D. carries only shear
   Answer: B. The whole depth has yielded so the section rotates at constant moment M_p. (Key point: Meaning)

2. Where do plastic hinges typically form?
   A. At points of zero moment
   B. Anywhere along the beam equally
   C. At points of maximum moment, such as fixed supports and under concentrated loads
   D. Only at simple supports
   Answer: C. Hinges form where the moment is maximum: fixed supports, joints, concentrated loads, and zero shear under a UDL. (Key point: Location)

3. When does a structure collapse in terms of plastic hinges?
   A. When the first hinge forms
   B. When enough hinges form to make it a mechanism
   C. When hinge length equals the span
   D. When moment is below M_p everywhere
   Answer: B. A mechanism forms when enough hinges turn the structure into a mechanism. (Key point: Collapse)

## Related in Theory of Structures II

- [Plastic hinge versus elastic hinge](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Adefine-plastic-hinge-compare-elastic) — Theory of Structures II, chapter 5
- [Plastic hinge and its length](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Adefine-plastic-hinge-length) — Theory of Structures II, chapter 5
- [Shape factor, plastic hinge properties, T-beam](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Ashape-factor-t-beam-plastic-hinge) — Theory of Structures II, chapter 5
- [Elastic vs plastic analysis; plastic hinge](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Aelastic-vs-plastic-analysis-plastic-hinge) — Theory of Structures II, chapter 5