PSI - Issue 64

Alessandro Prota et al. / Procedia Structural Integrity 64 (2024) 1041–1048 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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insulated, waterproofed, and furnished with a lightly reinforced screed. For the second floor, horizontal bracing system were adopted to ensure the load transfer to the lateral load resisting system. In the initial design report, steel components were fabricated using Fe360 steel grade, equivalent to S235 according to UNI EN 10025 standards (CEN, 2019). It's essential to note that the average material properties for steel, commonly applied in evaluating existing structures, may deviate considerably from the nominal values. To address this variability, a mean yield strength of 310 MPa is justifiably utilized according to Da Silva et al. (2009). Adopting a knowledge level of "LC2" and considering uncertainties, a confidence factor of 1.20 is applied to adjust material properties (CEN, 2005; CS.LL.PP., 2018; CS.LL.PP., 2019). 2.3. Non-linear static analyses Utilizing SAP2000 software, a three-dimensional finite element model (FEM) was created for static non-linear analyses, as detailed by Computers and Structures Inc. (2022). The structural elements, comprising beams, columns, and diagonal elements, were represented using frame elements. The model accounts for the self-weight of beams, columns, slabs, and partitions as the dead load, denoted as "G." To replicate the in-plane behavior of the slab, a rigid diaphragm is employed. Indeed, the composite slab with reinforced concrete at the first level provides this rigidity, while additional measures (such as the introduction of horizontal bracing system) were taken at roof levels to achieve similar behavior . In this analysis, live loads denoted as "Q" with a magnitude of 3 kN/m 2 were applied to each story, adhering to the guidelines outlined in CS.LL.PP. (2018) for structures falling under Consequence Class (CC) 3. To capture the non - linear response of various steel components, a concentrated plasticity model was implemented. Zero-length plastic hinges were introduced at the ends of bending elements and in the middle of bracing elements, calibrated in accordance with the American Society of Civil Engineers (ASCE, 2017) standards (see Fig. 4).

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Fig. 4. Plastic hinges: (a) bending element; (b) bracing elements

Eight pushovers were conducted using two different force distributions in the two main horizontal directions, as suggested by the Italian code (CS.LL.PP., 2018; CS.LL.PP., 2019). The first distribution (“First Mode”) aligns with an acceleration pattern proportional to the shape of the fundamental vibration mode in the building's considered direction. Conversely, the second distribution (“Uniform”) is derived from a uniform acceleration pattern along the building's height. Such distributions were selected in accordance with the regulations, taking into account that, as a result of the modal analysis performed on the existing structure, the fundamental mode of vibration in the considered direction has a mass participation of not less than 75%. Figure 5 illustrates the pushover curves, depicting base shear versus top displacement. This figure also outlines the capacity points corresponding to each of the evaluated limit states. Specifically, the performance criteria for both local (element plastic deformation) and global (Maximum Inter-storey drift) requirements related to the required limit state (Operability OP, Damage Limitation DL, and Life Safety LS) for a school-type building were considered (ASCE, 2017). In particular, for OP LS, the maximum inter-