Designers often specify the use of distributed steel (welded wire fabric or small-diameter reinforcing bars) in the upper half of concrete floors on ground. It's used to prevent random cracks from widening. Keeping cracks tightly closed improves load transfer, helps to prevent faulting (differential displacement of the slab at the crack), and makes spalling less likely at the crack edges. The amount of steel needed is usually calculated using the subgrade-drag equation. But does this equation give the right amount of steel for best floor performance?


Subgrade-drag theory assumes that when a slab of concrete dries and contracts, it draws itself over the subgrade from each end toward the center. Frictional resistance of the subgrade prevents full contraction. This resistance causes a tensile stress that increases as drying progresses and is highest at slab midlength. Adding reinforcing steel doesn't prevent cracking; it simply helps to control crack width.

Even if designers believe that more distributed steel provides superior crack control, the lack of published data on floor performance at different reinforcement percentages may make it difficult to justify the additional cost. However, some of the data suggest that minimum values of 0.10% to 0.15% are needed for best performance. Perhaps it's time to modify the design approach for floors by at least setting a minimum allowable reinforcement percentage (adjusting for differences in minimum yield strength of the bars or wire). Assured benefits include better control of steel position during construction and a larger safety factor against faulting and unacceptably wide cracks.