What is Development Length?
It is the minimum length of the bar which must be embedded in concrete beyond any section to develop its full strength. This is also called as an anchorage length in case of axial tension or axial compression and development length in case of flexural tension or flexural compression.
It is provided at column beam joint or column footing joint. Here you can see in the picture this is development length for beam-column joint. This the main bar in the beam which is extended from here to here to safely transfer stresses from Beam to the column and this portion of length is called as development length.
Why we provide development length?
Development length creates a safe bond between bar surface and concrete it also ensures during ultimate load conditions the reinforcement bar should not sleep through the complete 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?
If we don’t provide development or if we provide less development length against the required then The structures will be prone to encounter failure due to sleeping of joint in such cases the bars will not break first but the failure will happen at joints and laps prior to breaking of reinforcement bars. In short reinforcement bars will split from concrete.
Let us take the case of the beam-column joint in which development length is not provided. If we apply force more than the maximum permissible limit then the restraining force between the beam and column will not be sufficient to hold the beam at its position.
The beam will come out from the column if the development length is not provided during construction. The development is needed to provide support to the beam to reduces the chances of the beam coming out of the concrete column. Hence it acts as a supporting member for reinforced beam in the concrete column.
How to calculate Development Length?
For understanding the concept of bond and development length, let us consider a steel bar embedded in concrete. the bar is subjected to a tensile load T. due to the tensile force, the steel bar will tend to come out and slip out of the concrete. This tendency of slipping is resisted by the bond stress developed over the surface of the bar.
Bond stress is the shear stress developed along the contact surface between the reinforcing steel and the surrounding concrete which prevent the bar from slipping out of concrete. To avoid slipping
T ≤ τ bd × 2πφ/2 × Ld (surface area = 2πr × Ld)
T = σ st × π/4 ×φ2 ( T = σ st × Aφ)
σ st × π/4 ×φ2 ≤ τ bd × 2πφ/2 × Ld
Ld ≥ σ st φ/ 4 τ bd
Ld = Embedded length of steel bar
σ st = Permissible stress in steel
τ bd = Bond stress
φ = Diameter of bar
Ld is called as the development length. As per code IS 456:2000, development length is given by
Ld = σ st φ/4τ bd
This formula is used to calculate the required development length in mm for any given diameter of the bar. The same formula is used for limit state method as well as working stress method.
The permissible bond stress τ bd depends upon the grade of concrete and type of steel. The values of permissible bond stress are given in the table.
Also, read – What is Lap Length? – Complete Guide
Permissible Bond Stress For Plain Bars And Deformed Bars
|Grade of Concrete||τ bd For plain bars(N/mm2)||τ bd For deformed bars(N/mm2)|
|M40 and above||1.2||1.92|
a.) For deformed bars is 60% more than that of plain bars.
b.) 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.
Ld in compression = σ st φ/ 4(1.25) τ bd
= 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
Ld in tension = σ st φ/ 4 τ bd
Ld in compression = σ st φ/ 5 τ bd
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
When 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 with the necessary clearance. In such cases, there are two options
- Increase the size of the concrete member that is the column or beam
- Bundle the bars in groups of 2, 3 or 4 bars
Here if we consider option number 1 and if we increase the size of the concrete member then there will be a cost implication. So, it’s better to go with option number 2.
If we bundled the bars then these bars will have a low contact area then surrounding concrete when compared to the bar placed separately. This affects the bonding between concrete and bars which is often a concern, especially in beams. To fulfil this requirement the development length is increased suitably.
- If two bars are bundled it shall be increased by 10%
- If three bars are bundle then it shall be increased by 20% and
- If four bars are bundled it shall be increased by 33%
Factors affect Development length
1. compressive strength of concrete
The development length required for bars is inversely proportional to the compressive strength of concrete which means if more is the compressive strength then less would be required development length.
2. Density of concrete
If lightweight concrete is used then it must be increased.
3. Rebar clear cover
If we increase clear cover then it will decrease.
4. Rebar centre to centre spacing
If the bar spacing is increased then more concrete will be available for rebar to resist horizontal splitting. In beams, bars are closely spaced one or two bars diameter apart on the other hand for slab footing and certain other types of members bars spacing are higher and thus required development length is less.
5. Coating of Rebar
Sometimes in some project where the structure is subjected to corrosion environmental conditions there epoxy coated rebars are used instead of normal rebars. Studies have shown that in such cases the bond strength between concrete and rebar is reduced and thus more development length is required.
6. Rebar diameter
The most important and common factor that would influence development length is the diameter of reinforcement is used. It has been observed that smaller diameter bars require lower development length than that of the larger diameter bars.
So, friends, I hope I have covered all the information about development length in this article.
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