Ground Motions and Structures - Earthquake Engineering for Structural Design - page 359

Ground Motions and Structures 359

The structural control technology can be divided into Tuned mass damper system
and Active mass damper system . The first is a system for absorbing the vibrations
generated by high winds or moderate earthquakes, obtained by adding a pendulum or
some mass on the building roof. The second uses a computer-controlled actuator,
trying to suppress the oscillation of the building by activating a corresponding
oscillation which reduces the ground motions effects and realizes the best performance
of the structure.
One of the most promising active control systems for protecting buildings against
strong earthquakes is the smart base isolation system (Yang, 1990, Wang and Dyke,
2007). It is a hybrid system consisting of a passive base isolation system (for instance
an elastomeric bearing) connected to an active control system. This hybrid system has
proved to be a very effective solution in reducing the response of buildings under
strong earthquakes, easier to implement than the one of pure active control.
The conventional approach to the earthquake resistant design of buildings is based
on the request of providing structures with strength, stiffness, ductility and robustness.
The new technologies, such as active control systems, seem to be much more effective.
But, despite the recent development, the structural control systems have some
functional limitation against the earthquake actions, due to the size of the buildings.

8.6SOIL-FOUNDATION-STRUCTURE INTERACTION

8.6.1MainEffectsofInteractions

There are significant number of controversies about the perceived effects of soil-
foundation-structure interaction on the overall performance of superstructures,
especially on soft soils. This beneficial role has been turned into a dogma: for instance,
according EC8-5: “For the majority of usual building structures, the effect of SSI (soil-
structure interaction) tend to be beneficial, since they reduce the bending moments and
shear forces acting in the various members of the superstructures…” (Gazetas,
2006).Therefore, this interaction has been initially considered as beneficial during
seismic motions, but the trend in research is changing and this causes different
thinking on this phenomenon (Grondin, 2004).
The response of a structure to a shaking earthquake is affected by the interaction
between three linked subsystems: soil, foundation and structure (Fig. 8.56a). A seismic
soil-foundation-structure interaction analysis evaluates the collective response of this
system to a specific free-field ground motion. It is considered as a natural source of
energy dissipation (Crouse, 2000).
Worldwide, the seismic structural design is based on standard procedures over-
simplifying the ground, where the type of foundation as well as its interaction with the
soil and the structure is not taken into account. So, the structure is considered as fixed-
base at the ground level (Fig. 8.56b), or at the bottom of foundation (Fig. 8.56c).