A simplified reinforcing steel model suitable for cyclic loading including ultra-low-cycle fatigue effects

This paper presents a new constitutive model for the simulation of reinforcing steel bars used in common reinforced concrete structures and it is designed to be used for general loading cases. The model includes the well-known Guiffrè–Menegotto–Pinto softened branch, although new expressions are proposed for the evolutions of the curvature-related parameter and of the yield surface. The constitutive relation is enhanced with an innovative and simplified proposal for considering ultra-low-cycle fatigue effects. This phenomenon is particularly important for structures that undergo a small number of very large displacement cycles, e.g. when subjected to intense seismic events. It is known that in those situations the steel reinforcements experience a continuum and significant strength decrease that ultimately leads to premature failure induced by fatigue. del’s mathematical description and some relevant implementation issues are described. Its accuracy is assessed by means of a series of validation tests using experimental data available in the bibliography.