Dynamic Structure-Soil-Structure Interactions Modeling and Experiments

Abstract eng:
The seismic response in presence of multiple interactions between structures through the soil is extremely complex even when the structures and the soil behave linearly. However, two facts reduce the complexity of the problem. First, some structures are built according to a periodic pattern and second, the seismic spectrum generates wavelengths significantly than the characteristic size of the studied system. In this double circumstance, it is possible to address the multiple interactions in the rigorous framework of the homogenisation method. In this paper, two examples of this approach are considered, namely the dynamics of soft soil layer reinforced by a periodic distribution of piles, and the influence of the city on the seismic response at the settlement sites scale. In both cases the theory is validated by experiments on analogous specimens performed on a shaking table (European project SERIES). The soil with piles is treated as a composite made of a soft matrix - the soil, working in shear as usual - periodically reinforced by linear slender elastic inclusions, i.e. the piles considered as beams and thus working in bending. It is shown that the overall behaviour of such a system couples the shear and bending effects, with simple expressions of the effective parameters. The experiments comfort this theoretical modelling. These results make questionable the usual description based on Winkler model, where the soil is assumed to react in extension-compression (with parameters adjusted in a empirical way). The seismic response of a densely urbanized city resting on a homogeneous plane half space is studied in an idealized situation : the city is seen a periodic distribution of linear oscillator describing the first eigen mode of the buildings. The homogenisation analysis shows that the city behaves as a resonating surface characterized by an impedance, whose the properties are inherited from that of the oscillators. This enables simple parametric studies of the actual city effects on the ground motions. The experimental program conducted on a set of 37 oscillators lying on an elastic layer provides results in accordance with the theoretical city impedance model. Direct engineering applications concern the effective impedance of groups of few (say more than 5) similar buildings.

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