For pressure pipes, the pipe load determines the main stress situation in the cross section, with tensile stress being dominant. When a pipe is subjected to internal pressure, its diameter increases. The Poisson effect describes the phenomenon where, as a material expands in one direction (circumferentially, due to increased diameter), it contracts in the perpendicular direction (axially).
Consequently, under internal pressure, a pipe not only increases in diameter but also shortens in length. This axial shortening due to the Poisson effect necessitates the use of tensile resistant end-joints for long pipes installed in one continuous piece. Methods like butt welding provide this required tensile resistance, unlike standard rubber ring seal joints which might allow for some movement.