Propriétés dynamiques des bâtiments beyrouthins: résultats instrumentaux des vibrations ambiantes

Dynamic parameters of buildings in Beirut (Lebanon) were investigated based on ambient vibration method for the risk and vulnerability assessment. Indeed the country is threatened by a high seismic hazard due to its location near major faults, combined to a high seismic risk caused by urban, economic and political chaos in addition to the lack of implementing a seismic design code (introduced recently in 2005). Researchers do not miss any opportunity to trigger the alarm and increase studies in the seismic field to reduce this risk. For this study, 330 RC buildings were measured, dynamic parameters extracted and a statistical analysis performed to identify correlations with geometrical characteristics. The regressions show that only the building height or number of floors N is statistically relevant for the estimation of the fundamental period T and the correlation between them is linear: T = N/25 for rock sites and N/18 for soft sites. The significant difference leads to the conclusion that the natural period of a structure depends on the type of soil. Moreover measured frequencies are compared to those estimated in seismic building codes. The latter significantly overestimates the recorded periods by 80%, while the measured damping exhibits a striking correlation with the frequency; the taller the building the lower is the damping. However results obtained in other areas based on ambient vibration are coherent. Part of the large discrepancy with building code recommendations may be due to the very low level of excitation. However it is a significant variation to be consistent with the observed frequency shifts for severely damaged structures.