---
title: "Characteristic strength and characteristic load"
subject: "Design of RCC Structure"
chapter: 1
key: "semester_7/design_of_rcc_structure:characteristic-strength-and-characteristic-load"
source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/characteristic-strength-and-characteristic-load
site: BCT Engineering
---

# Characteristic strength and characteristic load

Design of RCC Structure · Chapter 1: Concrete Structures and Design Methods

- Asked in: 2082 Baisakh, 3 marks; 2078 Bhadra, 1+1 marks; 2079 Bhadra, 2 marks (asked 3 times)
- Video: [🔥Characteristic Strength || Characteristic Load || Design Strength || Design Load ||](https://www.youtube.com/watch?v=rZwhyn9XRvY) — 𝗖𝗶𝘃𝗶𝗹 𝗡𝗲𝘅𝘂𝘀 𝗯𝘆 𝗘𝗿 𝗡𝗮𝗱𝗲𝗲𝗺
- 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/characteristic-strength-and-characteristic-load
- Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-1

## Question

Define/explain characteristic strength and characteristic load.

## Short answer

Characteristic strength is the value below which not more than 5% of test results fall. Characteristic load is the load with 95% probability of not being exceeded. Design strength is fk/gamma_m, and design load is gamma_f times characteristic load.

## Answer

#### Characteristic strength

The **characteristic strength** of a material is the value of strength below which not more than 5 percent of the test results are expected to fall (95 percent reliability). It is the basis of design in limit state method (IS 456:2000, cl. 36.1).

- Concrete: characteristic compressive strength $f_{ck}$ is the 28-day strength of 150 mm cubes. For M20, $f_{ck} = 20$ N/mm$^2$.
- Steel: characteristic strength $f_y$ is the yield stress (or 0.2% proof stress). For Fe415, $f_y = 415$ N/mm$^2$.
- Statistically, $f_{ck} = f_m - 1.65\,s$, where $f_m$ is the mean strength and $s$ the standard deviation.

#### Characteristic load

The **characteristic load** is the value of a load which has a 95 percent probability of not being exceeded during the life of the structure (IS 456:2000, cl. 36.2). Since statistical data are rarely available, the loads given in the codes are taken as characteristic loads: dead load from IS 875 (Part 1), live load from IS 875 (Part 2), wind load from IS 875 (Part 3), and earthquake load from IS 1893.

#### Use in design

Design values are obtained with partial safety factors:

- Design strength $= \dfrac{f_k}{\gamma_m}$, with $\gamma_m = 1.5$ for concrete and $1.15$ for steel.
- Design load $= \gamma_f \times$ characteristic load, with $\gamma_f = 1.5$ for DL+LL, for example.

## Check yourself

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

1. What does characteristic strength mean?
   A. The mean strength of test results
   B. The strength below which not more than 5 percent of test results are expected to fall
   C. The strength above which only 5 percent of results fall
   D. The maximum strength found in the tests
   Answer: B. Characteristic strength has 95 percent reliability: no more than 5 percent of results are expected below it. (Key point: Definition of characteristic strength)

2. Characteristic load is the load that has what probability of being exceeded during the life of the structure?
   A. Not more than 5 percent probability of being exceeded
   B. 50 percent probability of being exceeded
   C. 95 percent probability of being exceeded
   D. It is certain to be exceeded once
   Answer: A. It has a 95 percent probability of not being exceeded, i.e. 5 percent chance of exceedance. (Key point: Characteristic load)

3. For M20 concrete, what is the design strength used with gamma_m?
   A. 20 x 1.5 N/mm2
   B. 20 / 1.15 N/mm2
   C. 20 / 1.5 N/mm2
   D. 20 N/mm2 with no factor
   Answer: C. Design strength = fk / gamma_m, with gamma_m = 1.5 for concrete (1.15 for steel). (Key point: Design values)

## Related in Design of RCC Structure

- [Idealized stress-strain, characteristic strength](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aidealized-stress-strain-characteristic-strength) — Design of RCC Structure, chapter 3
- [Design strength and design load in LSM](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adesign-strength-and-design-load-in) — Design of RCC Structure, chapter 3

## Related in other subjects

- [Characteristic and design strength and loads](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Acharacteristic-strength-loads) — Design of Steel and Timber Structure, chapter 3
- [Characteristic strength at 95% confidence](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acharacteristic-strength-16-17) — Concrete Technology and Masonry Structure, chapter 5
- [Characteristic strength of concrete cubes](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acharacteristic-strength-cubes-26) — Concrete Technology and Masonry Structure, chapter 5
- [Characteristic strength from test results](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acharacteristic-strength-30-28) — Concrete Technology and Masonry Structure, chapter 5