PSI - Issue 44

Maria Teresa De Risi et al. / Procedia Structural Integrity 44 (2023) 958–965 De Risi, Del Gaudio, Scala, Verderame/ Structural Integrity Procedia 00 (2022) 000–000

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the shear-sensitive elements which exhibit a shear failure during the pushover analyses before the achievement of the 1 st JF “C” or the 1 st DF, whichever occurs first (see Table 1). Therefore, since no beams or columns SF occurs before such target points for both case-studies, only joint strengthening is designed and implemented. Anyway, it is worth noting that such kind of design could lead to significant overstrength, especially when the 1 st DF rules the design target, in those low-medium hazard sites where the 1 st DF occurs after the achievement of the displacement demand. This conservativeness is herein acknowledged and accepted to pursue a strengthening design that results independent on the building location (i.e., independent on the hazard). “Tensile” SFs in joints are here solved by means of a proper number of pre-stressed steel strips (CAM® technology), very effective in preventing brittle failure of BCJs even in case of high ductility demand at the elements-joint panel interfaces (Verderame et al., 2022; De Risi et al., 2022b). The number of steel strips required for BCJs has been evaluated with a “post-cracking” approach, namely considering the steel strips as an added transverse reinforcement resisting to the difference between the joint shear demand and the joint shear carried by the diagonal concrete strut in compression after diagonal cracking. This procedure is consistent with codes prescriptions for the design of stirrups in joints (NTC 2018; CEN, 2005a) and dealt with in detail in Verderame et al. (2022). Joint shear demand used for design is assessed depending on the maximum tensile demand in converging beams (depending on the beam-column strength hierarchy), eventually limited to the compressive strength of that joint. Nominal yield strength assumed for the reinforcing strips is 400 MPa; their transverse section is 0.9×19 mm 2 . Fig 5b shows the results of this design in terms of number of steel strips - to be distributed among 2 or 3 layers along the joint height - required to make “ductile” all joint typologies (note that joints #1,2,3,7,8,9 in Fig.5b represent themselves and their symmetric joints in the building plan – see Fig. 2). (a) (b) Fig. 5. a) Possible target points of “local strengthening” interventions; b) CAM intervention against shear failures of BCJs. To check the strengthening effectiveness, SI values have been re-assessed after the implementation of local interventions, based on the same pushover curves, as explained above, for all the considered sites in Italy. First, SI DL values shown in Fig 4c,d do not change at all due to the intervention typology that does not modify the building stiffness nor the infill in-plane displacement capacity. Resulting SI SD values and their increment with respect to the “as-built” condition (ΔSI SD ) are shown in Fig. 6. Significant increments in SI SD can be observed, especially in low-medium hazard sites and especially for 2-storey buildings, where there isn’t any limitation due to JFs “C”, as observed above. Post-retrofit SI SD has a minimum value of 0.61 and 0.17 for 2-storey and 4-storey building, respectively; median value of ΔSI SD is 2.98 and 0.51 for 2-storey and 4-storey, respectively. It is worth noting that the higher the PGA D,SD the lower the increment of ΔSI SD that can be reached with the described intervention, as shown in the next section. 5. Cost of the strengthening intervention A preliminary analysis of the cost of the intervention described above has been performed, based on the Price List of the Italian Genio Civile DEI 2021 (II semester). Considered activities for strengthening are: installation and final removal of scaffoldings, installation of CAM strengthening as designed in section 4, removal and replacement of plaster and finishing in the intervention areas, landfill transportation and disposal, and final painting. Note that any removal and replacement of portions of infill around beams and columns are necessary to apply CAM on joints.