PSI - Issue 44

Alessandra Gubana et al. / Procedia Structural Integrity 44 (2023) 512–519 A. Gubana and A. Mazelli / Structural Integrity Procedia 00 (2022) 000–000

514

3

Material properties are obtained from the available original drawings and the construction supervision documents provided by the technical office of the hospital (Gubana et al. 2019), due to the impossibility of performing in situ tests. Different concrete and steel classes were used in the two units. The medium strength of concrete is 28.1 MPa for unit 1 and 23.8 MPa for unit 2. The medium yield strength of steel is 497.3 MPa and 507.2 MPa for unit 1 and 2 respectively. All the reinforced bars are ribbed.

Fig. 1. Typical plan of the case study building (dimensions in m). Stair and lift cores are labelled A to H.

Preliminary elastic analyses were performed to have a first comprehension of the structural behaviour. The total seismic weight of the building is 285,000 kN and its principal modes have periods of 0.80 s and 0.64 s in X and Y directions respectively. 3. Non Linear Static Analyses Pushover analyses were performed in X and Y direction, considering two groups of forces. Group 1 curves were obtained under a modal force distribution, while Group 2 curves under constant forces at all levels. A complete 3D model was built for the pushover analyses. A lumped plasticity model was adopted for beams, and the moment–curvature nonlinear relationships were obtained by theoretical calculations for all the sections. Walls were modelled as frame elements connected by rigid links. Nonlinearities in columns and walls were modelled by fibre hinges, which capture the cracking of the section. RC sections were divided into 15 to 45 concrete fibres and in as many steel fibres as reinforcement rebars. Every concrete fibre follows an unconfined Mander constitutive model, while the steel has an elastic behaviour with a low hardening percentage (2%). The coupled axial and flexural behaviour of the section was obtained by integration of the uniaxial stress in all the fibres on the whole section. All columns and walls were provided with lumped shear hinges in the middle of the elements. The brittle shear behaviour was obtained using a steep unload slope after reaching the shear resistance of the section. The residual shear strength was set at 40% (Biskinis et al. 2004). The model was considered fixed at the base. Fig. 2 shows the pushover curves in X direction for both the load groups. F b is the total base shear and d c is the displacement of the control point, set on the centre of mass of the roof. The shear walls have no adequate reinforcement to resist the high shear demand, and they reach quickly the collapse condition at a pseudo acceleration in correspondence of the first mode S a (T 1 ,5%) = 0.011÷0.014 g (black diamond in Fig. 2) (Mazelli 2020, Gubana et al. 2019). The verifications were conducted by means of the N2 method (Fajfar and Gašperšič 1996)(Fig. 3).