Local and Global Ductility of Wide-Beam RC Frames

Abstract eng:
Italian and Spanish seismic codes prescribe lower behaviour factor (q) for wide-beam reinforced concrete moment resisting frames (WBF) with respect to conventional q factors adopted for deep-beam frames (DBF). Conversely, other relevant seismic codes worldwide, such as Eurocode 8, consider WBF capable of high ductility performances, provided that design rules related to (i) stress transfer in connections, (ii) lateral stiffness and (iii) energy dissipation are complied. On the other hand, local ductility of wide beams (WB) appears to be systematically lower than that for deep beams (DB). A parametric comparative numerical analysis of deformations of DB and WB, following the current approach, shows that WB have larger ultimate chord rotation but lower ductility in terms of chord rotation than DB despite the similar ductility in curvature, which is mainly due to a lower plastic hinge lengths in WB. Aimed at verifying whether such disadvantage is overcome by modern codes or not, several archetype European RC residential buildings are designed alternatively with DB or WB. Seismic performances are assessed with different degree of detail. Results suggest that WBF provide at least similar global seismic capacities than DBF, especially in frames whose design is ruled by damage limitation limit state. Hence, any reduction of q in Mediterranean codes for WBF appears to be at least obsolete.

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