GNGTS 2014 - Atti del 33° Convegno Nazionale

measurements were taken only at the highest and lowest accessible floors. Ambient noise measurements were also taken on a historical building, a coastal watchtower (Fig 2d), located along the northern coast of the island of Malta. The data analysis on modal behaviour was carried out using the Grilla software supplied with the Tromino instruments. Reliable estimates of the building’s fundamental frequency, as well as higher mode frequencies in some cases, were obtained by computing both horizontal- to-vertical spectral ratios (Nakamura, 1989) at a given level of the building, as well as the horizontal-to-horizontal spectral ratio of a given 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 DampingAnalysis (NonPaDAn) method (Mucciarelli and Gallipoli, 2007) was also used to obtain an estimate of the damping at the fundamental frequency of each building investigated, although the results are not presented here.. Results and discussion. Some examples of the results from individual buildings are presented in Fig. 2.The Portomaso Business Tower, shown in Fig. 2a, is the tallest building in Malta, located in the heart of St Julian’s, that has been open since 2001. It is 98 m tall, spread over 23 floors. To a first approximation, a building like the Portomaso Tower is similar to a cantilever beam fixed in a vertical position to the ground. However, its construction and architecture make it different from a simple cantilever because from the first to the sixth floor it is attached to another building with different dimensions.Readings were taken on all floors of the tower, placing the instrument in the stairwell. The Hi/Ho amplitude spectral ratio as a function of the frequency for the longitudinal direction is shown in Fig. 2a. The fundamental (lowest) frequency is 0.65 Hz, with the transverse frequency being slightly lower (0.59 Hz) as expected. On the other hand, the fundamental frequency measured from the H/V plot at the highest floor is 0.72 Hz. Higher mode frequencies are also observed in both longitudinal and transverse directions, and are also clearly identified by plotting the modal shapes at the given frequencies. Such measurements permit the observation of modal behaviour of tall structures relatively easily, however the interpretation and analysis of such behaviour is not simple and is best carried out in parallel with numerical modelling of the building. Another tall building studied in Malta is the Fortina Spa Resort located in Sliema (Fig. 2b). It is 52 metres tall spread over 17 floors with a basement underneath. This building has a regular oval in-plan shape and from the ground floor till the ninth floor it is joined to another building, similar to the Portomaso tower. The fundamental frequency, which is the lowest frequency of the building, is clearly identified in this case for both the longitudinal direction at a frequency of 1.47 Hz and transverse direction at a frequency of 1.05 Hz. Several higher frequency modes are observable in both the longitudinal and transverse directions, but no discussion of such modes will be attempted here. Fig. 2c shows results from the Porto Paolo apartment block. This building is situated in St Pauls’ Bay, Malta and is 12 floors high, spread over 38 m with three levels underground car park. It is an isolated building however it has a different structure than usual since from the ground till the fifth floor it has a larger in-plan area than from the sixth till the thirteenth floor. The Hi/Ho spectral ratio plots in the longitudinal and transverse directions give very sharp peaks representing the fundamental frequency of the building at 2.88 Hz in longitudinal (longest side) and 1.71 Hz in the transverse (shortest side) direction of the building. The H/V method gives fundamental frequencies of 2.80 Hz and 1.70 Hz for the longitudinal and transverse directions respectively. Fig. 2d shows the analysis for the Ghajn Tuffieha watchtower on the Northwest coast of Malta. Thesekind of structures were built by the Knights of Malta in the 17 th century for impending incursions into the Maltese territory. The watchtowers were occupied by local guards who would alert the army of any intruding visitors. The response of the structure is relatively simple, and the measured fundamental frequency lies between 5 – 6 Hz. The period - height relationships for the set of buildings covered in this study for orthogonal GNGTS 2014 S essione 2.3 391