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DYNAMIC AUSCULTATION OF BUILDINGS AND SEISMIC INTEGRITY THRESHOLD ASSESMENT

Recent results of in situ measurements and their interest for a seismic assessment of existing buildings are presented and analysed. The methodology is exposed on three steps. First an experimental program was performed in order to justify the use of vibration data collected in situ for identifying the actual dynamic behaviour of usual intact buildings built according to the common practise. The response to ambient vibrations, harmonic excitation and shock loading is recorded on intact buildings but also after their structure or their vicinity was modified. Taking advantage of the demolition, the tests enable to determine the actual influence of the light work elements, full pre-cast facade panels, bearing masonry walls, and the presence of neighbouring joined buildings. These experiments realized on seven real buildings show that information gathered from ambient measurements provide reliable and efficient data of real interest for a clear understanding of the actual building behaviour. Second it is shown that the experimental modal characteristics obtained on regular concrete buildings are described successfully by suited classical or uncommon continuous beam modelling consistent with the internal structure. In a given direction of motion the key identification parameter of the relevant modelling is the frequency distribution of the two (or three) first eigenfrequencies. Third, the advantage of integrating these previous developments in the vulnerability assessment is presented and discussed. Choosing the maximum tensile strain of concrete as damage criterion for key structural elements, a maximum level of the ground acceleration (in regulation meaning as French PS92) can be determined. This so-called Seismic Integrity Threshold is directly related to the onset of structural damages. This new approach is illustrated by using the in situ records of one of studied buildings. This work underlines the advantages of using ambient vibrations survey for the vulnerability assessment of existing buildings.