A lap is when two pieces of 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. CRSI's "Reinforcement Anchorage and Splices" includes tables of required lap splice lengths based on these variables.
Contact splices--in which the bars touch and are wired together--are preferred because they are more secure against displacement during construction. Non-contact lap-spliced bars should not be spaced too far apart.
For lap splice design and construction, the ACI 318 Building Code requires the Engineer to indicate locations and lengths of all lap splices on the structural drawings.
Where reinforcing bars of two sizes are lap-spliced in tension, industry practice is to use the larger of the tension lap splice length for the smaller bar, or the tension development length for the larger bar. When bars of different sizes are lap-spliced in compression, the lap splice length must be the larger of the compression development length of the larger bar or the compression lap splice length of the smaller bar.
Lap splices of #14 and #18 bars should not be used, except in compression only to #11 and smaller bars. Lap splices of bundled bars should be based on the lap splice length recommended for individual bars of the same size, and individual splices within the bundle should not overlap each other. The length of lap should be increased 20% for a 3-bar bundle and 33% for a 4-bar bundle. Bar laps should be securely wire-tied together to maintain the alignment of the bars and to provide minimum concrete cover.
Reinforcing Bars: Anchorages and Splices (Purchase Publication, myCRSI)
Reinforcing Bars: Anchorages and Splices is the definitive source for information on development and splicing of reinforcing bars. Includes extensive tables of development and lap splice lengths. Also includes expanded information on headed bars. Based on ACI 318-08 and AASHTO bridge specifications.