Table of Contents
What Is Development Length?
Development length is defined as the minimal size of the bar that must be set in concrete beyond one segment in order to build its maximum strength, but is also regarded as an anchorage length in situations of axial tension or axial compression.
Why We Provide Development Length?
Such length is expected to provide fixed stability to the beams and to pass stress into another concrete. A development length is given at the column beam or column footing joint, and this length provides a protective connection between the bar surface and the concrete.
The primary reasons for giving this development length;
- To establish a stable connection between the bar surface and the concrete.
- No loss occurs as a result of bar slippage during the ultimate load conditions.
- The extra length of the bar given as production length is responsible, for example, at column, for moving the stresses produced in any section to the adjoining parts. beam junction the extra length of bars given from the beam to the column
- A significant feature of safe building practises is ensuring appropriate production, and adequate development length in reinforcement bars shall be given in accordance with the steel grade considered in the design.
- The stresses produced are easily transmitted by the steel bond, and this length is given at the beam and column junction.
- If we do not have construction length, the designs would be vulnerable to failure due to joint sleeping.
What Will Happen If We Don’t Provide Development Length?
The beam will come out of the concrete column if the development duration is not given at the time of installation. As a result, this length is needed to provide protection for the beam and reduce the risk of the beam dropping out of the concrete column.
How to Calculate Development Length?
Limit State Method For M25 Grade Concrete And Fe – 415 Grade Steel.
- Diameter of bar = 12mm
- Stress in bar = 415 N / mm2
- Design bond stress = 2.24 N / mm2 ( as per 26.2.1.1 of IS 456 – 2000 for deformed bars )
Ld = (Øσs) / (4τbd)
Ld = [ (σs) / (4τbd) ] x Ø
Ld = [ (415 ) / ( 4 x 2024) ] x Ø
Ld = 46.316 x Ø = 46 Ø
Therefore, for bar of 12 mm diameter, development length ( Ld) = 46 x 12 = 552 mm is required
Working Stress Method For M25 Grade Concrete And Fe – 415 Grade Steel
- Diameter of bar = 12 mm
- Stress in bar = 230 N/ mm2 ( as per table 22, Annex – B of IS 456)
- Design bond stress = 1.44 N/ mm2 ( as per 26.2.1.1 of IS 456 – 2000 for deformed bars )
Ld = (Øσs) / (4τbd)
Ld = [ (σs) / (4τbd) ] x Ø
Ld = [ (230 ) / ( 4 x 1.44) ] x Ø
Ld = 39.93 x Ø = 40 Ø
Therefore, for bar of 12 mm diameter, development length ( Ld) = 40 x 12 = 480 mm is required
Permissible Bond Stress for Plain Bars and Deformed Bars
Permissible Bond Stress For Plain Bars
Grade of Concrete | Permissible Bond Stress |
M 20 | 0.8 |
M 25 | 0.9 |
M 30 | 1.0 |
M35 | 1.1 |
M40 and above | 1.2 |
Permissible Bond Stress For Deformed Bars
Grade of Concrete | Permissible Bond Stress |
M 20 | 1.28 |
M 25 | 1.44 |
M 30 | 1.6 |
M35 | 1.76 |
M40 and above | 1.92 |
- For deformed bars is 60% more than that of plain bars.
- It is easier to pull a bar than to push it inside. Therefore permissible bond stress for plain and deformed bars in compression is taken 25% more than that for the bars in tension.
- L_{d } in compression = σ_{ st }φ / 4(1.25) τ bd
- _{Ld in compression = }σ_{ st }φ / 5 τ _{bd}
- _{ }The development length for steel bars of different grades are computed by the following formula and data are given in the table
- L_{d } in tension = σ_{ st }φ / 4_{ }τ _{bd}
- L_{d } in compression = σ_{ st }φ / 5_{ }τ _{bd}
Development Length for Single Bars
Development Length For Fe 250 plane Single Bars.
Ld in tension (mm) | σ_{st }= 130 N/ mm^{2} | σ_{st }= 140 N/mm^{2} |
M20 | 41 φ | 44 φ |
M25 | 39 φ | 39 φ |
M30 | 30 φ | 35 φ |
Ld in compression (mm) | ||
M20 | 33 φ | 36 φ |
M25 | 30 φ | 32 φ |
M30 | 27 φ | 28 φ |
Development Length For Fe 250 Deformed Single Bars
Ld in tension (mm) | σ_{st }= 230 N/ mm^{2} |
M20 | 45 φ |
M25 | 40 φ |
M30 | 36 φ |
Ld in compression (mm) | |
M20 | 36 φ |
M25 | 32 φ |
M30 | 29 φ |
- φ is the diameter of the bar.
- As bundled bars come into contact, the production duration is determined by the length of the individual bars and expanded as follows:
- 10% if two bars come into touch.
- 20% if three bars are in touch.
- 33% when four bars are in touch.
Development Length for Bundled Bars
- This may not be practicable to position the bars separately if a significant number of bars are needed to be supplied depending on the design. In such cases, there are two options: maximise the scale of the concrete members, such as columns or beams.
- Bundle the bars in groups of two, three, or four.
- If we take Choice 1 and raise the size of the concrete member, there would be a cost effect. As a consequence, the second alternative is preferable.
- When the bars are bundled, they have a lower surface area with the underlying concrete than if they were mounted separately.
- This length is sufficiently extended to satisfy this criteria.
- If two bars are bundled, the development length must be expanded by 10%.
- If three bars are bundled, the development length is expanded by 20%.
- If four bars are bundled, the development length will be increased by 33%.
Factors Affect Development Length
Here are the resulting factors affecting this length such as;
- Compressive Strength of Concrete.
- Density of Concrete.
- Clear cover for Rebar.
- Rebar Centre to Centre Spacing.
- Coating of Rebar.
1. Compressive Strength of Concrete:
The required development length for bars is inversely proportional to the compressive strength of concrete, but if the compressive strength is greater, the required development length is smaller.
2. Density of concrete:
If lightweight concrete is used, the construction time must be extended.
3. Clear cover for Rebar:
If we raise the clear cover, this duration would reduce.
4. Rebar Centre to Centre spacing :
Unless the bar spacing is raised, more concrete would be required for rebar to withstand horizontal splitting. In pillars, the bars are placed one to two bar diameters apart, while in slab footings, the width is larger.
5. Coating of Rebar :
In certain projects where even the building is exposed to oxidation climate factors, epoxy coated rebars have been used. In these situations, the bond strength amongst concrete and rebar is limited, necessitating a longer construction time.
Development Length Formula
The development length is determined using the formula below;
Ld = (Øσs) / (4τbd)
Where ,
- Ld denotes the embedded length of the steel strip.
- σ s is the maximum allowable stress in steel.
- τ bd denotes bond tension and is the diameter of the bar.
This formula is used to measure the necessary development duration in mm for any given diameter of the bar, and it is used for both the limit state method and the working stress method.
Anchorage Length
- Anchorage length is the length needed for stress growth in rebars; this is accomplished by supplying the required development length or hook/bends if adequate length cannot be attained.
- If adequate construction length cannot be provided within the support/fixed end, anchorage length is provided. For a 90-degree bend, the L value is usually known to be 8 times the diameter.
- While a 135 degree bent requires 6 times the diameter of the bar and a 180 degree bend requires 4 times the diameter of the bar.
Why We Provide Development Length?
It creates a safe bond between bar surface and concrete. It also ensures that during ultimate load conditions, the reinforcement bar should not slip through the concrete. It transfers stress or loads from Beam to column smoothly now you understood why we provide development length.
What Will Happen If We Don’t Provide Development Length?
The beam will come out from the column if the development length is not provided during construction. The development length is needed to provide support to the beam to reduces the chances of the beam coming out of the concrete column.
How to Calculate Development Length?
For bars in compression, 1.25 times the above given values shall be used. Generally, in practice, the development length requirement is expressed as ’41 times Ø’or ’41 Ø’ where 41 is the factor calculated using the above formula& Ø is the dia of the bar.
Permissible Bond Stress for Plain Bars and Deformed Bars.
Permissible Stresses in Concrete (Refer to Table 21, IS 456) Notes: (i) The bond stress given above for tension is increased by 25% for bars in compression. (ii) The bond stress for plain bars is increased by 60% for deformed bars.
Development Length for Single Bars
1. φ is the diameter of the bar.
2. In case of bundled bars in contact, the development length is given by that for the individual bars and increased as follows:
- 10% for two bars in contact.
- 20% for three bars in contact.
- 33% for four bars in contact.
Development Length for Bundled Bars
Development length of individual bars within a bundle, in tension or compression, shall be that for the individual bar, increased 20 per cent for 3 bar bundles and 33 per cent for 4 bar bundles.
Factors Affect Development Length
- Compressive strength of concrete.
- Density of concrete.
- Rebar clear cover.
- Rebar centre to centre spacing
- Coating of Rebar.
- Rebar diameter.
Anchorage Length
Meaning of anchorage length is the length required for development of stress in the rebars, this is obtained by providing the required development length or hook/bends if sufficient length cannot be achieved.
Development Length in Footing
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). Fig 1: Development length in Footing.
How to Calculate Development Length of Steel Reinforcement?
σ s is permissible stress in steel. φ is the diameter of the bar. For any given diameter of the bar, this formula is used to calculate the required development length in mm and for limit state method as well as working stress method the same formula is used.