Ground Motions and Structures - Earthquake Engineering for Structural Design - page 361

Ground Motions and Structures 361

Figure 8.57 Soil-structure system: rocking and translation components
(FEMA273, 1997)

soil.
The main soil-foundation-structure interaction effects can be important, due to the
following aspects (Kramer and Stewart, 2004):
Foundation stiffness and damping. The inertia developed during the vibration of the
structure gives rise to base shear, moment and torsional excitation, and these loads in
turn cause displacements and rotations of the foundation. Moreover, these motions
give rise to energy dissipation via radiation and hysteretic soil damping, and this
energy dissipation can significantly affect the overall system damping.
The variation between free-field and foundation-level ground motions results from
two processes. The first is known as kinematic interaction and it is caused by the
presence of stiff foundation elements on or in soil, producing foundation motions,
which deviate from free-field motion as a result of the foundation embedment effects.
The second process is related to the inertial interaction effects and consists of the
relative foundation displacements and rotations described above.
Foundation deformations. Flexural, axial and shear deformations of foundation
elements occur as a result of loads applied by the superstructure and the supporting soil
medium. These deformations can also significantly affect the overall system behavior,
especially with respect to damping.
The effects on the superstructure due to the soil-foundation-structure interaction can
be summarized as follows: decreasing of the natural frequency (increase the natural
period) of the system, increasing of damping, increasing of lateral displacements and
secondary order effects, changing of the seismic base shear, depending on the
frequency content of the input motion and the dynamic features of the soil-foundation-
structure system. The current seismic design philosophy regards this dynamic
interaction to be beneficial to the structure behavior. The flexibility of the soil-
foundation system reduces the overall stiffness of global system and therefore
attenuates the peak loads caused by a given motion. This might be true in many cases.