1.If you are working off a 1999 or earlier version, the max ratio is 0.75 or 0.5 (seismic zones) of the balanced ratio. Balanced ratio is ((0.85?1f’c)/fy)(87000/(87000+fy)). The reinforcement limit of 0.75 of the balanced ratio results in a net tensile strain at nominal strength of 0.00376. 2002 and later versions require a net tensile strain at nominal strength of 0.004 for flexural members with an axial load less than 0.10f’cAg. I haven’t read 318-05, but I do know the ACI has proposed newer ways to actually measure ductility. For low rise and uniform-shape buildings, I typically use a reinf ratio of 0.01. This results in a fairly ductile and economical design.
2.Sections 10.3.4 and 10.3.5 of ACI 318-02 require different values of steel strains at nominal strength and reduce the strength reduction factor for the lower values.As a rule of thumb, if your steel ratio is greater than .017, you're probably over-reinforced and should explore a deeper member.
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