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).
Reason for providing Development length
- To develop a safe bond between the bar surface & the concrete so that no failure due to slippage of bar occurs during the ultimate load conditions.
- Also, the extra length of the bar provided as development length is responsible for transferring the stresses developed in any section to the adjoining sections (such as at column beam junction the extra length of bars provided from beam to column).
Provision of appropriate development is an important aspect of safe construction practices. Proper development length in reinforcement bars shall be provided as per the steel grade considered in design.
Otherwise in scenarios where less development length against the required is provided the structures will be prone to encounter failure due to slippage of joints, bonds, anchors & Laps, in such cases the bars will not yield first but the failure will happen at joints & laps prior to yielding of reinforcement bars.
Calculation of Development Length
Ø = nominal dia of reinforcement bar
?s = Stress in bar at the section considered at design load
?bd = Design bond stress
The above given formula is used to calculate the required development length in mm for any given dia of bar, same formula is used for limit state method as well as working stress method. The only change in calculation in both methods is due to the different value of design bond stress; the values of design bond for Limit State & working stress are as follows;
Table No 1: Design Bond Stress in Limit State Method
|Design Bond Stress in Limit State Method|
|M20||M25||M30||M35||M40 and above||–|
|Concrete Grade||1.2||1.4||1.5||1.7||1.9||For Plain Bars in Tension|
|Design Bond Stress (?bd,N/mm2)||1.92||2.24||2.4||2.72||3.04||For deformed bars in tension|
Table No 2 : Design Bond Stress in Working Stress Method
|Design Bond Stress in Working Stress Method|
|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|
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.