PSI - Issue 44

Greta Agata Venneri et al. / Procedia Structural Integrity 44 (2023) 291–298 Greta Agata Venneri et al. / Structural Integrity Procedia 00 (2022) 000–000

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achieved. Indeed, it is a common engineering practice to assume beam-to-column joints and column bases as ideally pinned or fully rigid. In Italy, this assumption has been acquiesced by national technical codes, which have been conceived to be more focused on the behavior of simple connections, rather than on the performance of the joints in terms of global behavior under bending forces. However, the assumption of having rigid joints is simply an idealization of the actual behavior: in reality, most joints, although originally designed as continuous -rigid and full strength- joints, exhibit some degree of flexibility that, if ignored, can lead to significant errors in the evaluation of the structural response (Sivakumaran 1988; Awkar and Lui 1999). Hence, the prediction of the real mechanical behavior of the joints is a factor that cannot be neglected, either at the design stage or at the safety assessment phase. A reliable procedure that provides the real behavior in terms of moment-rotation curve is the Component Method, adopted by the Eurocode 3 - Part 1-8 (CEN 2005), according to which the joint is schematized as an assemblage of “components”. Each component is characterized by its own strength and stiffness, working in tension, compression or shear (Jaspart 2000; D’Alessandro et al. 2018). The moment-rotation relationship given by the method allows joints to be classified as “hinged”, “full-strength” or “partial-strength”, based on the “m” ratio between M j,Rd and the resistance of the weakest connected element -usually the beam in beam-to-column joints- and the classification as “hinged”, “semi-rigid” or “rigid” based on the initial elastic flexural stiffness S j,ini . Several researchers investigated the seismic behavior of multi-storey frames with semi-rigid joints (Nader and Astaneh 1991; Lui and Lopes 1997; Awkar and Lui 1999). Analytical studies showed that the flexibility of the connections could reduce the frame stiffness and thus to increase the vibration period; as such, lower inertial loads are attracted, this potentially leading to a favorable response at the ultimate limit state. On the other hand, the lower stiffness of the frame entails a lower safety with respect to the serviceability limit state, because of the higher story drift provoked by the joins deformability and the lower restrain on beams and columns. Based on this premise, the scope of this paper is to evaluate the influence of partial-strength/semi-rigid joints, which were originally and erroneously designed to have rigid and infinitely resistant beam-to-column joints and column bases, on the seismic performance of multi-storey moment-resisting steel frames. To this purpose, a numerical study on two steel buildings placed in Toscana (Italy) is presented. Finite element models have been developed, by modelling the joints according to both the theoretical and the actual behavior in terms of strength and stiffness. To assess and to compare the seismic vulnerability of the buildings, fragility curves for different limit states have been generated by performing nonlinear time history analyses using a suite of 125 natural ground motions, so to account for the record-to-record variability as source of uncertainty. 2. The considered case studies The examined multi-story steel buildings, which henceforth will be indicated as “Case study 1” and “Case study 2”, are two hospitals placed in Toscana (Italy). The original design documents of these structures were provided to the authors of the paper by AIRES INGEGNERIA S.r.l., based in Caserta (Italy). Both buildings were designed in the late nineties in compliance with the Italian Building Code D.M. 16/01/96, considering both gravitational and lateral loads. For these buildings, the structural system was originally designed as a three-dimensional moment-resisting frame, with all structural joints assumed to be rigid and full-strength. Figs. 1(a) and 2(a) show the 3D model of the frames, respectively for the “Case study 1” and the “Case study 2”. At the same manner, Figs. 1(b) and 2(b) show the beam-to-column joints, whereas Fig. 1(c) and 2(c) show the column bases. Case study 1” is a three-storey building with a rectangular floor plan and lateral balconies. It consists of three bays at 6.4 m in the North-South direction and in the East-West direction. The interstorey height is of 3.8 m. The columns are characterized by square hollow 260x260x10 mm cross-section. The decks consist of composite rc-steel slabs of height H = 9.5+4 = 13.5 cm. The primary girders in both directions are made of IPE 450 cross-section for the first floor and IPE 400 for the second and third floor; the secondary beams have IPE 300 and IPE 360 cross-sections. Members and plates are made of steel with Fe 37 B (nominal yield stress f y = 235 MPa; nominal strength f u = 360 MPa) and Fe 44 B (nominal yield stress f y = 275 MPa; nominal strength f u = 430 MPa, Young’s modulus E = 210000 N/mm 2 ) grades. All beam-to-column joints are unstiffened extended end plates. Column bases are made of a base plate bolted to the concrete foundation and welded to the column, with vertical ribs in each edge. 8.8-M16 bolts were used for the beam-to-column joints; 8.8-M20 anchor bolts were used for the column bases. Foundations consist of RC plinths connected by RC beams.