Prediction of foundation stiffness and damping ratio under horizontal translation and rocking motion

In this paper, the results of snapback, cyclic and forced-vibration tests performed by laboratory and full-scale facilities are collected and analysed to investigate the nonlinear response of shallow foundations. The outcomes of tests on a full-scale soil-foundation-structure prototype, performed during the EU-research project SISIFO, were exploited to fill the lack of data on foundation damping and stiffness associated with the translation motion and enrich the database relevant to low-amplitude rocking motion. Predictive equations are calibrated to estimate the nonlinear variation of the foundation stiffness and damping ratio with the increasing amplitude of the foundation translation and rocking motions. In the latter case, the availability of numerous experimental data allows differentiating the predictive damping equation according to the so-called ‘critical foundation-soil contact area’, which is the most influencing factor on the energy dissipation capacity of the foundation. To prove the reliability of the proposed approach, two different applications with an increasing level of complexity are discussed. First, simplified analytical formulas are employed to calculate the fundamental period and damping ratio of the prototype tested during the SISIFO project. Then, a flexible-base model of the same structure is calibrated to simulate the experimental forced-vibration tests. The outcomes of both applications successfully match the experimental measurements.