Finite-Element Modeling of Failure of PCCP with Broken Wires Subjected to Combined Loads

Abstract

Nonlinear finite-element analysis is used to predict the performance of prestressed concrete pressure pipe with broken wires in a pipeline. A nonlinear finite-element model of the pipe is developed and subjected to combined effects of internal pressure, pipe and fluid weights and earth load as it loses prestress due to gradually increasing number of broken wires. The model incorporates a nonlinear stress-strain relationship for concrete that includes nonlinearities due to compressive crushing, tensile softening and cracking. The pipe model consists of shell elements with many integration points through the pipe wall to allow capturing of its non-linear behavior in pipe wall bending as the concrete cracks or crushes. The model first subjects the pipe to the combined effects of prestress pressure, pipe and fluid weights, earth load, and internal pressure, and then the prestressing is removed gradually. Analysis results show crack patterns, crack width, crack depth, pipe deflection, change in the stress in the wires, and concrete crushing as a function of prestress loss length. The results of analysis for a large diameter non-cylinder pipe with low internal pressure a fixed internal pressure and increasing prestress loss show that the final failure mode of the pipe is in fact in form of progression of wire breaks due to the increase in the stresses in the wires, rather than concrete crushing or major leakage. The results of analysis for a smaller diameter PCCP subjected to a prestress loss and increasing internal pressure show that interlocking strength of cracked outer core and steel cylinder ultimate strength, rather than progression of wire breakage, governs the strength of pipe. For pipe with large number of wire breaks, structural cracking can occur exposing the steel cylinder to corrosive soil environments. Corrosion of the steel cylinder can result in leakage and premature failure of the steel cylinder. The results of the nonlinear finite element analyses are used to validate engineering criteria and procedures used for risk analysis and repair priorities determination of pipes with broken wires and for comparison with confirmation testing of the pipe with broken wires.