ACI 318 10.3.5: Upper Limit on As, εs > `epsilon`=0.004
To ensure adequate ductile behavior in beams, ACI 318 requires the steel to have significant strain at the ultimate strength condition (i.e. when the concrete strain is 0.003). Section 10.3.5 prescribes the limit for this state.
For practical purposes, it is generally good to keep the steel strain above `epsilon`=0.005 or the reduction factor φ will need to be reduced.
The tensile strain can be derived from strain diagram once the location of the neutral axis, c, is determined.
This requirement effectively puts an upper limit on the amount of steel that can be used in the beam. As the amount of steel is increased in a given section the strain in the steel decreases.