How to calculate the development length for different grades of concrete as per IS 456?

 As per IS-456: 2000 clause 26.2.1, the development length Ld is given by

In IS-code 456, it is mentioned that the value of bond stress should be increased by 60% for high-yield strength deformed bars, & it should be further increased by 25% for the calculation of development length in compression.

I have made a table of design bond stress for the limit state method & working stress method as follows.

Design bond stress (τbd )
For Limit state method
Concrete grade	M20	M25	M30	M35	M40 & above
For plain bars in tension	1.2	1.4	1.5	1.7	1.9
For deformed bars in tension	1.92	2.24	2.40	2.72	3.04

 

Design bond stress (τbd )
For Working stress method
Concrete grade	M20	M25	M30	M35	M40	M45	M50
For plain bars in tension	0.8	0.9	1.0	1.1	1.2	1.3	1.4
For deformed bars in tension	1.28	1.44	1.6	1.76	1.92	2.08	2.24

 

Note: For the compression, I have further increased the value of design bond stress by 25% and made a table.

Design bond stress (τbd )N/mm2
For Working stress method
Concrete grade	M20	M25	M30	M35	M40	M45	M50
For plain bars in compression.	1.0	1.125	1.25	1.375	1.5	1.625	1.75
For deformed bars in compression.	1.60	1.80	2.0	2.20	2.40	2.60	2.80

 

Design bond stress (τbd )N/mm2
For Limit state method
Concrete grade	M20	M25	M30	M35	M40 & above
For plain bars in compression.	1.5	1.75	1.875	2.125	2.375
For deformed bars in compression.	2.4	2.8	3.0	3.4	3.8

 

 Now, let us consider 2 examples to calculate the development length for different grades of concrete.

Before proceeding further, Go through the article👇

👀. What is development length in construction? / Reasons for providing development length & factors affecting it.

1. Example - 1

Given data

Calculate the development length for rebar in tension, by the limit state method for the below-given data.

Grade of concrete = M30

Reinforcement bar = Fe500 (σs = 500 )

Diameter of the bar = 16mm. (∅ )

The development length Ld =  [ ∅ × σs ÷  4 × τbd]

                                                   

Ld = [16 × 500 ÷ 4 × 2.40]

( From the above-given table, τbd = 2.40 N/mm2 for M30 in tension, for the limit state. )

      = [8000 ÷ 9.6]

   Ld = 833mm.

                                              

2. Example - 2 :

Given data

Calculate the development length for rebar in compression, by working stress method for the below-given data.

Grade of concrete = M25

Reinforcement bar = Fe415 (σs = 415 )

Diameter of the bar = 12mm. (∅ )

The development length Ld =  [ ∅ × σs ÷  4 × τbd]

                                                    

Ld = [12 × 415 ÷ 4 × 1.8]

( From the above-given table, τbd = 1.8 N/mm2 for M25 in compression, for the working stress method. )

    = [4980 ÷ 7.2]

  Ld  = 692mm.

For you 👇

❤ What is 0.42D in a crank bar (bent-up bar) cutting length formula? - its derivation.

I hope you understood the concept. If you have any queries, you can ask me in the comment box.

To go through the articles on Estimation & calculation in civil engineering, click here.

Thank you for going through this article❤. Have a good day😄

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