On the beam and shell modes of buckling of buried pipelines

This study is concerned with buckling failures of pipelines in seismically active areas. Such buckling is caused by compressive forces developed primarily due to large ground movements. By reviewing case studies of such failures two main modes of buckling were identified. They were given the names of 'beam' and 'shell' modes of buckling. Smaller diameter pipes buried in shallow trenches tend to behave more like beams. Under axial compression a section of the pipe tears through the foundation and lifts off the ground. Larger diameter pipes buried in deeper trenches behave more like shells. Axial compression causes shell buckling characterized by a number of circumferential and axial waves.;In the 'beam' mode of buckling the pipeline was modelled as a long, heavy beam resting on a rigid (or deformable) foundation. The large deflection inelastic deformation of the uplifting mechanism was studied numerically using the finite difference method. The uplifting response of the pipeline was found to be characterized by a limit load instability. The limit load was found to be very sensitive to the initial geometric imperfections present.;The 'shell' mode of buckling was analyzed by modelling the pipe as a long, elastic-plastic cylindrical shell radially supported by a nonlinear spring foundation. Axisymmetric initial geometric imperfections were introduced to the shell. The problem was formulated through the Principle of Virtual Work using Sanders shell kinematics. It was solved using a Ritz procedure and Newton's method. The analysis includes bifurcation buckling calculations from axisymmetric into non-axisymmetric configurations. The shell response calculated was found to be characterized by limit load and bifurcation buckling instabilities. Both were found to be highly imperfection sensitive. The presence of the supporting soil delayed the onset of buckling but had a small effect on the critical load calculated.;The critical conditions under which the beam and shell modes of buckling occur were compared for a number of practical examples. The possibility of the two modes interacting was also considered. This interaction can occur due to combined axial load and bending moment which develop during the uplifting process in the beam mode. The resultant combined stresses can lead to local shell buckling.