Soil Structure Interaction in Displacement Based Design

The research investigates how to deal with Soil-Structure Interaction (SSI) in the Direct Displacement Based Design (DDBD) procedure of precast concrete industrial and commercial buildings, whose structural layout is tipically made by cantilever columns connected at the top by prestressed precast beams supporting prestressed precast concrete roof elements. In these particular structures, neglecting the SSI could lead to an underestimation of the seismic displacements and therefore to unexpected damage, especially for non-structural elements like cladding panels. The SSI is considered in the DDBD procedure adding the foundation flexibility and the foundation damping limited to the foundation rocking motion. Extending results available in the literature, two possible methods are considered in order to account for SSI in DDBD: the first method considers an elastic foundation with a reduced stiffness to account for the increase in soil strains due to the seismic event, while the second method considers the actual foundation stiffness and damping nonlinear dependence associated to a soil macro-element available in the literature. The results validation, by means of Non Linear Time History (NLTH) analyses, shows the good performance of the design procedure albeit in particular conditions, depending on the foundation and super-structure damping ratio and stiffness ratio, the foundation inertia, not considered in the design procedure, might play a significant role. In those situations the design should be validated with NLTH analyses