A “preloaded” joint is one where the bolt(s) have been tensioned beyond what it takes to just barely make the two sides touch. When a preloaded joint is subsequently subjected to an external “separating” load, part of that load goes into trying to pull those sides apart, and part of it goes into the bolt itself. The ratio of those two load paths depends on the stiffness ratio between the bolt (on the one hand) and the two sides (on the other hand). Conceptually, the bolt and the two sides operate as if they’re two springs in parallel1.
When/as/if the two sides do loose contact during service, all the subsequent load goes into the bolt. In Primary Structural applications, such separation is itself almost always considered a failure of the joint, because things can get ugly shortly thereafter. Preloading is (therefore) an important technique for establishing certainty of knowledge about the maximum load going into the bolt during service2.
Given joint “relaxation” and temperatures, it is possible that the maximum combined load on the bolt occurs at the time it is tightened for service. Given the uncertainties about temperatures, friction and the effect of long-term compression, it is also possible that the maximum combined load case is when the bolt is removed from the joint.
All of the preceding is summarized from NSTS 08307A “Criteria for Preloaded Bolts”3. It should be noted that the reference requires a very detailed analysis of the life cycle of loads, such as noted in the preceding paragraph, to properly determine the limit load case for every bolt in every joint in the entire spacecraft. Because the loads are, in large part, dominated by the selection of material and configuration, the reference effectively requires an iterative process between derivation of load requirements at the piece-part level and the design with respect to those loads.
Footnotes