---
title: "Shear carrying mechanism and behaviour of concrete"
subject: "Design of RCC Structure"
chapter: 5
key: "semester_7/design_of_rcc_structure:shear-carrying-mechanism-and-behaviour-of"
source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/shear-carrying-mechanism-and-behaviour-of
site: BCT Engineering
---

# Shear carrying mechanism and behaviour of concrete

Design of RCC Structure · Chapter 5: Design for Shear and Torsion

- Asked in: 2079 Baisakh, 4 marks; 2078 Bhadra, 4 marks (asked 2 times)
- Video: [8 - Concrete Contribution to Shear Strength](https://www.youtube.com/watch?v=1dERfmpwJmg) — David Garber
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/shear-carrying-mechanism-and-behaviour-of
- Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-5

## Question

Discuss the shear carrying mechanism of reinforced concrete / behaviour of concrete under shear with neat sketches. Explain the different conditions.

## Short answer

Shear carried by uncracked concrete (20-40%), aggregate interlock (35-50%), and dowel action (15-25%). With stirrups, truss analogy applies: concrete struts and stirrup ties. Failure modes depend on shear span ratio a/d: deep beam (<1), shear compression (1-2.5), diagonal tension (2.5-6), flexure (>6), web crushing if τv > τc,max.

## Answer

When a beam is loaded, shear force and bending moment together produce **diagonal tension** in the web. Concrete is weak in tension, so inclined cracks form at about 45$^\circ$ near the supports. After cracking, shear is carried by several mechanisms working together.

#### Shear transfer mechanisms (beam without stirrups)

1. **Uncracked concrete compression zone:** carries about 20-40% of the shear.
2. **Aggregate interlock (friction) across the crack:** rough crack faces transfer shear, about 35-50%.
3. **Dowel action of longitudinal bars:** bars crossing the crack resist transverse movement, about 15-25%.

```text
          P
    ______|_______________
   |      |     inclined crack
   |  Vcz |  /   Vay (interlock)
   |______|_/____________
   o===o==/===o===o===o===   Vd (dowel)
  R
```

#### With shear reinforcement

Stirrups (and bent-up bars) cross the diagonal crack, carry tension, and hold the crack width small. This also improves aggregate interlock and dowel action. The behaviour is modelled as a **truss** (Ritter-Morsch analogy): concrete struts in compression and stirrups as vertical ties. Design shear strength:

$$
V_u=V_c+V_{us},\qquad V_c=\tau_cbd,\qquad V_{us}=\frac{0.87f_yA_{sv}d}{s_v}
$$

#### Failure conditions (depends on shear span ratio $a/d$)

| $a/d$ | Mode | Description |
|---|---|---|
| $<1$ | Deep beam action | Arch action; splitting of the strut, very strong |
| 1 to 2.5 | Shear compression | Crack propagates; concrete in compression zone crushes |
| 2.5 to 6 | Diagonal tension | Inclined crack forms suddenly and splits the beam; brittle |
| $>6$ | Flexure | Flexural failure occurs before shear |
| Any (thin web) | Web crushing | Excessive shear stress ($>\tau_{c,max}$), crushing of the web |

Shear strength increases with concrete grade, percentage of tension steel $p_t$ (dowel action, smaller crack width) and axial compression, and decreases with depth (size effect). Since shear failures are brittle, beams are designed so flexural failure occurs first.

## Check yourself

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

1. Which are shear transfer mechanisms in a beam without stirrups?
   A. Stirrup yielding only
   B. Bond slip
   C. Creep
   D. Uncracked compression zone, aggregate interlock, dowel action
   Answer: D. The three contributors are concrete in compression, interlock and dowel action. (Key point: Mechanisms)

2. In the truss analogy with stirrups, what role do stirrups play?
   A. Vertical tension ties
   B. Compression struts
   C. Compression chords
   D. Dowels
   Answer: A. Concrete forms diagonal struts while stirrups act as ties. (Key point: Truss analogy)

3. Which a/d range gives sudden diagonal tension failure?
   A. Below 1
   B. 2.5 to 6
   C. Above 6
   D. 1 only
   Answer: B. Diagonal tension is brittle and occurs for a/d between 2.5 and 6. (Key point: Failure mode)

4. Why design for flexural failure first?
   A. Shear is ductile
   B. Flexure is cheaper
   C. Shear failure is brittle
   D. Stirrups are costly
   Answer: C. Brittle failure gives no warning, so flexure must govern. (Key point: Design philosophy)

## Related in Design of RCC Structure

- [Significance of under and over-reinforced sections](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Asignificance-of-under-and-over-reinforced) — Design of RCC Structure, chapter 4
- [Define balanced, under, over-reinforced sections](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adefine-balanced-under-over-reinforced-sections) — Design of RCC Structure, chapter 4
- [Under, over and balanced sections](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aunder-over-and-balanced-sections) — Design of RCC Structure, chapter 4, asked 6×
- [Derive moment capacity of doubly reinforced WSM](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aderive-moment-capacity-of-doubly-reinforced) — Design of RCC Structure, chapter 2
- [Design steps of doubly reinforced section](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adesign-steps-of-doubly-reinforced-section) — Design of RCC Structure, chapter 4
- [Balanced, under, over sections with strain diagrams](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Abalanced-under-over-sections-with-strain) — Design of RCC Structure, chapter 4

## Related in other subjects

- [Ductile behaviour of reinforced and unreinforced masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aductility-reinforced-unreinforced-masonry) — Design of Timber and Masonry Structures, chapter 6
- [Seismic behaviour of unreinforced and reinforced masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aseismic-behavior-urm-versus-reinforced) — Design of Timber and Masonry Structures, chapter 7
- [Reinforced concrete column stresses in steel and concrete](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Arc-column-8-bars) — Strength of Materials, chapter 3
- [Physical properties of OPC and concrete behaviour](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acement-physical-properties) — Concrete Technology and Masonry Structure, chapter 1