---
title: "Define redundant force"
subject: "Theory of Structures II"
chapter: 2
key: "semester_5/theory_of_structures_ii:define-redundant-force"
source: https://www.bctengineer.com/q/semester_5/theory_of_structures_ii/define-redundant-force
site: BCT Engineering
---

# Define redundant force

Theory of Structures II · Chapter 2: Force method

- Asked in: 2073 Shrawan, 1 marks
- Video: [SA24: Force Method (Part 1)](https://www.youtube.com/watch?v=4Sp3ArX1hUg) — Dr. Structure
- 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-redundant-force
- Whole chapter: https://www.bctengineer.com/md/semester_5/theory_of_structures_ii/chapter-2

## Question

Define and explain the term redundant force.

## Short answer

Redundant force is a reaction or internal force not required for static equilibrium, so it cannot be found from equilibrium equations alone. The number of redundants equals the degree of static indeterminacy. They are released to form the primary structure and solved from compatibility equations.

## Answer

A **redundant force** (or redundant) is a reaction or internal force that is not needed for the static equilibrium of a structure, so it cannot be found from the equations of equilibrium alone. The number of redundants equals the degree of static indeterminacy $D_s$.

The redundants are chosen and released to form the primary (determinate) structure, are treated as unknown external loads $X_1, X_2, \ldots$, and are solved from the compatibility equations $\sum f_{ij}X_j + \Delta_{i0} = 0$. Example: the prop reaction $R_B$ of a propped cantilever, or the horizontal thrust $H$ of a two-hinged arch.

## Check yourself

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

1. What is a redundant force?
   A. A force found by equilibrium alone
   B. A reaction or internal force not needed for static equilibrium
   C. The largest load
   D. A zero force member
   Answer: B. Redundants cannot be found from equilibrium equations alone and are not needed for stability. (Key point: Definition)

2. How many redundants does a structure have?
   A. Equal to D_s
   B. Always one
   C. Equal to the number of joints
   D. Equal to 3
   Answer: A. The number of redundants equals the degree of static indeterminacy. (Key point: Number)

3. How are redundants determined?
   A. From equilibrium only
   B. By guess
   C. From compatibility equations Σ f_ij X_j + Δ_i0 = 0
   D. By ignoring them
   Answer: C. They are treated as unknown loads on the primary structure and solved from compatibility. (Key point: How found)

## Related in Theory of Structures II

- [Redundant truss: force in member BF](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Atruss-redundant-bf-member-force) — Theory of Structures II, chapter 2