PSI - Issue 44

Gianluca Salamida et al. / Procedia Structural Integrity 44 (2023) 139–146 Gianluca Salamida et al. / Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction

The latest earthquakes that struck Italy (L'Aquila 2009, Emilia 2012, central Italy 2016) demonstrated the importance of the issue of seismic vulnerability of existing buildings. A large part of the existing building stock was built without seismic design and this makes it vulnerable to future events. After 2012 Emilia earthquake, several studies involved the types of damaged buildings. In particular, fragility models, which express the probability of achieve certain damage levels as a function of a ground motion Intensity Measure (IM), have become important tools in the seismic vulnerability assessment of existing buildings. Buratti et al. (2017) developed fragility models for precast Reinforced Concrete (RC) buildings. Based on Emilia post-earthquake surveys and observational damage, Ioannou (2021) and Simoni et al. (2021) developed fragility models for masonry buildings. Verderame et al. (2014) and Salamida and Buratti (2021) have focused on RC frame residential building with masonry infills, obtaining fragility models for a number of representative buildings. The present work aims to contribute to the assessment of the seismic vulnerability of residential RC buildings. Analysing the main features of the existing building stock in the Emilia area, a 4-storey representative building was defined and studied through non-linear static analyses. A Latin Hypercube Sampling was used to consider uncertainty in material properties. To account the effects of ground motion properties, displacement demand was evaluated by means of non-linear dynamic analyses on equivalent Single Degree of Freedom (SDoF) systems, considering a set of recorded accelerograms. Through multi-stripe analyses, fragility curves associated with the probability of exceeding a series of damage states defined according to Grünthal (1998) were obtained as a function of an IM. Since, as discuss by Buratti (2012), IMs have different performance depending on the system under investigation, part of the work is focused on the comparison of the efficiency on various IMs, in order to assess the most suitable one for the considered building. 2. Case study building Based on evidences of the study on a part of the existing building stock, the geometry of a four-storey building was defined, which is considered to be representative of a large part of the building stock. The case study building has frames with four bays in the main direction, X, and with three bays in the secondary direction, Y, as shown in Table 1; a symmetrical layout was assumed. Slabs are assumed to transfer loads to the frames in the X direction. In order to conduct non-linear static analyses, a two-dimensional finite element model was developed for the structure under consideration. In particular, since the frames have generally different features and behaviours in different directions, two plane models were created, to simulate the building response for X and Y directions. In order to define the non-linear behaviour of the frame members, a simulated design was carried out. Information about the age of construction, obtained from the study on the existing building stock, have made possible to place the building in the period of greatest interest, i.e. between the 50s and 70s, and to associate it with an appropriate regulatory context. The allowable stress method was used in the simulated design, according to the 1976 Italian structural design code requirements. The case study structure was designed for gravitational loads only, without seismic design details. Slabs permanent loads and live loads were assumed as 5 kN/m 2 and 2 kN/m 2 , respectively. The concrete class C20/25 was adopted; smooth reinforcement bars of the FeB32k class were used, with an allowable stress of 155 MPa. Cross sections of the columns were defined in function of the axial load calculated from the tributary area, assuming a stirrup spacing corresponding to 25 cm. Beams dimensions in the longitudinal direction are 30x50 cm for the perimeter frames (and 30x45 cm at the roof level), while the thickness of the internal beams is equal to the thickness of the slab, i.e. 24 cm, with 75 cm width; the frame also have internal beams in Y direction, with a reduced width. Table 1. Main geometrical features of the building under study. Number of storeys n° bays (X) Span length X [m] Lx [m] n° bays (Y) Span length Y [m] Ly [m] Plan surface [m 2 ] 4 4 4.5 18 3 4 12 216