What is the difference between lap length and development length in steel reinforcement?

What is the difference between lap length and development length in steel reinforcement?
Development Length is essentially provided to create a bond between steel and concrete in order to transfer the bar force without slip to prevent the bar from pulling out under tension.

Image A: The development length is provided in the beam and column junction. Due to this the stresses developed are easily transferred by the bond created by steel and concrete.

Image B: The development length is not provided in the beam and column junction. Due to this the stresses developed in the beams will not be able to transfer it to the column.

As per IS 456 for Limit state method, the development length Ld is given by

Ld = d x 0.87 x fy / τbd

Where,

d = Diameter of the bar.

fy =Characteristic strength of the stirrup reinforcement in N/mm2

τbd = Design bond stress.

Development length
When you have a reinforcement bar, that is no longer required to carry the tensile forces, that is the tension is zero at that section, then you will have to develop the bar further so that the grip between the steel and concrete makes it a continuous structure. Now we have to provide this development length because if we do not, then the bar does not have any restrain and we know that if a member does not have any restrain than it does not resist any forces. So that is the concept of development length.

Lap Length: Lap length is the minimum length that must be provided if two bars are joined together such that the forces can be transferred easily.

Image A: Check the lap splice provided in the beam. The length of the overlap changes depending on the flexural tension, direct tension and compression, In our practical site condition it is taken about 50 x diameter of bar.

Image B: Check the image if bars are not overlapped no forces are transferred.

Image A: The overlapping is mostly done in columns as the columns can reach to more than 100m. To make the job simpler for workers the bars are cut at every second storey of the building.

Image B: If the overlapping is not done in the column the forces are not going to be transferred.

Important Things to remember:

• Lap splices shall not be used for bars more than 36mm dia, bars shall be welded in such case.
• Lap splices shall be as far from the section of maximum stress.

Lap Length
Lap length is provided when you require tension reinforcement but the length of the bar is not sufficient to continue through the entire member, for example columns. Now, columns are typically 100' tall. Neither can we have a 100' long bar, nor is it easy or possible to cage, so we tend to cut bars to every 2 story. Now, at the location of discontinuity we want the tension forces to transfer from one piece of bar to another. So for that we tend to provide second bar near to the one that is discontinued and the amount of overlapping between the two bars is called a lap length.