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

968

3

around the unreinforced joint panel, like “externally added” stirrups (Figure 1b). The number of strips needed as reinforcement was evaluated with a “post-cracking” approach, i.e., 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, consistent with the procedure suggested by codes (NTC 2018; CEN, 2005a). Further details are provided in (Verderame et al., 2022). As a result, (5+5+5) steel strips were adopted. The first strengthened specimen (“CAM1”) was realized with a “standard” strengthening layout. The second strengthened specimen (“CAM2”) had a different, less invasive layout characterized by steel strips passing through a diagonal hole crossing the joint core, aimed at avoiding internal disturbance in the building (it will be disregarded in this work for the sake of brevit). Concrete compressive strength (f c ) was equal to 32.2 MPa, the yield (f y ) and ultimate (f t ) strength of reinforcing steel was equal to 503 MPa and 627 MPa for 16 mm diameter bars used as longitudinal reinforcement and 460 MPa and 541 MPa for 8 mm diameter bars used as transverse reinforcement, respectively, and, finally, steel strips used for strengthening had a 0.2% proof stress (CEN, 2004) equal to f 0.2 = 409 MPa. Note that the tensile stress applied to the steel strips in the prestressing installation phase was about equal to f p = 120 MPa. Tests were carried out in displacement control, with a hydraulic actuator imposing a cyclic displacement path at beam’s end corresponding to the drift protocol reported in Figure 1c, with the drift calculated as the ratio between the imposed displacement and L b . A constant axial load was applied to the column, equal to 290 kN, corresponding to an axial load ratio ν=0.10. The response of specimen NS was controlled by joint panel failure following beam yielding (BJ-failure), while specimen S showed a ductile response, controlled by beam flexural failure (B-failure). Specimen CAM1 showed a ductile response (B-failure), very similar – and even slightly better, thanks to the beneficial active confinement due to the prestressing of steel strips – to specimen S.

(a) (c) Fig. 1. (a) Geometry and reinforcement details; (b) 3D view of the strengthening specimens; (c) applied drift pattern. (b)

2.2. New experimental campaign Based on the response of specimens observed in the previous experimental campaign, it was decided to investigate, with further experimental tests, the influence of a higher amount of longitudinal reinforcement in beam, expected to lead – through the increase in joint shear demand – to a joint panel failure prior to beam yielding (J-failure) in the unreinforced non-strengthened specimen, and the influence of a lower value of the concrete compressive strength, that can be more representative of existing RC buildings and can lead, under the same axial load, to a higher axial load ratio, potentially modifying the type of joint panel failure. Then, a reference specimen (“2NS”) was realized, as-built (without strengthening) and without stirrups in the joint panel, with (5+5) 16 mm bars as longitudinal reinforcement in beam and (4+4) 16 mm in column. Two specimens strengthened with CAM ® technology were realized. The layout of the strengthening was identical to specimen CAM1 described above, and the same design procedure for the strengthening was followed. The first strengthened specimen (“CAM3”) was identical to 2NS; for this specimen,