PSI - Issue 64

Alessandra De Angelis et al. / Procedia Structural Integrity 64 (2024) 327–334 De Angelis et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The masonry infill walls are one of the most common types of non-structural elements adopted worldwide for the realization of external and internal walls in reinforced concrete (r.c.) framed structures. The easy construction, the ability to create any type of opening together with the possibility to be changed during the life of the building, without making any changes to the structural elements, are ones of the advantages that have led to their spread. Commonly, they are considered as non-structural elements and consequently their contribution is neglected in the design of new r.c. and steel framed buildings but they are considered only in terms of added masses and loads. This approach is generally conservative to the ultimate limit states since it is assumed that the infill walls are completely damaged but it is not at the operational limit states. Therefore, the strategy of infills’ modelling , to consider their contribution in term of stiffness and strength to the performance of buildings, subject to lateral loads, takes on a fundamental role. For these reasons, in recent decades, in-site vibration tests have gained increasing interest as a methodology to deepen the knowledge of the dynamic of infilled frame buildings and allow the improvement of modeling strategies. The application of this type of tests to real building allowed to address the contribution of infill walls starting from the interpretation of test results through numerical models as in Bovo et al. (2020) and De Angelis & Pecce (2020). Also other author as Yun et al. (2020) and Zhou et al. (2017) used the experimental results of in-situ tests based on vibrations to calibrate FE models and to investigate the infill mass and stiffness contributions on the global dynamic response of buildings. Moreover, as underlined by Ventura & Schuster (2011) the in-situ tests based on vibrations allow the identification of the dynamic behavior that characterizes the building at the time the tests are carried out. Therefore, both structural and non-structural elements can be taken into account and the actual distribution of mass and stiffness in the structure can be considered. An important contribution was made by Vanni et al. (2022) proving the usefulness of the dynamic characterization of buildings based on the use of ambient vibration tests (AVTs) at different construction phases to progressively validate the construction stages. In this paper, instead the AVTs coupled with the progressively updating of the numerical model, during some key stages of the rehabilitation works of a real building, are used to detect the influence of masonry external and internal infill walls on the dynamic behavior of r.c. frame buildings. The experimental campaign was carried out with reference to a case study, i.e., an infilled frame building where rehabilitation works are underway to redevelop and improve the energy and structural performance. In particular, three ambient vibration tests, i.e., on the as built structure, after the demolishment of the internal partition walls and after the partial demolishment of the external infill walls at the semi-basement and in the area of the staircase, have been carried out. The 3D finite element model of the building has been successfully updated based on the global modes identified by the in situ tests, pointing out the important role of the non-structural components for this type of building in each stage. 2. In situ dynamic tests The aim of the in situ dynamic tests is to identify the dynamic behavior of the tested structure through the measurement of accelerations (or velocities) due to different types of excitations. The most adopted test is the so called ambient vibration test (AVT), whose input excitation is provided by the environment actions, i.e., micro tremors, wind and anthropogenic activities. AVTs allow the characterization of the dynamic behavior of the building in terms of modal parameters, namely frequencies, damping ratios and mode shapes and it can be used to have information about the global behavior of the building in terms of mass and stiffness distribution. In this section, after a description of the building selected as case study, the results of a first AVT, named AVT n.1 in the following, performed on the building in the as-built condition is recalled and used as reference. Then two AVTs, named AVT n. 2 and n. 3, carried out during two key stages of the rehabilitation works are described and finally the evolution of the building modal parameters is used to highlight the role of internal and external masonry infill walls. 2.1. Description of the case study in the as-built condition and results of AVT n.1 The selected case study is an existing building designed only for gravity loads located in the Campania Region, southern Italy. The building has 44.10×14.10 m rectangular plan and total height of 15.40 m (with three storeys above the ground level and a semi-basement). The structure is constituted by r.c. frames (3 in the longitudinal