A lap is when two pieces of reinforcing bar (rebar) are overlapped to create a continuous line of rebar. The length of the lap varies depend on concrete strength, the rebar grade, size, and spacing.
D is the diameter of the bars to be lapped together. So if you want to lap 20 mm dia column bars , you need to provide a lap of minimum 45 x20 = 900 mm. Overlap length in columns, beams and slabs is the distance intersection between two joining reinforcement bars.
The factors that are seriously considered in the analysis of the lap welded joints are: The bar diameters (d= 8, 10, 12, 14, 16, 18, and 20 mm), the length of the weld (4d and 5d), the distance “a” between the ends of the welded parts and the free length of bars “l” for l=300, 400, 500, 600, 700, 800, 900, and 1000 mm.