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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1. Introduction Existing RC buildings are often vulnerable to seismic loading, thus requiring retrofitting strategies to mitigate such vulnerability. This is certainly true for existing RC buildings realized in Italy before ‘70s (De Risi et al., 2022a). More in detail, before 1976, only 16% of the Italian municipalities were characterized as seismic-prone areas. This means that most of existing buildings realized before ‘70 have been design totally neglecting seismic actions. Overall, in Italy, more than 2 billion of RC buildings were designed for gravity loads only (GLD), mainly characterized by a number of stories (Ns) ranging from 2 to 4 (Fig. 1). Such “low-standard” RC buildings often exhibited, after past earthquakes (e.g., L’Aquila, 2009; Centre Italy, 2016), shear failures in beams/columns and beam-column joints, which strongly require proper retrofitting strategies. Obviously, a given building, designed for gravity loads only, can exhibit different kind of failures and extension of damage depending on the (current) seismic hazard of the site where it is located. In this context, this work presents the as-built assessment of Italian pre-70 RC GLD 2-storey and 4-storey buildings. These case-study buildings are located in several sites in Italy, characterized by different seismic hazard, and they have been assessed, as commonly performed in design practice, in a code-based approach according to Italian seismic prescriptions, post-processing the occurrence of shear failures. For each building/site, the capacity has been evaluated at Damage Limitation (DL) and Severe Damage (SD) limit states (LSs). Their retrofit has been designed aiming at the resolution of the detected shear failures at SD LS. Resulting seismic performance are compared to seismic demand in all the considered sites to quantify the effectiveness of the adopted retrofitting strategy depending on the seismic hazard throughout all the Italian territory. (a) (b) Fig. 1. Number of existing RC buildings in Italy depending on their age of construction (a); percentage of 1-, 2-, 3-, ≥4-storey RC GLD buildings 2. Selected case-study buildings Two Moment Resisting Frame (MRF) RC buildings are investigated as case-study structures, characterized by 2 or 4 stories. Each building has four 2D resisting frames in the longitudinal (X) direction, and, as typical in pre-70 existing MRF buildings in Italy, only two (external) frames in the transverse (Z) direction. The case-study structures have the same floorplan (see Fig. 1), symmetric in both directions. The interstory height is 3 m. Both buildings are intended to be representative of pre-70 Italian buildings, thus reinforced with plain reinforced bars (Verderame et al., 2010). “Maximum allowable stress method” is adopted for their “simulated design”. Maximum allowable concrete strength was assumed equal to 5 MPa or 6 MPa, for purely compressive loads or bending actions, respectively (according to R.D. 1939). A maximum allowable strength equal to 140 MPa has been adopted for reinforcing bars in the hypothesis that steel typology “AQ42” was used, as typical in the reference time period in Italy (Verderame et al., 2010). GLD buildings have been designed to sustain gravity loads only and to comply code prescriptions about minimum amount of longitudinal and transverse reinforcement as defined by the Italian R.D. 1939 (De Risi et al., 2022a). Floors are 20 cm thick, with 4 cm RC slab. The floor weight is sustained by the longitudinal frames only. An infill weight (per panel unit surface) equal to 2.00 kN/m 2 and 2.36 kN/m 2 for longitudinal and transverse direction, respectively, is assumed for exterior frames. Stirrups with 250 mm maximum spacing and 8 mm diameter have been used in beams; stirrups with 140 mm spacing and 6 mm diameter have been used in columns. All the beams resulted 0 200 400 600 800 N° of buildings (x10 3 ) Age of construction GLD RC 0% 50% 100% % buildings Age of construction 1 2 3 ≥4