The net forces of acting on the body need to be zero. This is also applicable for the simply supported structures. This means that the support reaction is also counted in making the net force zero.

The resultant of a distributed load is a single vector that represents the net force applied by the distributed load. It is equal to the area of the load diagram and acts at the centroid of that area. 

To determine the resultant force, the magnitude of each differential force acting on infinitesimal areas must be summed and integrated over the load-bearing surface area. The magnitude of this resultant force is equal to the total volume under the distributed-loading diagram.