A Development length can be defined as the amount of reinforcement (bar) length needed to be embedded or projected into the column to establish the desired bond strength between the concrete and steel (or any other two types of material)

Watch this Video till the end to get the full details about development length-

When you open a structural Drawing- You Find L_d

Sometimes It may be-

L_d=30d

L_d =40d

L_d =50d

But the question is where it is coming from or where it is Derive From and Depends on which factors. 

By the Thumb rule, these are taken in the site work as anything but that's not the correct way to take it, maybe failure occurs.  

Because it depends upon both the Grade of Concrete and The type of Steel you use in the construction members. so we can't take the value of development length by guessing work. 

All we know from this article. 

Now to find the development length which is actually used in the construction site, IS 456:2000 will help you to find the Required Data- 

For Formula to Calculate Development Length of Bar as per IS 456:2000, Go to clause No 26.2.1

σ_s- Is actually the Stress in bars which depends upon the grade of steels like Fe250, Fe415, Fe500

For Design bond stress, go to IS 456:2000 Clause No - 26.2.1, you’ll find this Table

These are all data for Plain bars but we Generally use steels in the market or for House Construction are Deformed Bars.

Development Length for Tension 

These are all data for Plain bars but we Generally use steels in the market or for House Construction are Deformed Bars.

For Design bond stress, go to IS 456:2000 Clause No - 26.2.1, you’ll find this Table

For Plain Bar (clause 26.2.1.1)-No Ribs in the Bars.

When you Purchase Bars from the Market, You get Bars with Ribs- Which are called Deformed Bars and we calculate Values for Those Bars. 

But Before calculation, knowing all the important factors to be considered.

But Deformed bar calculation is not given in the IS Code but they have outlined two specifications.

You’ll Find This In clause No. 26.2.1.1 Here are the important factors to be considered.

In short- 

• For Deformed Bar = Tension increased 60 % = Tension * 1.60 
• For Compression = Tension increased 25 % = Tension * 1.25

Compression members (Column)

The value of Bond stress can be increased by 60% when high yield strength deformed (HYSD) bars are used. 

It can be further increased by 25% when we calculate the development length for bars in compression. 

 Design Bond stress (τ_bd) in Limit state Method for Plain Bars in Compression shall be as Below- 

For Plain bars in compression for M20= t_bd (of plain bar) x 1.25 = 1.2*1.25= 1.5 For Deformed bars in compression for M20= t_bd (of plain bar) x 1.25 = 1.92*1.25= 2.4

Similarly, you can calculate the Values for all grades of Concrete.

Usage of Formula for Development Length and its Calculation by Limit State Method-

Here you’ll calculate the development length for the plain bar in tension, by the limit state method.

Development Length Calculation for Fe 250 based on IS 456:2000

Grade of concrete = M20 

Reinforcement bar = Fe250 (fy = 250) 

Stress in Bar = 0.87 * fy= 0.87 X 250 

For M20 Concrete Grade, Design bond stress for the plain bar in Tension 

τ_bd =1.2 N/mm² 

Therefore, by using the Formula for Development Length, Put the above data in Following Formula-

Check with all the Diameters of the Bars, You’ll find the Following Results

Development Length for Fe 415

Data Given:- 

Grade of concrete = M25 

Reinforcement bar = Fe415 (Fy = 415) 

Stress in Bar = 0.87 * 415= 0.87 X 415 

For M20 Concrete Grade, Design bond stress for the Deformed bar in Tension 

τ_bd =1.2 N/mm²  * 1.6 = 1.9 N/mm²

Therefore, by using the Formula for Development Length Here 

Similarly checking with all the Diameters of Bars, You’ll find the Following Results

That’s How You can find the Development length values for Different grades of concrete and Steel. 

As per the General Thumb rule, the development length requirement is expressed as ’40Ø’ to ’50 Ø’. But as a Civil Engineer,  you must use the correct calculation. 

Let's say You suggest clients use a 12mm diameter deformed Bar for Beam construction. So you must use M25 (1:1:2-ration of Cement, sand, and aggregate), and The Required Development length (Tension Deformed Bar) for the Bar will be 41*12=492 mm.

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