A development length can be characterized as the total reinforcement bar length needed to be planted or projected into the column or beam. A development bar is required to build the desired bond strength between the concrete and steel. development bar length is also called anchorage length.
What is the reason for providing development Rebar?
- The Development bar is provided to create a strong bond between the concrete and reinforcement bars. To avoid structure failure, during the final settlement of the concrete.
- It also controls the stress developed in any adjacent section of column and beam, for instance, additional development bar length is provided from beam to column or column to beam sections.
Development Length Formula | How To Calculate Development Length
According to IS 456:2000 Limited State Method (LSM), the formula to calculate the rebar development is:
(Ld = σ st φ/4τ bd)
Where,
Ld = Embedded length of steel bar
σ st = Permissible stress in steel (0.87)
τ bd = Bond stress of a concrete grade
φ = Diameter of bar
As per the thumb rule, the bar length is calculated between 45D-55D. Normally we consider 50D on Construction site, so if we have a diameter of φ 16, the bar development length would be 50 x (16mm)=800 mm
Relevant Content: Rebar Detailing and Estimation
Development Length in Beam & Column
Development bar length is provided in the beam and column to control the failure due to the slippage of joints in the structural members.
In such a Scenario, reinforcement bars will be segregated from the concrete and the bars will not be brake itself rather the failure will happen at joints and laps provided.
The development length is required to support the beam and reduces the chances of beam splitting from the column and cause a major failure.
The formula to calculate the development bar length in beam and column for design purposes is as under;
(Ld = σ st φ/4τ bd)
Where
Ld = Embedded length of steel bar
σ st = Permissible stress in steel (0.87)
τ bd = Bond stress of a concrete grade
φ = Diameter of bar
Development Length Table
To calculate the required development bar length for any dia in mm the above formula can be used. the same formula can be used for the limit state method (LSM) and working stress method (WSM). the only difference in the calculation of both methods is the value of design bond stress.
Design Bond Stress in Limit State Method
| M20 | M25 | M30 | M35 | M40 | ||
|---|---|---|---|---|---|---|
| Concrete Grade | 1.2 | 1.4 | 1.5 | 1.7 | 1.9 | For Plain Bars in Tension |
| Design Bond Stress | 1.92 | 2.24 | 2.40 | 2.72 | 3.04 | For deformed bars in tension |
Design Bond Stress in Working Stress Method
| M20 | M25 | M30 | M35 | M40 | M45 | M50 | ||
|---|---|---|---|---|---|---|---|---|
| Concrete Grade | 0.8 | 0.9 | 1 | 1.1 | 1.2 | 1.3 | 1.4 | For Plain Bars in tension |
| Design Bond Stress (N/mm2) | 1.28 | 1.44 | 1.6 | 1.76 | 1.92 | 2.08 | 2.24 | For deformed bars in tension |
Factors Affecting Development Length
There are some factors affecting the development length of the rebar.
a. Density Of Concrete
b. Clear Cover
c. Rebar Spacing
d. Compressive Strength of Concrete
e. Rebar Coating
a. Density Of Concrete
Rebar development length depends on the density of concrete, if lightweight concrete is used, then the rebar development length must be increased.
b. Clear Cover
if the concrete clear cover is increased then normal then the development bar length will be increased as well to provide more support.
c. Rebar Spacing
the bars are closely spaced to resist the horizontal or vertical splitting, if the bar spacing is increased then more concrete will be poured resulting in more structural weight and more compression. therefore, if the spacing is changed center to center then rebar development length will also be affected.
d. Compressive Strength of Concrete
the length of the development bar is inversely proportional to the compressive strength of concrete, if the concrete compressive strength is more then less development bar will be provided in the structural elements. concrete general compressive strength various from 15 mpa(2200 psi) to 30 mpa (4400 psi). compressive strength of the concrete also depends on many factors such as water-cement ratio, the strength of cement, quality of concrete components, and quality control procedures during the concrete casting.
e. Rebar Coating
if the weather conditions are not suitable and the rust and corrosion are more obvious than epoxy-coated rebars are used, the epoxy coating reduces the bond strength of concrete and epoxy-coated bars, therefore, in such cases, more developmental length bars are required.
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