what is development length & Lapping length ?
Development length
Development length is certain minimum length of the bar required on either side of a point of maximum steel stress, in order to transfer the bar force to surrounding concrete through bond without slip ,so as to prevent bar from pulling out under tension. This is also called anchorage length .
Development length is provided in Slabs, Continuous beams and at column-beam joints.
As per IS 456:2000, in the tension or compression member at any section shall be developed on each side of the section by providing appropriate development length. If provided, the bar won’t be slipped away from the support
Formula for development length of steel ?
Example :
Assume Grade of concrete= M 30
yield strength of the steel= 415 N/ mm 2
Apply value in above formula
Lap length=(415 x D) / (4 x 1.50) = 69 D
Where D is Diameter of steel bar
Lapping length
Lap length is the minimum length that must be provided if two bars are joined together such that forces can be transferred safely
In other words , Lap length is provided when you require tension reinforcement but the length of the bar is not sufficient to continue through the entire member
Lap length in tension:
The lap length including anchorage value of hooks shall be
- For flexural tension – Ld or 30d whichever is greater is considered.
- For direct tension – 2Ld or 30d whichever is greater is considered.
The straight length of lapping of bars shall not be less than 15d or 20cm.
Lap Length in Compression:
The lap length is equal to the development length calculated in compression but not less than 24 d.
Thumb rule for Lap length for M 20 concrete:
- Columns – 45 d
- Beams – 60 d
- Slabs – 60 d
Where d is diameter of bar