PSI - Issue 44

Marco Gaetani d’Aragona et al. / Procedia Structural Integrity 44 (2023) 1052–1059 M. Gaetani d’Aragona, M. Polese, A. Prota / Structural Integrity Procedia 00 (2022) 000–000

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longitudinal reinforcement bars (e.g., sufficient embedment length, anchorage of the beam reinforcement in the core). According to Calvi et al. (2022), to account for the contribution of the joint panel in the sub-assembly composed by columns, beams, and beam-column joints converging in the joint panel it is possible to introduce the concept of equivalent column moment (ECM) capacity. The ECM represent the equivalent moment acting to the column corresponding to the development of specified failure in members composing the sub-assembly, and it is obtained via equilibrium considerations depending on the sub-assembly geometry. For RC members reinforced with smooth rebars, the behavior can be particularly sensitive to degradation and bar slippage due to cyclic load reversals. For this reason, bar-slip effect is considered in both columns and beam-column joints (2.3). The contribution to lateral strength of infill panels encased in exterior frames is properly simulated (2.4) considering constitutive models proposed in literature. Masonry infills with different consistency, namely weak (W), medium (M), and strong (S) infills made of vertically hollowed brick units (Hak et al., 2012) are considered. The presence of openings in the infill panel to accommodate windows or doors alters the behavior mechanism of the masonry panel in terms of stiffness, strength, and energy dissipation (Gaetani d’Aragona et al., 2017b), depending on the size and the position of the openings. Thus, the infill envelope is conveniently modified to account for the presence of openings. Once the envelope curves at the member level (structural and non-structural) are characterized, the interstory backbones can be generated in both main building directions under the assumption of shear-type behavior. However, due to the presence of masonry infills encased in the perimeter frames, premature triggering of brittle failures at the top of the column or in the beam-column joint panel can occur due to local interaction phenomena (4.1). To derive the interstory backbones, the interaction between different RC members at the sub-assembly level is studied and suitably composed at the story level (4.2). It is considered that a rate of the forces acting in the panel, which amount depends on the geometrical and mechanical characteristics of the infill-frame system, is transferred to the frame. These additional shear forces acting on columns or beam-column joints of perimeter frames accommodating infill panels are considered during the analysis since these can precipitate the occurrence of brittle failures.

(a) Equivalent beam-column joint envelope

(d) Top sub-assembly envelope

M c,top

ECM

x H c /2

q

q

Top section

+

(e) Bottom sub-assembly envelope

(b) Column envelope

M c

M c,bot

x H c /2

q

q

(f) Column shear-displacement envelope V

(c) Bar-slip envelope

Bottom section

M c,top/bottom

q

D

Fig. 2. RC member behavior considering different sub-assembly failure modes for the top and the bottom of the column: (a) beam-column joint, (b) column and (c) bar-slip failure; and (d) obtained top and (e) bottom sub-assembly envelopes obtained composing the behavior as in-series. Column (f) shear-displacement behavior obtained composing top (d) and bottom (e) sub-assembly envelopes. From [xxx]. Finally, to account for typical collapse modes of existing RC frames, two different definitions of story-level collapse, namely side-sway (SSC) and gravity load collapse (GLC), are introduced (4.3). The first occurs when all columns and infill panels in a single story have reached their residual lateral capacity at the same time. The GLC occurs when the vertical load demand in a single story (the sum of gravity loads transferred from upper stories) exceeds the total vertical load capacity of the columns.