Development Length
|Development length is the minimum length of the bar that must be embedded in concrete beyond any section to develop its full strength. In the case of axial tension or axial compression, development length is also known as an anchorage length.
If the development length is not provided at the time of construction the beam will come out of the concrete column. So, to give support to the beam and to reduce the chances of the beam coming out of the concrete column this length is needed.
Development length is provided to provide fixed support to the beams and for transferring the stress into the concrete, this length is required.
At column beam joint or column footing joint, development length is provided and between bar surface and concrete, this length creates a safe bond.
The Formula of Development Length:
By using the formula below development length is calculated;
Where,
Ld is the embedded length of the steel bar.
σ s is permissible stress in steel.
τ bd is bond stress &
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 the limit state method as well as the working stress method the same procedure is used.
Reasons for Providing Development Length:
There are the following reasons for providing this length as given below;
- Between the bar surface and the concrete to develop a safe bond.
- Due to slippage of bar occurring during the ultimate load conditions no failure occurs.
- For transferring the stresses developed in any section to the adjacent sections the extra length of the bar provided as development length is responsible for example at column beam junction the additional length of bars provided from beam to column.
Importance of Development Length:
An essential aspect of safe construction practices is providing suitable development, and as per the steel grade considered in the design, proper development length in reinforcement bars shall be provided.
The stresses developed are easily transferred by the bond created by steel and in the beam and column junction, this length is provided.
The structures will be prone to encounter failure due to sleeping of joints if we don’t provide development length.
Calculation:
The formula for calculating development length is;
To calculate the required development length in mm for any given diameter of the bar, the above-given formula is used and for the limit state method as well as the working stress method, the same procedure is used. In both methods, the only change is due to the different values of design bond stress.
For Limit State & working stress the values of the design bond are;
Concrete Grades | τ bd For Plain (N/mm2) |
τ bd For Deformed Bars in Tension (N/mm2) |
M20 | 1.2 | 1.92 |
M25 | 1.4 |
2.24 |
M30 | 1.5 | 2.4 |
M35 | 1.7 |
2.72 |
M40 & Above | 1.9 |
3.04 |
Design Bond Stress in Working Stress Method;
Concrete Grades | τ bd For Plain (N/mm2) | τ bd For Deformed Bars in Tension (N/mm2) |
M20 | 0.8 |
1.28 |
M25 | 0.9 |
1.44 |
M30 | 1 | 1.6 |
M35 | 1.1 |
1.76 |
M40 | 1.2 | 1.92 |
M45 | 1.3 |
2.08 |
M50 | 1.4 |
2.24 |
Development Length for Bundled Bars:
If there is a large number of bars required to be provided based on the design, it may not be possible to place the bars separately. Then there are two options in such cases;
- The size of the concrete members like a column or beam should be increased.
- In the group of 2, 3, or 4 bars bundle the bars.
There will be a cost implication if we consider option number 1 and if we increase the size of the concrete member. So, it’s good to go with the second option.
By bundling the bars these bars will have a low contact area than the surrounding concrete as compared to the separately placed bar.
This length is increased suitably to fulfil this requirement.
- The development length shall be increased by 10% if two bars are bundled.
- This length shall be increased by 20% if three bars are bundled.
- This length shall be increased by 33% if four bars are bundled.
Factors Affecting Development Length:
There are the following factors affecting this length such as;
- Compressive Strength of Concrete
- Density of Concrete
- Rebar Clear Cover
- Rebar Centre to Centre Spacing
- Coating of Rebar
Compressive Strength of Concrete:
If more is the compressive strength then less would be required for development length because the development length required for bars is inversely proportional to the compressive strength of concrete.
The Density of Concrete:
The development length must be increased if lightweight concrete is used.
Rebar Clear Cover:
This length will decrease if we increase clear cover.
Rebar Centre to Centre Spacing:
More concrete will be available for rebar to resist horizontal splitting if the bar spacing is increased. The bars are closely spaced one or two bars diameter apart in beams and the spacing of the bars is higher for slab footing.
Coating of Rebar:
In some projects where the structure is subjected to corrosion environmental conditions, epoxy-coated rebars are used. In such cases, the bond strength between concrete and rebar is reduced and thus requires more development length.