---
title: "Equivalent strut behaviour of infill wall"
subject: "Concrete Technology and Masonry Structure"
chapter: 7
key: "semester_5/concrete_technology_and_masonry_structure:infill-equivalent-strut"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/infill-equivalent-strut
site: BCT Engineering
---

# Equivalent strut behaviour of infill wall

Concrete Technology and Masonry Structure · Chapter 7: Introduction to masonry structures

- Asked in: 2080 Baisakh, 3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/infill-equivalent-strut
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-7

## Question

What is the equivalent strut behaviour of an infill wall? Is it advantageous?

## Short answer

Under lateral load, a masonry infill wall in an RC frame acts as a diagonal compression strut between opposite corners after tension corners separate. This strut increases lateral stiffness and strength. It is advantageous if infill is uniform, but irregular placement can cause soft-storey or short-column effects.

## Answer

When a masonry infill wall sits inside an RC frame and the frame sways under lateral load, the frame and the infill separate at the tension corners, and the infill touches the frame only at the two opposite (compression) corners. The wall then acts like a **diagonal compression strut** between these corners. This is the **equivalent strut (equivalent diagonal) model**.

```text
  +------------+
  |\           |
  |  \  strut  |
  |    \       |      width w = 0.175 (alpha_h)^-0.4 d
  |      \     |      d = diagonal length of infill
  +------------+
```

The strut has a thickness $t$ equal to that of the wall and a width $w \approx 0.175\,\alpha_h^{-0.4}\,d$, where $\alpha_h = h\left[\dfrac{E_m t \sin 2\theta}{4E_f I_c h_m}\right]^{1/4}$ ($h$ = column height, $E_m$ = masonry modulus, $E_f$ = frame modulus, $I_c$ = column moment of inertia, $h_m$ = infill height, $\theta$ = angle of the diagonal with the horizontal). It is modelled as a pin-ended diagonal brace in the frame analysis.

**Is it advantageous?** Yes, in general: the strut greatly increases the lateral stiffness and strength of the frame, reduces drift and dissipates energy, so some damage to the infill can be tolerated. But the benefit depends on the infill being well built and uniformly distributed. Irregular infill can cause soft-storey, short-column and torsion problems, and the wall may fail out of plane. These effects must be considered in design.

## Check yourself

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

1. How does an infill wall behave when an RC frame sways?
   A. It acts as a tension tie
   B. It acts as a diagonal compression strut between two opposite corners
   C. It stays bonded to the whole frame
   D. It acts as a cantilever
   Answer: B. The wall separates at tension corners and touches at two opposite corners. (Key point: Strut model)

2. On what does the strut width mainly depend?
   A. Relative stiffness (alpha_h) and diagonal length d
   B. Only the wall colour
   C. Only the mortar age
   D. The roof type
   Answer: A. w = 0.175 (alpha_h)^-0.4 d. (Key point: Strut width)

3. Is infill beneficial?
   A. No, it always weakens frames
   B. Yes in all cases without conditions
   C. Yes generally, it raises lateral stiffness and strength, but irregular infill can cause soft storey and short column problems
   D. It has no effect
   Answer: C. Benefits depend on good construction and uniform distribution. (Key point: Advantage)

## Related in other subjects

- [Out-of-plane bending of a masonry wall](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aout-of-plane-wall-flexure-check) — Design of Timber and Masonry Structures, chapter 6
- [Torsion of a rectangular hollow thin-wall box](https://www.bctengineer.com/question.md?key=semester_5%2Fmechanics_of_solids%3Athin-walled-box-torsion-numerical) — Mechanics of Solids, chapter 9