How old am I? I’ll give you an idea: we had been the last batch way back university days that used an older version of ACI that doesn’t interpolate the strength reduction factor depending on the strain at the extreme bar/s in tension. (Feels like it was a dang long time ago!)
Just because it’s bending, the old code automatically sets the strength reduction factor to 0.90. It’s not the case anymore. Before, what limits your flexural strength is the rho max and the reducing effective depth d as the flexural bars increase.
The recent codes however, take into account the strain of the extreme bar/s in tension and it doesn’t matter if the member is in pure bending or a combination of axial and bending.
Cropped from ACI 318M-11
The figure above, R.9.3.2 should be able to explain this in detail. Strains of below and equal to 0.002 will have a reduction factor of 0.65. Strains of 0.005 and beyond will have a reduction factor of 0.90. Strains in between will be linearly interpolated. Now an example by hand was done below for a member in pure bending.
Hand calculations by yours truly
For emphasis, this doesn’t only apply to members primarily in bending but is also applicable to members in combined axial and flexure.