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

# Define primary structure

Theory of Structures II · Chapter 2: Force method

- Asked in: 2073 Shrawan, 1 marks
- Video: [Force Method vs Displacement Method | DSI vs DKI Explained | Structural Analysis](https://www.youtube.com/watch?v=J-JxZL9Pcl0) — Civil Engineering by Arsalan 
- 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-primary-structure
- Whole chapter: https://www.bctengineer.com/md/semester_5/theory_of_structures_ii/chapter-2

## Question

Define and explain the term primary structure.

## Short answer

Primary structure is a statically determinate and stable structure obtained by removing redundant forces from an indeterminate structure. The removed redundants are treated as unknown loads and determined from compatibility conditions. It is not unique but must remain stable.

## Answer

The **primary structure** (also called the released or basic determinate structure) is the statically determinate and stable structure obtained from an indeterminate structure by removing the redundant forces, that is, redundant reactions or internal forces such as a support, a member, or a moment restraint. The removed redundants are then applied as unknown loads and found from compatibility conditions.

A structure with $D_s$ degrees of indeterminacy needs $D_s$ releases. The primary structure must remain stable, and it is not unique. Example: for a propped cantilever, the primary structure may be the cantilever (prop removed) or a simply supported beam (fixed-end moment released). In the force method every analysis starts by choosing it, finding $M_0$ for the loads and $m_i$ for unit redundants.

## Check yourself

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

1. What is the primary structure?
   A. The original indeterminate structure
   B. The determinate, stable structure obtained by removing redundants
   C. A mechanism with no supports
   D. The structure with extra members
   Answer: B. It is the statically determinate, stable structure left after the redundants are released. (Key point: Definition)

2. Which statement about the primary structure is correct?
   A. It must be unstable
   B. It is unique
   C. It must remain stable and is not unique
   D. It has D_s redundants
   Answer: C. Different sets of releases give different primary structures, but each must be stable. (Key point: Stability and uniqueness)

3. For a propped cantilever, which can be a primary structure?
   A. Fixed-fixed beam
   B. Cantilever with prop removed
   C. Beam with extra support
   D. Continuous beam
   Answer: B. Removing the prop leaves a cantilever; alternatively releasing the fixed-end moment leaves a simply supported beam. (Key point: Example)

## Related in Theory of Structures II

- [Define force method and primary structure](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Adefine-force-method-primary-structure) — Theory of Structures II, chapter 2

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