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For a rigid joint its corresponding orientations of the beam-ends, orientations of the beam-ends connected to that joint, maintain their relative difference in the transient phase. For a straight joint, the two corresponding orientations of the beam-ends coincide with each other throughout the process. Therefore, by assigning orientations as in Fig. 3a, we can assign the pre-stress of an initially straight joint. The assignment of an initially angled joint is shown in Fig. 3b.

Fig. 3 (a) In the area enclosed by the dashed line, the orientations of the consecutive beam ends are equivalent. This builds up the pre-stress of moments of an initially straight joint; (b) In the area enclosed by the dashed line, the orientations of the two consecutive beam ends are the same as its own beam orientation. Hence no pre-stress of moments and torsion is built.