INELASTIC BUCKLING OF SOIL-STEEL STRUCTURES (DISCUSSION AND CLOSURE)

The buckling of composite soil-steel structures is examined, taking into consideration the formation of plastic hinges in the conduit walls. A structural model is applied in which the soil is replaced by discrete normal and tangential springs acting at the nodal points of a closed polygon of beam elements representing the conduit. The coefficient of the soil springs is taken to be dependent on the type of soil as well as on the direction of displacement and depth of soil at the surface of contact with the conduit. A nonlinear matrix analysis is applied to examine the stability problems in the conduit. Numerical examples show two distinctive modes of failure. A snap-through failure is observed in conduits with relatively large spans or shallow cover or both, whereas short-span conduits with deep cover exhibit no sudden buckling but instead displacements with a higher rate of increase after each loading step. The analyses show reasonable agreement with the results of the failure load obtained from the Ontario Highway Bridge Design Code for the case of cylindrical conduits. However, it is found that the code overestimates the failure load for the horizontal ellipse, whereas it underestimates that for the vertical ellipse.