پاسخ لرزه ای سیستم های سازه و خاک
Abstract
1. Introduction
2. Constitutive model for the sand behavior
3. Numerical modeling
4. Input motions
5. Numerical cases
6. Procedure of analysis
7. Results
8. Conclusions
Acknowledgement
References
Abstract
The soil liquefaction is a major cause of damage in structures during earthquakes. This damage varies from small settlements to complete failure due to the loss of bearing capacity. To deal with these problems, piled foundations have been utilized in the presence of liquefiable soils in seismic zones. More recently, rigid inclusion foundations have been also considered. A fundamental approach to study the soil-foundation-structure interaction requires the determination of the influence of the kinematic and inertial effects in the system. In order to investigate the effects of this interaction, numerical models with a 3-storey reinforced concrete building founded on pile systems (soil-pile-structure) and rigid inclusion systems (soil-inclusion-platform-structure) were analyzed. Finite difference numerical models were developed using Flac 3D. The SANISAND constitutive model was utilized to represent the behavior of the liquefiable soil layer. This model predicts with accuracy the soil response for various soil densities, stress levels and loading conditions. The linear elastic perfectly plastic constitutive model with a Mohr-Coulomb failure criterion was used to represent the behavior of the non-liquefiable soil layers. Different relative density values of the sand layer were considered. Two earthquake signals were used to study the influence of the frequency of the systems excitation. For each case, the spectrum response, shear forces and rocking of foundations were obtained. Maximum shear strains and excess pore pressures were presented at different depths. The efforts and displacements in the rigid elements (piles or rigid inclusions) were also compared for the different systems. The results show that the relative density, the pile length and the frequency of the input motion greatly influence the response of the reinforced systems.
Introduction
Liquefaction is one of the significant and complex issues in geotechnical engineering because it has been reported as the main cause of damage and failure in buildings and other structures under earthquakes (Niiagata 1964, San Fernando 1971, Loma Prieta 1989, Kobe 1995). The liquefaction takes place due to the accumulation of pore pressure in loose saturated sand deposits under dynamic loading, which in turns produce an important reduction in the strength of the underlying soils. The loss of bearing capacity and the foundation settlements are the consequences of this soil strength reduction. To deal with these problems, piles foundations supporting structures are often used in seismically zones. More recently, rigid inclusion system has also been utilized. This method is similar to the pile foundation system; however, in this case the rigid elements are separated from the structure with the use of an earth platform (Fig. 1). The arching effect in the platform, caused by shearing mechanism due to differential settlements of soil and piles, allows a transfer of the load to the rigid inclusions. The remaining loads are transmitted to the underlying soil. The earth platform constitutes a zone of energy dissipation for seismic loading. The response of these foundation systems in the presence of liquefiable soil considers simultaneously, i) the dynamic loading acting in the rigid elements (piles/inclusions) due to the surrounding soil and the presence of the structure; ii) the shear strength reduction and the degradation of the soil stiffness due to the soil nonlinearities and the generation of pore pressure [1].