Answer:
RF‑CONCRETE Surfaces always checks first whether the required reinforcement is sufficient to absorb the shear loading.
When calculating with the first option, "Use Required Longitudinal Reinforcement", you can see that the required reinforcement is sufficient for a large part of the area to ensure the shear capacity. For this reason, the required reinforcement is used instead of the basic reinforcement when determining VRd,c.
Here is a brief explanation of the options in RF-CONCRETE Surfaces 1.3 Longitudinal Reinforcement:
Option 1
The program checks whether the statically required longitudinal reinforcement is sufficient to avoid the shear reinforcement. If not, the shear reinforcement is required.
Option 2
If the statically required reinforcement is not sufficient, the higher value of the basic reinforcement and the statically required reinforcement are applied. If it is sufficient, the value is defined as statically required longitudinal reinforcement (because it is necessary to avoid shear reinforcement).
In cases where neither the statically required longitudinal reinforcement nor the basic reinforcement is sufficient to cover the shear reinforcement, only the required longitudinal reinforcement is applied for the calculation of VRd,c, and the additionally required shear reinforcement is calculated.
Option 3
Asl is increased for the calculation of VRd,c until either the longitudinal reinforcement is sufficient, or the maximum reinforcement ratio is reached.
The results display is designed to output only the statically required reinforcements. For this reason, only the statically required reinforcement is applied for VRd,c. It is necessary to use the maximum value of the statically required and the basic reinforcement to avoid the shear reinforcement.
In cases where shear reinforcement is mathematically required, the amount of provided longitudinal reinforcement in the formulas no longer has any influence. Therefore, it would not be useful to additionally consider the basic reinforcement in VRd,c.
The procedure details of the program can be well understood in the design details.
