A common analysis method of soil-structure systems in seismic design procedures such as FEMA-440 is to replace the entire soil-structure system by a fixed-base oscillator with an equivalent fundamental period and damping ratio to consider inertial effect of soil-structure interaction. It is generally believed by researchers that ignoring kinematic effect of soil-structure interaction is conservative to determine response of structures and FEMA-440 supports this idea by defining a reduction factor applying to elastic response spectra. Also, the improvements of nonlinear static procedures in FEMA-440 are achieved for fixed-base structures and the soil effects are not perfectly obtained in coefficients and relations of these procedures. Thus, it seems necessary to assess the accuracy of proposed procedures of FEMA-440 to include soil-structure interaction. In this paper, the accuracy of equivalent replacement oscillator and nonlinear static Procedures of Equivalent Linearization and Coefficient methods, defined in FEMA-440, to analyze soil-structure systems with surface and embedded foundations are evaluated. Both kinematic and inertial effects of soil-structure interaction are investigated by conducting a parametric study using 20 ground motions recorded on soft soil site E, on which the more SSI effects are probable. 4339 difference between the motion experienced by the essentially rigid foundation (the foundation input motion (FIM)) and the free-field motion (FFM). This effect is called the kinematic interaction (KI) effect and happens even if the foundation has no mass. In other words, the FIM is the result of geometric averaging of the seismic input motion in the free field (Meek and Wolf [8]).2) The flexibility of soil affects the response of the structure subjected to FIM. In fact, the soil-structure system behaves as a new system with different dynamic properties (longer natural period and usually higher damping). This effect is called inertial interaction (II) effect.Numerous researches on the effects of SSI have been carried out over the past few decades, but generally excluded the nonlinear behavior of structures. Replacing the entire soil-structure system with a fixed-base oscillator to consider II effect is a common analysis method in seismic design procedures. Current SSI-related regulations in seismic codes, such as ATC3-06 [9] and NEHRP [10] are based only on the knowledge of the II effect on elastic response of structures while the KI effect is traditionally ignored. The variations of the equivalent natural period and damping ratio of equivalent replacement oscillator have been studied by other researchers such as Veletsos and Meek [11], Veletsos and Nair [12], Wolf [13] and Aviles and Perez-Rocha [14].However, the yielding behavior of structures has recently been given more attention by some researchers. Bielak [15] first studied this matter by investigating the harmonic response of a bilinear structure supported on a visco-elastic half-space and found that the resonant structural deformation could...
In this article, seismic zoning maps of Golestan based on modified methodology of probabilistic seismic hazard analysis have been prepared. For this purpose, first, major seismotectonic provinces of Iran surrounding the study region are delineated and seismicity parameters are evaluated for each province. By determining the main active faults of the region and preparing the earthquake catalog containing 19 historical events and 270 instrumental events, 25 potential seismic sources are modelled as area sources in the region. In order to properly reflect the inhomogeneity of seismicity in time and space, and to avoid underestimation of potential hazard of large magnitude earthquakes, the annual mean occurrence rate of earthquakes in each seismotectonic province should be allocated to each magnitude interval in the corresponding potential seismic sources, using the spatial distribution function. In this research, different kinds of seismological, tectonic and geophysical data are used to indicate the possible future earthquake activities in the interest region, providing basis for evaluation of spatial distribution function. Seismic hazard assessment is carried out by considering a universal attenuation relationship and using the SEISRISKIII computer program for the area in a radius of 200 km from centre of Gorgan (the centre of Golestan province). The study region is divided into a series of grid points and seismic hazard analysis for every grid point is carried out using characteristics of seismic activity in seismotectonic provinces and potential seismic sources. Horizontal Peak Ground Acceleration (PGA) for different seismic hazard levels is evaluated by modified probabilistic estimation. Finally, seismic hazard zoning maps of Golestan for 10% Probability of exceedance (PE) and 2% PE in 50 years and for three types of the site soil are prepared and compared with the seismic hazard macrozonation of Iran based on standard 2800.
In this research, both kinematic interaction (KI) and inertial interaction (II) effects of soil-structure interaction (SSI) on inelastic seismic demands of structures are investigated. Site effect is also considered only by applying ground motions recorded at site classes D and E (as defined in NEHRP[1] and FEMA-440 [2]) that on them SSI effect is considerable. Carrying out a parametric study, the structure and underlying soil are modeled as a Single Degree Of Freedom (SDOF) structure with elasto-plastic behavior and a mathematical simplified 3DOF system, based on the concept of Cone Models, respectively. Also the foundation is considered as a rigid cylinder embedded in the soil with different embedment ratios. Then the whole soil-structure systems are analyzed under 30 ground motion recorded at site classes D and E and a comprehensive parametric study is performed for a wide range of non-dimensional parameters defining SSI problem. Results indicated that ignoring SSI causes considerable and in some cases un-conservative differences in seismic demands of structures. In the case of embedded foundation, it is observed that rocking input motion due to KI plays the main role and increase the structural demands especially in deep foundation embedment and slender buildings located on soft soils. Consequently, comparing the results with and without inclusion of SSI effects reveals that both II and KI effects of SSI play an important role in analyses or design procedures and ignoring them may cause un-conservative results in cases of deep embedded foundation and slender structures.
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