EVALUATION OF INTERACTION EFFECTS ON THE SYSTEM PERIOD AND THE SYSTEMDAMPING DUE TO FOUNDATION EMBEDMENT AND LAYER DEPTH

On the basis of some simplifying assumptions, a parametric analysis is made of the interaction effects on the effective period and damping o f structures with embedded foundation in a soil layer, A simplified th ree-dimensional interaction model is used, in which the depth of a cyl indrical foundation, the degree of contact between the ground and the footing walls and the depth of a homogeneous stratum over rigid rock a re considered variable. The soil is replaced with impedance functions that are taken from a data base obtained with an appropriate numerical technique, so that suitable springs and dashpots dependent on the exc itation frequency are used. The system period and system damping are d etermined from the steady-state response of an equivalent single oscil lator with flexible base subjected to a harmonic motion with constant amplitude, by equating its resonant response with that of a replacemen t oscillator with rigid base excited with the same motion. The influen ce of the foundation embedment and soil layer is investigated for seve ral depths of both the footing and the stratum. It is confirmed that t he system period decreases and the system damping increases with the f oundation embedment only for sidewalls extending along the entire foun dation depth. For embedded footings without sidewall or with sidewall in null contact with the surrounding soil, the effective system parame ters behave opposite to those corresponding to the interface condition of total contact. Also, the system damping increases significantly wi th the layer depth, while the system period is practically insensitive to variations of this characteristic parameter. Finally, introducing additional permissible simplifications, an improved approximate soluti on for the effective period and damping of coupled systems is presente d, which differs from previous analogous approximations in that dampin g factors of second order are not neglected and the foundation depth i s explicitly considered.