There are several different types of loading a rigid bolted joint can undergo. In many cases, considerations may need to be made for several different load types for the same joint, as few real world applications experience only one force type.
When the applied load of a joint is in line with the fastener axis, the joint is experiencing tensile loading. By assembling the joint and tightening a bolt in a tapped hole or nut, the tensile prestress elongates the fastener. In this state, the tensile force of the bolt is in equilibrium with the resultant compressive forces of the joint parts. This is where bolt preload comes into play. When the bolt is preloaded so that the tensile loading exceeds the applied tensile loading of the joint, the fastener, and therefore the parts, never experience any increase in tensile force.
If the actual applied tensile load on the joint exceeds the allowable tensile load on the fastener system, there will be a decrease in the compressive load on the parts and a reduction of the clamp load.