Elastic Interactions between Bolts Explained for Single Pass Flange Bolting using DuraSquirt(R) DTIs. https://www.appliedbolting.com/index-flange.php In the early 1990s, BP America and The Pressure Vessel Research Council (PVRC) sponsored the University of Akron Ohio, to develop a way to improve flange bolt-up procedures, using experimentally determined elastic interaction coefficients . The study resulted in a way to fully torque a flange, in a single tightening pass, while achieving uniform bolt stresses around the flange. The groundbreaking procedure was made possible, by determining and compensating for elastic interaction between all bolts, within a flange. Elastic interaction is the phenomenon that occurs between a group of bolts as tightening initializes and progresses throughout the group. As each bolt is tightened, other bolts in the group may tighten or loosen, due to dynamically changing forces within the flange assembly.

https://invidious.fbxl.net/watch?v=yMrieQoErcQ

It turns into an ad at the end, but the fundamental stuff they talk about beforehand is a really interesting and easy to understand summary of a fundamental issue with tightening bolts on a flange.