ABSTRACT
When the bolts have been tightened, the joint is said to be "preloaded." The tensile forces in the bolts oppose the equal and opposite compressive forces in the joint members. These forces may be reduced a bit by various relaxation mechanisms-embedment, metal creep, gasket creep, and/or differential thermal expansion (which can either increase or decrease the forces established during assembly)-but in general they will still be acting when the joint is put in service. If tensile service loads are then placed on the joint, they can either increase or decrease the tensile and compressive forces in bolts and joint members in ways that are not at all obvious. By tensile loads I mean loads that act more or less parallel to the axes of the bolts. Shear loads, acting at right angles to the axes of the bolts, will generally not change the preloads in bolts and joint members.
