For a doubly reinforced concrete beam, under what specific condition will the compression steel yield before the concrete reaches its ultimate strain?

For a doubly reinforced concrete beam, the compression steel will yield before the concrete reaches its ultimate strain under the specific condition that the strain in the compression steel (εs') is equal to or greater than the yield strain of the steel (εy) at the very moment the extreme compression fiber of the concrete reaches its ultimate compressive strain (εcu). The ultimate compressive strain of concrete, εcu, is typically taken as 0.003 in design codes like ACI 318, representing the point where the concrete is considered....