Please take a look at the program manual, RamConcreteShearWall.pdf, Section 7.4 Reinforcement Ratio Limits for some details on how we check ACI 318-11 Section 21.9.2.1. If that does not answer your question, let me know.
I've copied a part of that below, but the formatting is not good:
When the provisions of Chapter 21.9 are considered (Special Reinforced Concrete Structural Wall),
Sections 21.9.2.1 and 21.9.4.3 are also checked.
In these sections ACI uses the following terms:
ρl = Ratio of area of distributed longitudinal reinforcement to gross concrete area perpendicular to that reinforcement.
ρt = Ratio of area of distributed transverse reinforcement to gross concrete area perpendicular to that reinforcement.
Despite the above definitions given by ACI, ρl is also used within ACI 318-11 to denote horizontal reinforcing (e.g., 11.9.9.2), and ρt is also used to denote vertical reinforcing (e.g., 11.9.9.4). To resolve this, the following approach is used within the Shear Wall Design module:
In cases where ρlis referenced and the text within the ACI code section specifically mentions “vertical” reinforcing, ρlis calculated using the area of vertical reinforcing resulting from the Bar Pattern. If “vertical” reinforcing is not specifically referenced, the area of reinforcing is calculated using the area of the reinforcing bars that intersect and are normal to the Section Cut Segment under consideration.
In cases where ρt is referenced and the text within the ACI code section specifically mentions “horizontal” reinforcing, ρt is calculated using the area of horizontal reinforcing resulting from the Bar Pattern. If “horizontal” reinforcing is not specifically referenced, the area of reinforcing is calculated using the Bar Pattern that is parallel to the Section Cut Segment under consideration.
Sections 11.9.9.3 and 11.9.9.5 both specify spacing limits that are a function of the overall length of the wall, lw.
RAM uses the Section Cut Segment length for this value. The limitation of this assumption is that for very short wall segments, the limit spacing will likewise be small. Since the calculated spacing limit is compared to the nominal bar spacing and not the true bar spacing, false check failures may be reported for short wall segments.