Evidence of coupled soil-structure interaction and site response in continuous viaducts from ambient vibration tests

The scope of this research was to identify the significance of soil-structure interaction and site response on the dynamic behaviour of continuous multispan reinforced concrete viaducts, based on ambient vibration measurements and numerical simulations, using finite element models. For this purpose, an 875 m long bridge, located in Central Italy, founded on piles in eluvial-colluvial soil deposit was instrumented and ambient vibration tests together with geophysical investigations were performed. Experimental modal properties were evaluated by means of the operational modal analysis on accelerometric data and the role of soil-structure interaction in the interpretation of tests was detected by means of finite element models characterised by different accuracy in addressing the interaction problem. In the soil-structure interaction models the local site condition in correspondence of each bridge piers (resulting from geotechnical and geophysical investigations) were taken into account in the definition of the soil-foundation impedances. Comparison between the experimental results obtained from ambient noise measurements on the free-field and on the bridge deck permits the identification of both the predominant period of the site and the fundamental periods of the structure. In addition, comparisons between results obtained from the different numerical models with the measured dynamic response of the viaduct, in terms of fundamental frequencies and mode shapes, allow the identification of the contribution of different soil-structure interaction aspects such as the pile-soil-pile interaction, the radiation problem, the pile cap embedment as well as the variability of the soil stratigraphy along the longitudinal direction of the viaduct.