---
title: "Shear strength of concrete"
subject: "Concrete Technology and Masonry Structure"
chapter: 5
key: "semester_5/concrete_technology_and_masonry_structure:shear-strength-concrete"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/shear-strength-concrete
site: BCT Engineering
---

# Shear strength of concrete

Concrete Technology and Masonry Structure · Chapter 5: Testing of concrete and quality control

- Asked in: 2067 Magh (old course), 5 marks
- Video: [01 shear behavior unreinforced concrete](https://www.youtube.com/watch?v=5z-1z6i5whQ) — Mukai UWYO Structural Concrete
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/shear-strength-concrete
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-5

## Question

Describe the shear strength of concrete.

## Short answer

Concrete shear strength is its resistance to sliding, but in beams failure occurs by diagonal tension. Nominal shear stress τ_v = V_u/(bd) is compared with design shear strength τ_c (IS 456 Table 19, depends on steel percentage and grade). Shear reinforcement is provided when τ_v > τ_c, and τ_v must not exceed τ_c,max. Punching shear stress is 0.25√f_ck.

## Answer

**Shear strength** is the resistance of concrete to failure by sliding of one part over the adjacent part under forces acting parallel to the plane. In practice pure shear is rare; shear in beams occurs together with flexure, and failure is by **diagonal tension**.

- **Direct (pure) shear strength** of plain concrete is about 20 to 30% of the compressive strength (higher than the tensile strength). It is hard to test because shear and bending are mixed (the double shear test on a prism is used).
- **Beam shear:** the nominal shear stress is $\tau_v = V_u/(bd)$ (IS 456, Cl. 40.1). The design shear strength of concrete $\tau_c$ depends on the grade and the tension steel percentage $100A_s/bd$ (Table 19; for M20 it ranges from 0.28 N/mm$^2$ at 0.15% steel to 0.82 N/mm$^2$ at 2.5% steel and above). If $\tau_v > \tau_c$, shear reinforcement is provided (Cl. 40.4); $\tau_v$ must not exceed $\tau_{c,max}$ (Table 20, 2.8 N/mm$^2$ for M20), otherwise the section is changed.
- **Factors:** compressive strength, steel percentage (dowel action), aggregate interlock, depth of beam (size effect) and axial load (compression raises the shear strength, tension reduces it).
- **Punching shear** in footings and flat slabs: $\tau_c = k_s \cdot 0.25\sqrt{f_{ck}}$ (Cl. 31.6.3).
- Shear failure is brittle and sudden, so design is conservative, with minimum stirrups (Cl. 26.5.1.6).

## Check yourself

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

1. Pure shear strength of plain concrete is about what fraction of compressive strength?
   A. 80 to 90%
   B. 5 to 8%
   C. 20 to 30%
   D. 100%
   Answer: C. Direct shear is about 20 to 30% of compressive strength. (Key point: Direct shear)

2. How is the nominal shear stress in a beam found?
   A. tv = Vu x bd
   B. tv = Vu/(pi d)
   C. tv = Mu/(bd^2)
   D. tv = Vu/(bd)
   Answer: D. IS 456 Cl. 40.1 uses tv = Vu/bd. (Key point: Nominal shear stress)

3. What is done if tv > tc?
   A. Provide shear reinforcement; section changed if tv exceeds tc,max
   B. Nothing, concrete is safe
   C. Reduce steel
   D. Increase span
   Answer: A. Stirrups carry the excess; exceeding tc,max needs a bigger section. (Key point: Shear reinforcement)

4. Which factor raises shear strength?
   A. Axial tension
   B. Axial compression
   C. Larger beam depth
   D. Lower tension steel percentage
   Answer: B. Axial compression raises, tension reduces shear strength. (Key point: Factors)

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