What happens when the thrust line coincides with the bottom Vern axis of beam? (Prestressed concrete)

Basically, shear force is the combination of flexural compression which result as C and tends to form an inclined thrust line known as arch. For example consider a simple strut and tie model. Here it may be technically expected that the shear force is supported by the longitudinal resultant.  

NαVcotθNa=αVcotθ

Shear force is the product of shear stress and area. Thrust line can also affect the stability. 

As we know that the shear force is always combined with flexural compression which as resultant C which leads to form an inclined thrust line which is arch.

NαVcotθNa=αVcot⁡θ

Here a simple strut and tie model is consider, which may be practically expected that the longitudinal resultant Na supports the shear force αV, and Za temporarily denotes the whose acting point  

Thank You.

Hi, Shear force is combined with flexural compression which as resultant C,  leading to form an inclined thrust line which is arch. For the present approach to a simple strut and tie model, it may be technically expected that the shear force αV is supported by the longitudinal resultant Na, and the whose acting point is temporarily denoted by Za. That is, 

NαVcotθNa=αVcot⁡θ

Shear force is a product of shear stress and area.

Refer above the figure for understanding.