Investigation the accuracy of FEMA-440 procedure to analyze soil-structure systems

Abstract

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...

Soil-Structure Interaction: Comparative Review of International Codes and Other Literature with Indian Seismic Code

“…The non-linear nature at the interface of structure and soil is also considered. Khanmohammadi, L. et al [10] performed the analysis in the time domain. The frequency domain considers only linear behaviour.…”

Seismic Isolation Retrofitting of Typical Multi-Span Steel Girder Bridges in New York State

“…In particular, FEMA 440 considers the reducing effect of kinematic interaction by modifying a free-field response spectrum or time history suite to estimate FIMs ( 27 ). However, it should be pointed out that some studies found that the rocking components of FIM may increase the structural demands and lead to non-conservative designs for soil-structure systems with deep embedded foundations ( 28 , 29 ). Since the primary objective of this study is to investigate the beneficial effect of using LRB isolators for bridge seismic retrofit, FFMs at foundation levels are used as input motions by ignoring that the foundation would alter the incoming ground motion because of its inherent stiffness and ground motion incoherence.…”