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.

Lap length term is used when another bar is to be joined to an existing bar and is usually specified as 60 D for Horizontal bars and 45 D for vertical bars. Development length is the L shaped leg provided to a bar . This sounds so complicated so these images will help you understand it better, and the link bellow will clarify it all.

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