GNGTS 2014 - Atti del 33° Convegno Nazionale

In particular, for heights up to 30 m, the experimental period – height relationships are very similar to the relationships provided by Gallipoli et al. (2008) for Italian buildings, Gallipoli et al . (2010) for European buildings (consisting of buildings from Italy, Slovenia, Croatia and Republic of Macedonia), Panzera et al. (2013) for Catania, and Longo (2012) for Siracusa, as well as for Chiauzzi et al. (2012) for Canadian RC buildings. It is important to remember that in this study, a low energy input source (ambient noise) was used for estimating the fundamental period of a building. In addition, this might influence the linear elastic response, thus causing the fundamental period to be underestimated as it is not based on strong seismic actions. As observed by Celebi (1998), the first mode periods and damping ratios obtained from strong motion response were about 20 to 50 % more than those corresponding to ambient vibration data. In fact, longer periods of vibration due to a limited amount of cracking during earthquakes are expected. Conversely, Hong and Hwang (2000) observed a much lower period than expected during strong earthquake shaking in RC Taiwan buildings, designed according to the Uniform Building Code (UBC). It is therefore likely that the shaking level (whether it is weak or strong) may not be the only responsible factor for the observed differences between experimental and numerical period – height relationships. Another important factor would be attributed to the role of stiff masonry infill walls. Studies have shown ( e.g. Kose and Karslioglu, 2006; Pujol et al ., 2008; Kose, 2009) that continuous infill masonry walls aim to reduce the vulnerability of a RC structure. In fact, the presence of infill walls tends to strengthen the mass and lateral stiffness of the system (Panzera et al ., 2013), hence decreasing the fundamental period of the building and consequently changing its seismic response (Ricci et al ., 2009). Concluding remarks. The motivation for this work arose from the need to understand the local dynamic behaviour of Maltese buildings, especially in the case of earthquake ground shaking. Knowing the behavior of local buildings is essential for understanding how buildings will respond under earthquake ground motion and hence an essential part of seismic risk assessment. The dynamic behaviour of 21 Maltese buildings based on ambient noise measurements were analysed, usingbothH/Vspectral curves aswell as spectral ratiosHi/Ho. Fundamentalfrequencies were also deduced from non – parametric damping analysis (NonPaDAn), although these results are not presented here.These methods gave comparable results for the estimation of the fundamental frequency of buildings. However, in most cases, the Hi/Ho spectral ratio method appears to be the most reliable method for assessing frequency, especially for the identification of higher mode frequencies in tall buildings. Furthermore, with a short, single measurement of ambient noise at the top of a building, the non – parametric damping analysis (NonPaDAn) was able to provide the damping estimations at the relative fundamental mode of the building. Results from different methods show that for a rapid survey of a large number of buildings, and where only an approximate value for the fundamental frequency is required, it is enough to use one portable seismograph on the roof of the building and measure the fundamental frequency from the HVSR plots, since this is a good approximation to the one obtained from the Hi/Ho plots. This would mean that the study of one building can be done in about 15 minutes. The study presented in this paper has led to two simplified relationships, between the fundamental period of vibration in orthogonal translational directions and the height of 21 different buildings, situated in Malta. These relationships obtained through the Hi/Ho spectral ratio technique were fitted by a simple power law relationship for the transverse andlongitudinal directions. Moreover, experimental results show that the fundamental periods for Maltese buildings are very similar to other buildings particularly in Sicily and southern Europe, when measured from ambient noise data. However, significant differences exist from other relationships. In particular, the experimental period results show lower values than those proposed by the Eurocode 8 (CEN, 2003) for elastic behaviour of buildings, Goel and Chopra (1997) for US reinforced concrete frame buildings and Crowley and Pinho (2004) for cracked infilled reinforced concrete European buildings. The discrepancy with the EC8 recommendations is particularly important GNGTS 2014 S essione 2.3 393