This document discusses development length and lap length in structural design. It defines development length as the length of reinforcement embedded in concrete required to develop the bond stress. A formula for calculating development length is provided based on bar diameter, steel stress, and design bond stress in concrete. Design bond stress values for different concrete grades are also given. Lap length refers to the overlapping length of bars and must be equal to or greater than the development length or 24 times the bar diameter, whichever is greater.

1. PREPARED BY:
Asst. Prof. GAURANG PRAJAPATI
CIVIL DEPARTMENT
DEVELOPMENT LENGTH
Mahatma Gandhi Institute Of
Technical Education
& Research Centre, Navsari (396450)
ELEMENTARY STRUCTURAL DESIGN
6TH SEMESTER
CIVIL ENGINEERING

2. INTRODUCTION
 DEVELOPMENT LENGTH (Ld):
 A length of reinforcement embedded in concrete so that it can develop the bond
stress, is termed as Development length.
ESD
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DEVELOPMENT LENGTH

3. INTRODUCTION
ESD
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DEVELOPMENT LENGTH
Where,
Ld = Development length
𝜙 = Dia. Of bar
𝜎𝑠 = Stress in bar
= 0.87 fy
𝜏 𝑏𝑑 = Design bond stress in concrete
Ld =
𝝓 𝝈 𝒔
𝟒 𝝉 𝒃𝒅
IS 456
Clause No.: 26.2.1
Page No.: 42

4. DESIGN BOND STRESS IN CONCRETE (𝝉 𝒃𝒅)
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DEVELOPMENT LENGTH
Grade of concrete M 20 M 25 M 30 M 35 M 40 & Above IS 456
Clause : 26.2.1.1
Page No.: 43𝝉 𝒃𝒅 (N/mm2) 1.2 1.4 1.5 1.7 1.9
 For plain bars in tension are as follow:
 For Deformed bars (Fe 415 & Fe 500) (Tor Steel), these values shall be
increased by 60%. (multiplied by 1.6)
 For bars in compression (column), these values shall be increased by 25%.
(multiplied by 1.25)

5. DESIGN BOND STRESS IN CONCRETE (𝝉 𝒃𝒅)
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DEVELOPMENT LENGTH
 For Example:
Type of bars 𝝉 𝒃𝒅 (N/mm2)
 M 20 grade, Plain bars in tension 1.2
 M 20 grade, Plain bars in compression 1.2 × 1.25
 M 20 grade, Deformed bars in tension 1.2 × 1.6
 M 20 grade, Deformed bars in compression 1.2 × 1.6 × 1.25

6. LAP LENGTH:
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DEVELOPMENT LENGTH
 The length of bar extended over the other bars in beam, column, etc. is called lap
length.

7. LAP LENGTH:
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DEVELOPMENT LENGTH
 The lap length in compression shall be equal to Development length (Ld), but not
less than 24ф
Lap length = Ld ≮ 24 ф IS 456
Clause No.: 26.2.1
Page No.: 42