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.
Development length is the amount of reinforcement embedded in a column to achieve the suitable bond strength between the steel and concrete.
It has a crucial role to play in the stability of a structure as it helps to prevent joint slippage. The amount of development length required in a particular case depends on a variety of factors, such as concrete density and steel coating.
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
[table responsive=”yes” alternate=”no” fixed=”no”]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
[table responsive=”yes” alternate=”no” fixed=”no”]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.
[table responsive=”yes” alternate=”no” fixed=”no”]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
[table responsive=”yes” alternate=”no” fixed=”no”]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.
Frequently Asked Questions (FAQs) about Development Length in Reinforced Concrete Structures
What is development length in reinforced concrete?
Development length refers to the minimum length of reinforcement required to be embedded in concrete to ensure proper transfer of stress between the steel reinforcement and the concrete.
Why is development length important in construction?
Development length is crucial as it ensures structural integrity by preventing slippage of reinforcement under load, thereby enhancing the bond strength between steel and concrete.
How is development length calculated?
Development length can be calculated using formulas that consider factors such as bar diameter, permissible bond stress, and concrete grade. For example, Ld = (Øσs) / (4τbd), where Ø is the diameter of the bar, σs is the stress in the bar, τbd is the bond stress.
What factors affect the development length?
Several factors influence development length, including the compressive strength of concrete, density of concrete, clear cover for reinforcement, spacing between bars, and the type of bar surface coating.
What happens if the required development length is not provided?
Failure to provide adequate development length can lead to structural issues such as reinforcement slippage, which compromises the stability and load-bearing capacity of the concrete member.
How does development length vary for different grades of concrete and types of reinforcement bars?
Development length varies based on the grade of concrete (e.g., M20, M25) and the type of reinforcement bars (plain or deformed). Higher grades of concrete typically require shorter development lengths due to increased bond strength.
What are the considerations for bundled bars in terms of development length?
When bars are bundled together due to design requirements, the development length needs to be adjusted accordingly, typically by increasing it based on the number of bars bundled.
Is there a difference in development length calculation between limit state method and working stress method?
Yes, while both methods involve similar principles, the limit state method considers ultimate load conditions and allows for higher bond stresses, whereas the working stress method uses lower bond stresses based on serviceability criteria.
What is the difference between development length and anchorage length?
Development length refers to the length of reinforcement required within concrete to develop its full strength, while anchorage length refers to the additional length provided by bends or hooks at the end of bars to ensure proper stress transfer.
How can designers optimize development length in concrete structures?
Designers can optimize development length by selecting appropriate reinforcement types, optimizing concrete mix designs, minimizing clear cover distances, and considering alternative construction methods like bundled bars where feasible.