[en] Modern piping system design generally includes an analytical determination of displacements, rotations, moments, and reaction forces at various postions along the piping system by means of a flexibility analysis. The analytical model is normally based on a strength-of-materials description of the piping system as an interconnected set of straight and curved beams, along with ''flexibility factors'' that are used to compensate for inaccuracies in the model behavior. This report gives an in-depth evaluation of the various analytical descriptions of the flexibility factors associated with piping system branch connection and nozzles. Recommendations are given for developing needed improvements. 59 refs., 29 figs., 26 tabs