On the benefits of using in the joints of R/C frames subjected to seismic loading

The extensive research activity carried out in the last few decades on fibre-reinforced concrete is showing - beyond any doubt - that FRC has very interesting properties for structural applications. The dispersion of short fibres - made of steel, polymers, carbon, etc. - in the concrete mass brings in a 'crack bridging' effect, which prevents or delays cracking, and – at the same time – provides concrete with a ductile behaviour both in tension and in compression, given that relatively large amounts of stiff fibres are used (as for example in high-performance fibre- reinforced concretes - HPFRC). As a result, the use of such concretes significantly improves significantly the structural performance of R/C members, not only under static and fatigue loading, but also under dynamic and seismic loading. However, considering their higher costs and more complex technology, high-performance fibre-reinforced concretes are only suitable for critical areas of R/C beams and columns, as well as in the beam-column joints of R/C frames, where the actual Italian code (2008) may require an excessive amount of reinforcement, even in the design of low-ductility members. Within this context, this paper investigates the benefits of using high-performance fibre-reinforced concretes in the nodal and inelastic regions of R/C seismic-resistant plane frames are investigated in this paper, focusing on typical frames which are commonly found in residential buildings, with two-four bays and two-eight storeys. Ordinary mixes (C25/30 and C40/50) are adopted for frame members, while higher-performance fibre-reinforced mixes (FRC40/50 and FRC80/85) are used in critical areas and in the joints. The joint regions are modelled with or without rigid end-sections and in-plane stiffness of the floors is introduced as well. The frames are designed according to the actual Italian code (NTC08) and to EC8. In the nonlinear static analyses, considering either triangular or uniform load profiles, a diffused-plasticity model is adopted for frame members. In terms of global capacity and ductility, the use of HPFRC - instead of plain concrete – offers considerable benefits.