A study of the dynamic characteristics of Maltese buildings using a portable seismograph

Abstract

Maltese Islands are subjected to a considerable seismic risk. To reduce such risk, it is important to understand how buildings will respond to seismic disturbances and predict possible resonance phenomena. The aim of this project is to investigate the fundamental frequency and relative damping of a number of Maltese buildings located on limestone areas, utilising the information contained in ambient noise vibrations. The ambient noise was measured using Micromed Tromino® 3-component portable seismographs located at the top of a building and some other levels. The building response survey was carried out by recording a I 0 - 30 minutes of ambient vibrations on 21 different buildings around Malta with difference in shape, different construction material (reinforced concrete or masonry) and with height in the range of 1 - 23 floors. A reliable estimate of the building's fundamental frequency, as well as higher mode frequencies in some cases, was obtained by computing both horizontal-to-vertical spectral ratios at a given level of the building, as well as the horizontal-horizontal spectral ratio of the topmost level to the basement level. The latter was carried out along two axes corresponding to the long and short dimensions of the building. The Non-Parametric Damping Analysis (NonPaDAn) method was used to obtain an estimate of the damping at the fundamental frequency of each building investigated. A period-height relationship was developed for the chosen sample of local buildings, and compared with similar relationships in other countries. The methodology used was shown to provide a quick and reliable estimate of the dynamic behaviour of buildings, which has so far not been applied in Malta.