DUCTILITY-BASED DESIGN OF SECONDARY SEISMIC ELEMENTS IN REINFORCED CONCRETE STRUCTURES

The study of the effects of seismic actions on buildings is an open challenge for engineering and represents a constantly evolving and expanding field. The aim of this paper, which comes from the MS thesis work of the first author under the supervision of the second one, is the development of a ductility-based design approach for secondary seismic frame elements (beams and columns) in wall type reinforced concrete buildings subjected to seismic action. The main goal is to limit as much as possible the ductility checks, and satisfy them intrinsically through the control of the secondary members’ reinforcement and geometry. These are therefore allowed to plasticize in a controlled manner, avoiding also the occurrence of brittle collapse such as premature shear failures. The procedure is developed as much as possible in compliance with the provisions of Eurocode 2, which allows the plastic analysis of structures. After having briefly introduced, with reference to Eurocode 8, the seismic classification of secondary elements and the implications of this choice in the current literature, the typical plastic collapse mechanisms for a frame structure subjected to imposed deformations are analyzed. Subsequently, the concepts of curvature ductility and chord rotation ductility are investigated, and, basing on this, the procedure for the flexural sectional design is presented, with emphasis on the interrelations between ductility and strength. Finally, the design approach developed is applied to a Case Study, referring to a structure classified as a primary seismic walls system with, in addition, a secondary seismic frame part, and is validated by carrying out non-linear dynamic analyses with a three-dimensional model of the structure. The research numerical code adopted implements the well proven “RCIZ” Finite Element model for RC members which combines shear force-bending moment-axial force coupling. The results related to the global and local behavior of the building allow to draw interesting conclusions on the new design proposal, highlighting its positive aspects and limitations.