engleski

Seismic behavior of shallow founded inverted pendulum on soft soil

Seismic design of safe and economically justified structures represents a complex and challenging task of which most of the complexity lies in the foundation soil, an infinite medium that provides not only support for structures but also unpredictability and hazard entrenched in ground motions. In soil-structure interaction analyses in practice, due to its complexity and high computational cost, soil is often modelled (oversimplified) using discrete independent springs. Furthermore, current numerical procedures for both estimating seismic response of structures and for generating artificial ground motions are often based on the assumption of a fixed-base structure. Also, a common presumption in seismic design practice is that introduction of soil-structure interaction is beneficial on response of low to medium rise structures since it elongates the fundamental period and leads to higher damping of the system. Elongated fundamental period, as a direct consequence of the shape of the design response spectra prescribed in codes, would result in reduced inertia forces in the structure. Moreover, codes use the fundamental period calculated using empirical expressions which ignore the soil flexibility. It is shown that code-defined response spectra for soft soils particularly contradict reality, what is also confirmed by authors of this work. It is well known that elastic systems suffer the most from excitations containing cycles of nearly uniform amplitude and a nearly constant period equal to the fundamental period of the system. Thus, due to possibility of resonance occurrence, great care should be taken when conducting design of elastic systems using response spectra currently defined in codes. In addition, it is questionable whether current response spectra defined in codes are valid for deriving compatible artificial time histories. Results of research made by authors of this work considered on shallow founded linear-elastic inverted pendulum systems, as equivalent systems for real structures, lying on soft soil. Also, this paper shows how the soil-structure interaction affects the response of the pendulums and thus the shape of the response spectra. Soil was modelled as dry sand. Numerical modeling was done in Plaxis2D using an advanced non-linear soil model calibrated at the University of Dundee using centrifuge. Ground motions recorded on the Balkan Peninsula were used in this research.

seismic; shallow foundatin; soft soil; soil-structure interaction

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