DYNAMIC PROPERTIES OF BEIRUT BUILDINGS : INSTRUMENTAL RESULTS FROM AMBIENT VIBRATIONS

The Beirut (Lebanon) area is exposed to a high seismic hazard because of its location close to major faults (Yammouneh, Roum, Hasbaya, Rachaya and Serghaya). The lack, until very recently, of enforcement of a building code together with the concentration of population, politic and economic activities, and the consequences of the civil war on buildings, transform this high hazard into a very high risk. It is therefore mandatory to assess the level of hazard and vulnerability, in view of shaping a policy to stabilize and eventually reduce the risk. Besides investigations to identify the local hazard (Brax, 2013), and considering the peculiarities of the Beirut building stock weakened by a long civil war, it has been decided to investigate whether the dynamic properties of Beirut buildings exhibit special characteristics as compared to other cities of the Mediterranean area. An ambient vibration measurement campaign has been carried out for a set of 303 buildings ranging from 2 to 33 stories. Most of them are reinforced concrete frames, 182 are located on rock and 121 on soft soils. The recordings are basically 15 minutes long and have been processed using the Geopsy software, to extract the natural frequencies and the corresponding damping values. A statistical analysis was then performed to identify correlations between these dynamic parameters and the geometrical characteristics of the buildings, and to compare them with existing relations. The results show that the key parameter controlling the fundamental frequency is the building height (or number of floors), while horizontal dimensions of the building have an only marginal influence. A comparison between the two groups of measurements in rocky and soft soil highlights slightly different, but statistically meaningful, correlations between the natural period T and number of floors N (T=N/23 for rock sites and N/18 for soft sites). A comparison between the measured frequencies and those computed from conventional formulas recommended in classical building codes exhibits very significant differences: the latter lead to an about 80% overestimation of the actually measured periods, while the measured damping exhibits a striking correlation with the frequency (the higher the building the lower the damping, with typical values around 1% for 2 Hz / 12 story buildings and around 5% for 6-8 Hz / 3-4 story buildings). These results are very consistent with conclusions drawn from similar measurements in other areas. Part of the large differences with building code recommendations may be due to the very 1 PhD student, ISTerre, Grenoble University, Grenoble, France, Notre Dame University NDU, Louaize, Lebanon, christelle.salameh@ujf-grenoble.fr 2 Eng, IRD, ISTerre, Grenoble University, Grenoble, France, armand.mariscal@ird.fr 3 Professor and Chair, Notre Dame University NDU, Louaize, Lebanon, jharb@ndu.edu.lb 4 PhD, IFSTTAR,ISTerre, Grenoble University, Grenoble, France, pierre-yves.bard@ujf-grenoble.fr 5 PhD, IRD, ISTerre, Grenoble University, Grenoble, France, bertrand.guillier@ird.fr 6 PhD, IRD, ISTerre, Grenoble University, Grenoble, France, cecile.cornou@ujf-grenoble.fr 7 PhD, CNRS, ISTerre, Grenoble University, Grenoble, France, cvoisin@ujf-grenoble.fr