Nonlinear pile-head macro-element for the seismic analysis of structures on flexible piles

Performance-based design (PBD) procedures require accurate estimates of both maximum and residual displacements in structural systems. Macro-element models are already proven tools for designing structures on shallow foundations according to PBD, since they represent a very cost-effective solution in terms of balance between physical behaviour, simulation accuracy and computational cost. This work extends the macro-element approach to the analysis of laterally loaded pile-shafts and soil-pile-structure interaction. The lateral response of the entire soil-pile system to seismic actions is thus condensed at the pile-head, being represented by a zero-length element located at the base of the columns and subjected to the foundation input motion. The macro-element model is presented, based on the three fundamental features of the response of laterally loaded piles: initial elastic behaviour, gap opening/closure effects and failure conditions. These three characteristic behaviours are all made compatible by using an inelastic model which accounts for the evolution from initial nonlinear elastic behaviour to full plastic flow at failure. Such inelastic model is based on a bounding surface plasticity theory formulation that ensures a smooth transition from the initial elastic pile-head response up to nonlinear behaviour and plastic mechanism formation. In order to validate the macro-element, its response is favourably compared with numerical results from advanced simulations of pile lateral behaviour and with load tests on real piles.