We often get confused about using these terms – Development Length, Anchorage Length, and Lap Length.
What is the development length?
The steel reinforcement that is prolonged into concrete to avoid steel slippage and to transfer the load to the concrete bond smoothly is known as Development length or Anchorage Length.
The steel reinforcement provided which does not require to carry any tension stress but need to restrain is known as development length (L_{d}).
How is the development length calculated?
Development length differs according to the grade of concrete, steel reinforcement, and design load.
Development Length Formula
As per IS Code 456:2000, (Clause 26.2.1) Development length can be calculated using the below formula
L_{d} = Фσ_{s}/4τ_{bd}
Where,
Ф = diameter of the reinforcement bar
σ_{s} = Stress developed in a bar at section considered at load design
τ_{bd} = Design bond stress
Note- The formula can be used for both the Limit state and the Working stress method. It only differs in design bond stress value.
Development Length Calculation
Assume – Finding development length in Limit state method, For M30 grade of concrete & Fe500 steel grade
Diameter of bar = 12 mm
Stress in bars = 415 N/mm2 (For Fe415 Bars)
Design Bond Stress, τ_{bd} = 2.4 N/mm2
L_{d} = Фσ_{s}/4τ_{bd}
L_{d} = Ф*415/4*2.4 = 43Ф
L_{d} = 43Ф
According to IS 456:2000, the minimum development length should be 41Ф.
What is Lap Length?
The steel prolonged to overlap on another steel reinforcement for steel continuity and to transfer the load to another steel reinforcement is known as Lap Length.
The manufactured steel is mostly 12m in length. To prolong the steel reinforcement for the required length (more than 12m) and safely transfer the loads from one rod to another, lap length is provided.
Lap Length Reinforcement Details
According to IS 456:2000, the minimum length should be 24-55d, where d is the bar’s diameter. The lap length should not be less than 75mm.
What is the minimum lap length?
- Minimum Lap Length for Column = 24d – 40d
- Minimum Lap Length for Beam = 24d (for compression zone) & 45d (for tension zone)
- Minimum Lap Length for Slab = 50d
Difference between Development Length and Lap Length
Development Length | Lap Length |
Steel to Concrete – Reinforcement provided to prolong the connection of steel to concrete | Steel to Steel – Reinforcement provided for steel continuity and to transfer loads to another reinforcement. |
Development length does not carry any tension force. | Lap length carries tension force to another reinforcement |
Minimum development length should be 41Ф | Minimum lap length should be 24-55Ф |
Happy Learning 🙂