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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Nevertheless, lower strain values are observed – and quite similar (at peak) among different specimens, highlighting a conservativeness in design, probably due to an underestimation of the joint shear carried by the concrete strut.

strain, [-]

strain, [-]

strain, [-]

(a) (c) Fig. 5. Strain in the prestressed steel strips of specimens CAM1 (a), CAM3 (b) and CAM4 (c) corresponding to first sub-cycles at maximum negative and positive imposed drift, in bottom (N1), central (N2) and top (N3) layers. These considerations can apply to specimens CAM1 and CAM4, for which the effectiveness of the strengthening was observed. For specimen CAM3 (Figure 6b), the strain demand remains well below ε 0.2 , despite the severe damage – up to collapse – of the joint panel. A possible explanation to this failure can be found looking at the stress state in specimens with f c = 16.3 MPa, more specifically at the principal compressive stress, in the most unfavorable condition (maximum joint shear demand and maximum axial load), see Figure 6. In this case, the overcoming of the ηf c limit value (NTC 2018; CEN, 2005a) is not expected for the specimen with “low” beam longitudinal reinforcement (CAM4) but is expected for the specimens with “high” beam longitudinal reinforcement (2NS and CAM3), thus explaining the joint failure in compression, despite the presence of the strengthening. Note that, on the contrary, the fact that the principal tensile stress exceeds the concrete tensile strength only leads to an expected cracking condition, which is observed indeed for all specimens. (b)

shear stress, [MPa]

shear stress, [MPa]

(a)

(b)

Fig. 6. Mohr’s circle for concrete in the joint panel for the non-strengthened (black) and strengthened (blue) specimens, with f c = 16.3 MPa and with “low” (CAM4) (a) or “high” (2NS and CAM3) (b) beam longitudinal reinforcement. 4. Conclusions This study describes an experimental campaign, performed in continuation with a previous one, on unreinforced RC beam-column joints strengthened with external prestressed steel strips, through the CAM ® technology. Different parameters are investigated, i.e., the amount of longitudinal reinforcement and the concrete compressive strength. The as-built, non-strengthened specimens show a behavior controlled by joint failure, without (J) or after (BJ) beam flexural yielding. The presence of the strengthening is able to increase the joint shear strength, leading to a ductile, flexure-controlled response of the subassemblages, as for code-compliant beam-column joints, unless a joint failure in compression occurs. The analysis of the local response allows the analysis of the strain demand in the prestressed strengthening strips, highlighting the need for further investigation and possible improvement of the design approach currently adopted for dimensioning the strengthening intervention.