PSI - Issue 44

Maria Teresa De Risi et al. / Procedia Structural Integrity 44 (2023) 966–973 De Risi, Ricci, Verderame / Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction The response of beam-column joints without transverse reinforcement (“unreinforced”) can influence very significantly the vulnerability of existing reinforced concrete (RC) buildings subjected to seismic action, from local damage up to global structural collapse due to the loss of vertical-load-carrying-capacity. The response of these elements depends on different parameters (i.e., the geometry of the joint panel, the characteristics of the adjoining beam and column elements, the axial load level on column, the material properties), that can lead to different modes of failure and to a significant complexity of nonlinear response and capacity modeling. For these reasons, there are ongoing developments in literature and in codes regarding these issues, i.e., both nonlinear modeling approaches (e.g., Celik and Ellingwood, 2008, De Risi et al., 2017) and capacity models development (e.g., Park and Mosalam, 2012; Jeon et al., 2014), as well as continuous activities of experimental investigation (e.g., Pantelides et al., 2002; Masi et al., 2013; De Risi et al., 2016; Ricci et al., 2016, De Risi and Verderame, 2017). Of course, great attention is also addressed to the development and assessment of strengthening techniques for these elements, with different approaches, i.e., RC jacketing with traditional concrete (e.g. Karayannis et al., 2008) or steel fiber high-strength concrete (e.g. Tsonos, 2014) for increasing the joint resisting area, application of composite materials (e.g., Antonopoulos and Triantafillou, 2003; De Risi et al., 2020) or steel elements (e.g. Shafaei et al., 2014) for compensating the lack of transverse reinforcement. In this work, the application of the so-called CAM ® technology to the strengthening of unreinforced RC beam column joints is investigated. This technology is based on the use of externally-added prestressed steel strips, and was originally developed for the seismic strengthening of unreinforced masonry buildings, but successfully adopted also for existing RC buildings (Dolce et al., 2001a,b). A very similar strengthening technique for unreinforced RC beam column joints was presented in Liu and Yang (2020). This work represents the continuation of an experimental campaign carried out previously (Verderame et al., 2022), in which four full-scale beam-column joint subassemblages were tested, with or without CAM ® strengthening, highlighting the effectiveness of this technology in increasing the ductility of the response, also modifying the failure mode, preventing the collapse of the joint panel and leading the performance of an unreinforced beam-column joint strengthened with CAM ® very close to the response of a beam-column joint with code-compliant transverse reinforcement. The experimental study described in this work investigates the influence of a variation in some key parameters on the response of the subassemblages and the effectiveness of the strengthening. To this end, the same geometry of the specimens is still adopted, but (i) a lower concrete strength is adopted and (ii) the longitudinal reinforcement in beam is increased. Hence, the main results obtained from these experimental tests are described, along with the main results from the tests reported in (Verderame et al., 2022). To this end, the design procedure and the main characteristics of the specimens are illustrated; then, the experimental results both in terms of global (force displacement relationship for the subassemblages) and local (joint panel shear response, strain in prestressed steel strips) response are described, and, consequently, the effectiveness of the adopted strengthening technique is analyzed. 2. Description of the experimental campaigns 2.1. Previous experimental campaign (Verderame et al., 2022) In Verderame et al. (2022), four full-scale external RC beam-column joints were tested, with a geometry reproducing an interstorey height equal to 3.4 m (L c =3.4/2=1.7 m) and a bay length equal to 3.6 m (L b =3.6/2=1.8 m), with L c and L b intended to represent the shear span of column and beam elements, respectively. Beam’s and column’s sections were (300×500) mm 2 and (300×300) mm 2 , respectively. The reference specimen was designed to be representative of an existing three-storey building designed for moderate seismic loads according to past Italian technical codes (D.M. 1986; D.M. 1992). (3+3) 16 mm bars as longitudinal reinforcement in beam and column were adopted (Figure 1a). Two non-strengthened and two strengthened specimens were tested. The first, reference specimen (“NS”) was as-built (without strengthening) and without stirrups in the joint panel. The second specimen (“S”) was identical to specimen NS, but with code-compliant stirrups in the joint panel. Then, two specimens (identical to NS) strengthened with CAM ® technology were realized. The steel strips used for strengthening (with a resisting area equal to 19×0.9 mm 2 ) were arranged in three layers along the height of beam and were installed as closed prestressed strips