Development length and Lap length
Many students get confused between Development length and lap length terminologies. Lap length addresses the length of the bar needed to transfer the stresses to the other bar whereas Development length addresses the length of the bar needed to transfer the stresses to the concrete.
Let us look at them separately
Usually at the end of the section where you have forces in the bar, you need to make sure that bar pullout failure doesn’t happen. In order to resist forces in rebar it has to be embedded in concrete.
A development length is the amount of rebar length that is needed to be embedded or projected into concrete to create a desired bond strength between the two materials and also to develop required stress in steel at that section.
In the example below we require development length of 10db at the end of the section so that bond between concrete and steel remain continuous. As there is no space at the end of the section bar is bent. Notice that 90 degree configuration is used here more configurations can also be used. For further detail refer to ACI or Indian Standard Development Length section.
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.