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

• A simplified constitutive model for concrete reinforcing steel bars is presented.
• The main focus is drawn to the simulation of the low-cycle fatigue effect.
• The fatigue-related strength degradation and premature rupture can be simulated.
• The model’s mathematical description and its FEM implementation are described.
• The results obtained are compared with experimental data revealing a good accuracy.

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.The model’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.