Q.: As the strength of a reinforced concrete slab or beam affected by the spacing of rebar ties?
A.: Rebars are tied only to maintain bar positions during work done by other trades and during concrete placing. Tying adds nothing to the strength of the finished structure. In most cases, a tie at every fourth or fifth intersection is all that's needed. For preassembled mats, however, tie enough bar intersections to make the mats rigid enough for handling. When snap ties are used, tie every bar intersection around the perimeter and alternate intersections within the interior of the mat.
More information on bar placing, splicing, and tying is given in the Concrete Reinforcing Steel Institute (CRSI) book Placing Reinforcing Bars. The 193-page book is available for $10.00 ($6.00 for members) from CRSI, 933 North Plum Grove Road, Schaumburg, Illinois 60173; telephone 312-490-1700.