1) To develop a safe bond between the bar surface and the concrete so that no failure due to slippage of bar occurs during the ultimate load conditions.
2)The extra length of the bar provided as development length is responsible for transferring the stresses developed in any section to the adjoining sections.
Importance
Provision of appropriate development is an important aspect of safe construction practice, proper development length in reinforced bars shall be provided as per the steel grade considered in design otherwise in scenarios where less the against the required is provided the structure will be prove to the due to slippage of joints,bonds,anchors and laps, in such cases the bars will not yield first but the failure will happen at joints and laps prior to yielding of reinforcement bars.
Calculation of Development Length
As per the Indian Standard IS 456:2000, clause 26.2.1 the development length Ld is given by the following expression:
Where,
f= Nominal Diamater of Reinforcement bar
ss = Stress in bar at a section considered at design load
tbd = Design Bond Stress
The given formula gives the value of development length in mm for any given diameter of bar.
Same formula is used for limit state method and working stress method. The only difference is the different value of design bond stress tbd .
The value of design bond stress for limit state and working stress are as follows:
Note: For bars in compression 1.25 times the
above-given values shall be used.
Note: For bars in compression 1.25 times the above-given values shall be used.
Generally in practice the development length requirement is expressed as 41 times diameter or 41 * f
where, 41 is the factor calculated using the above formula and f is the diameter of the bar.
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