Bridge and viaduct structures supported on extended pile shafts are widely used for their economic and technical advantages. Meanwhile, the seismic response of structures supported on a monopile foundation is complex since the soil may experience large deformations, and hence exhibit strong nonlinear behaviour. Consequently, the pile-soil system would exhibit strong nonlinearity due to yielding of pile material and gapping at the pile-soil interface in addition to cyclic hardening/degradation of soil. This paper aims to evaluate the impact of the ground motion duration on the seismic performance of extended pile-shaft-supported bridges. A beam on nonlinear Winkler foundation model that is able to capture soil nonlinearities such as yielding, gapping, soil cave-in and cyclic hardening/degradation effects is used in the analysis. Incremental dynamic analyses are performed considering 4 real ground motion earthquake characterized by different durations to evaluate the effects of ground motion duration and soil nonlinearity on the performance of extended pile shafts. Various homogeneous soil profiles including saturated clay and sand in either dry or saturated state are considered. The results are presented in terms of incremental structural pseudo-acceleration curves and bending moments, and show that the ground motion duration may strongly affect the performance of bridges founded on soft degrading soils by increasing stresses within the extended pile-shaft because of a deeper point of virtual fixity than one obtained in non-degrading soils.