What is the function of interplane strut of a Biplane?
The wing strut prevents the lower wing from bending upwards due to the lift forces.



The biplane is a truss structure: rectangular shapes are "crossed out" by diagonal structural members, in order to create triangular shapes with only tension and compression in their structural members, no bending. If the diagonal is a rod, it can transfer both compression and tension forces and only one diagonal is required. If it is a cable, it can only transfer tension forces, and the other diagonal needs to be crossed out by a cable as well.

In flight, lift wants to bend the wings upward and lengthen the diagonal, which becomes tension loaded. The upper wing is then prevented from bending upwards and becomes compression loaded.

The interplane strut connects the lower wing with the upper wing, and prevents the lower wing from bending upwards. It is compression loaded: the diagonal exerts a downward force, lower wing lift exerts an upward force.

The method of how to compute the force is in this answer. In a truss structure the intersection points are modelled as hinges, and the structure members are dimensioned to carry the resulting tension/compression loads.