Ductility demand on steel reinforcing bars in concrete buildings

According to actual Italian and European standards for constructions, reinforced concrete buildings in seismic areas should be designed following the capacity design approach, in order to dissipate seismic energy through the development of plasticisations located in particular areas, providing global collapse mechanisms and avoiding premature failures of non-ductile elements (for example mechanisms of soft storeys). The areas in which plastic hinges are located, generally coinciding with the ends of beam elements, are known as dissipative zones. In common practice, the seismic design of new buildings is developed through the elaboration of simplified linear models that, in spite of their simplicity, do not take into account all the problems related to the non-linear response of materials and consequently are not really calibrated on the base of the effective ductility demand imposed by earthquakes. For this reason, in a European research project funded by the Research Fund for Coal and Steel (RFCS) named Rusteel "Effects of Corrosion on Low-Cycle Fatigue (Seismic) Behaviour of High Strength Steel Reinforcing Bars", a detailed investigation of the effective behaviour of reinforced concrete buildings under seismic action was executed through the consideration of accurate non-linear models and the execution of non-linear analyses. The main aim of the research project consists in the correlation between the ductility demand imposed by earthquakes and the ductility capacity of dissipative elements such as beams, whose hysteretic behaviour is strictly related to the ability of steel reinforcing bars (rebars) of sustaining a relative large number of high plastic deformations without showing evident losses of ductility and strength (low-cycle fatigue (LCF) condition). The evaluation of the ductility capacity of steel reinforcements shall consider the consequences, both in terms of strength and ductility, of corrosion phenomena resulting from aggressive environmental conditions. A detailed experimental test programme, including monotonic and LCF tests on uncorroded and corroded rebars, was executed in the framework of Rusteel and preliminary results are showed in thispaper. As regards the investigation of the ductility demand, different case studies were designed following the prescriptions imposed by actual standards and using dynamic modal analysis; refined non linear models implemented in OpenSees were then developed including the effects of the interaction between steel and concrete (bond-slip phenomena), through the development of a simplified constitutive stress-slip law. In this paper, preliminary results about both the mechanical capacity of corroded and uncorroded steel rebars under monotonic and cyclic loads and the ductility demand under seismic action are presented.