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# Development Length

## What is Development Length?

It is use to transfer the load from rebar to concrete. Development Length is also known as anchorage length.

Development length is the min length of the steel that must be placed in concrete over any section to develop full strength. it is also known as an anchorage length in case of tension or compression and development length in case of flexural tension or flexural compression

According to IS 456:2000, the tension or compression is developed in any type of steel at any section to resist these forces desirable development length will be provided. So that because of development length the structural members (beam, column, slab etc) won’t be slipped away from the support.

Development length will be used in a continuous beam, cantilever slabs and other serious joints such as beam to column joint. This length will be used as a bend where the holding member is thin.

Development Length

### Why Development Length is Provided?

It makes a safe bond between steel and concrete. development length also makes sure during optimum load conditions the steel bar should not sleep through the complete it transfers loads from Beam to column easily. That’s why development length is provided

### If Development Length is not Provided What Will Happen?

If development length is not provided or if the development length is less against the required, then The structures will be inclined to face failure because of sleeping of joint in such situations the steel will not cut or crack first but the failure will occur at joints and laps initial to cracking of steel. In short steel will split from concrete.

If we take an example of the beam column joint in which development length is not used. If the load is applied more than the max acceptable limit then the keep load between the beam and column will not be sufficient to carry the beam at its position.

If the development length is not used in construction, then beam will come out from the column. it is needed to give support to the beam to decrease the chances of the beam coming out of the concrete column. thus development length acts as a supporting member for reinforced beam in the concrete column.

### Calculation of Development Length:

To calculate the development length, the formula is given below:

Where

Ld = Development length

Ø = diameter of steel

σs = Stress in steel.

τbd = Design bond stress

Development length = (diameter of steel × stress in steel) ÷ (4 × design bond stress)

#### Development Length in Tension Zone:

To find the development length in tension zone the formula of development length will be used for example:

If

D =12 mm

stress in bar = 415 N/mm2

design bond stress for M25 CONCRETE = 1.4 N/mm2

then,

Ld = 12 x (415 ÷ 4) x 1.4

Ld = 889 mm say 1m

Development length is 1 m in tension zone for M25 concrete Mix

#### Development Length in Compression Zone:

To find the development length in compression zone 25 % of value will be deduct from the value of Development length in tension zone

For example, in compression zone development length for same detail will be

Ld = 1- 25% x 1

Ld = 1- (25 ÷ 100) x1

Ld = 1- 0.25

Ld = 0.75m or 750mm

Development length is 0.75 m in compression zone for M25 concrete Mix

The concept of bond and development length, a steel is placed in concrete. the steel is treated to a tensile load. Because of the tensile force, the steel will try to slip out of the concrete. This slipping is resisted by the bond stress developed above the surface of the steel.

Bond stress is the shear stress arise through the contact surface between the steel and the concrete that stop the steel from slipping out of concrete. To stop slipping

The acceptable bond stress depends upon the grade of concrete and type of bars. The values of acceptable bond stress are given below.

Also, read what is overlapping complete guide

### Acceptable Bond Stress for Plain Steel and Deformed Steel:

The acceptable bond stress for plain steel and deformed steel is shown that is given below in table:

 S.No Concrete Grade Plain Steel(N/mm2) Deformed Steel(N/mm2) 1 M20 0.8 1.28 2 M25 0.9 1.44 3 M30 1.0 1.60 4 M35 1.1 1.76 5 M40 and above 1.2 1.92

1.) Bond stress for Plain Steel is 60% less than that of Deformed Steel.

2.) To pull a steel it is very easy than to push it inside. so, acceptable bond stress for plain and deformed steel in tension is taken 25% less than that for the steel in compression.

Development Length in compression = σ × d × τ bd

The development length for steel of different grades are calculated by this formula and data are given in table.

Development Length in tension = σ × d × τ bd

Development Length in compression = σ × d × τ bd

Where

d is the diameter of steel

σs is the Stress in steel.

τbd is the Design bond stress

### Development Length for One Steel:

The development length for one single steel is given below:

 S.No Type of steel Bond Stress (N/mm) Development Length in            Tension(mm) M20 M25 M30 1 Fe 250 Plain Steel 130 41d 39d 33d 140 44d 39d 36d 2 Deformed Steel 230 45d 40d 36d

Table of development length in Tension

 S.No Type of steel Bond Stress σ (N/mm) Development Length Ld in Compression(mm) M20 M25 M30 1 Fe 250 Plain Steel 130 33d 30d 27d 140 36d 32d 28d 2 Deformed Steel 230 36d 32d 29d

Table of development length in Compression

1). “d” is the diameter of the steel.

2). Bundled steels in contact, the development length is given by that for the single steel and increased as follows:

• 10% for two steel in contact.
• 20% for three steel in contact.
• 33% for four steel in contact.

### Development Length for Tie Together Steels:

When the number of steel is more, needed to be used based on the design it may not be possible to place the steel individually with the requires clearance. In situation like this, there are two options that is given below:

1. Increase the size of the concrete member that is the column or beam
2. Bundle the steel in groups of 2, 3 or 4 steel.

If we bundled the steels, then these steel will have a less area contact then concrete when match to the steel put individually. This affects the bonding between steel and concrete that is frequently disturb, mostly in beams. To achieve this demand, the development length is increased properly.

• If two steels are bundled it shall be increased by 10%
• If three steels are bundle then it shall be increased by 20% and
• If four steel are bundled it shall be increased by 33%

### Factors affect Development Length:

The following factors affects the development length that is given below:

• Density of concrete
• Center to Center distance of Steel
• Concrete cover of steel
• Compressive strength of concrete
• Dia of steel
• Steel coating

#### 1.   Density of Concrete:

Density of concrete affects the development length If lightweight concrete is used, then the development length will be increased or if the high density of concrete is used.

#### 2.   Center to Center Distance of Steel:

If the distance of steel is increased, then more concrete will be needed for steel to withstand horizontal shearing. In beams, steel is nearly spaced one or two steel diameter separately on the other hand for different types of member’s steel spacing are higher and hence needed development length is less.

#### 3.   Concrete Cover of Steel:

Concrete cover of steel also affects the development length If the thickness of concrete cover is more than the development length will be decreased they are inversely proportional to each other.

#### 4.   Compressive Strength of Concrete

If the intensity of compressive strength is more than the development length will be less it means they are inversely proportional to each other or if the intensity of compressive strength is less than the development length is more.

#### 5.   Dia of Steel

Dia of steel affects the development length. It has been notice that smaller diameter of steel needed lower development length than that of the larger diameter steel. Dia of steel is directly proportional to development length

#### 6.    Steel Coating:

structure is subjected to corrosion environmental conditions in some projects there epoxy coated steels are used instead of conventional steel. the strength of bond between concrete and steel is reduced and hence more development length is needed.

### Opinions:

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