Generally, any bottom bar will be spliced near the support points. Any top bar will be spliced at the the mid-span of the beam. It provides for reliable calculations when you don’t have to figure in variables like the strength at the area of the splice.
Regardless, most codes call out splice lengths to be between 30 to 45 bar diameters (usually 45). If you’re using #8 (1″ diameter nominal) rebar in a design, the length of the splice (the overlap between the ends of two pieces) would be 30–45″ long.
