Difference Between Lap length and Development Length

There are two important terms used in reinforcement binding. Lap length and Development length. But many of us get confused about the difference between lap length and development length.

All these extra length or extension are provided to safely transfer of stresses from one member to another. Today in this article we are going to discuss the difference between these two terms. 

Difference Between Lap Length and Development Length

Lap Length

Difference Between Lap length and Development Length
  • As the name suggests Lap length means overlapping of two bars side by side to achieve the required design length.
  • Lapping length required to safely transfer stress from one bar to another.
  • It varies from member to member and depends upon the grade of concrete, bar size, concrete cover etc.
  • Lap length for tension member = 40 d
  • The Straight length of lapping should not be less than 15d or 20 cm.
  • Lap length for compression member = 50d but not less than 24d  (where d is the diameter of the bar)
  • In the case of bars having different diameter are to be spliced the length is calculated on the basis of the smaller diameter of the bar.
  • Lapping splice should not be used for the bars having a larger diameter than 36 mm. In that case, welding should be done. But if welding is not practicable then lapping may be permitted for the bars larger than 36 mm diameter. The additional spiral should be provided around the lapped bar.
  • Generally lap length for column = 45d for M20 grade
  • Generally lap length for beam = 60d for M20 grade
  • Lap length for Slab = 60d for M20 grade

Development Length

Difference Between Lap length and Development Length
  • It is a length of Steel bar needed to be embedded into the column to establish the desired Bond strength between concrete and Steel in simple language it is the length of Steel bar which holds to concrete members together. concrete members like beams, columns, footings etc.
  • It is a grip between steel and concrete and makes a continuous structure. It is also known as anchorage length
  • It transfers stresses from steel bar to concrete and makes a continuous structure. It is provided as a bend.
  • Such length is provided in a column footing joint, column beam joint and other critical joints.
  • Rebar is bent because there is no space available at the end section
  • Development length for tension = σ st φ/ 4 τ bd
  • Development length for compression = σ st φ/ 5 τ bd

Where

Ld   = development length of steel bar

σ st = Permissible stress in steel

τ bd = Bond stress

φ    =  Diameter of bar

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Also, read

Difference Between one way and Two way Slap

How to calculate unit weight of steel bar?

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