PSI - Issue 78

Maria Teresa De Risi et al. / Procedia Structural Integrity 78 (2026) 1151–1158 M.T. De Risi, C. Del Gaudio, G.M. Verderame / Structural Integrity Procedia 00 (2025) 000 – 000

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1. Introduction Reinforced Concrete (RC) buildings worldwide often are significantly vulnerable to seismic actions. Structures designed without or according to obsolete seismic codes very often experience shear failures in beams, columns, and beam-column joints during earthquakes. This issue is especially evident in moment-resisting-framed RC buildings realized in Italy before the 1970s (De Risi et al., 2022; 2023), when structural design was primarily guided by gravity loads actions only (GLD buildings). In Italy, before 1970, the municipalities classified as non-seismic prone area were about 6700, spanning various seismic hazard zones, and, currently, about 2/3 of the existing RC Italian buildings have been designed to sustain gravity loads only. Therefore, a significant portion of existing RC buildings often requires seismic improvement. Among all the possible retrofitting strategies, past works from the literature (Frascadore et al., 2015; De Risi et al., 2023) proved that solving shear failures can play a very important role in enhancing the seismic behavior of existing Italian RC structures. Among the existing strengthening methods, including RC or steel jacketing or FRP wrapping (see Fig. 1), this study evaluates the effectiveness of pre-stressed stainless steel strips (CAM® technology), which can be applied to beams, columns, or joints. The technology is based on the use of externally-added stainless steel strips, which are pre-closed and stressed, as a sort of active exterior stirrups (Dolce et al., 2001; Verderame et al., 2022); it enhances shear strength and displacement capacity by keeping constant the as-built lateral stiffness.

a) c) Fig.1. Main shear strengthening techniques: (a) steel jacketing, (b) FRP wrapping, (c) pre-stressed steel strips. b)

Such technique has been used in past research works for masonry structures (e.g., Dolce et al., 2001), but also for RC structures (see Helal et al., 2024; Ler et al., 2024), aiming at strengthening beam-column joints (Yang et al., 2019; Verderame et al., 2022; Chen et al. 2023), or shear critical columns/beams (e.g., Saatcioglu and Yalcin, 2003). In this work, experimental tests on shear-critical columns are presented, with and without the strengthening with pre-stressed steel strips. Tested specimens and experimental setup are first described. The main results are then shown in terms of global responses and observed damage evolution, comparing the as-built and retrofitted outcomes to quantify the effectiveness of the selected strengthening technique. Lastly, code prescriptions about the design of this technique are assessed based on the main experimental outcomes to provide suggestions for future optimization in the retrofitting design. 2. Description of the tested specimens Three identical shear-critical columns have been tested under seismic loading condition. They all are cantilever columns, with a 30x40 cm 2 rectangular cross-section, fixed within a rigid foundation block. Deformed bars with 24 mm and 6 mm of diameter have been used for longitudinal and transverse reinforcement (in the interested area at the base of the column), respectively, to force the occurrence of a shear failure in as-built condition, which are the main object of this study. 8 mm diameter stirrups are instead used in the upper portion of the column, close to the loading area (see Fig. 2).