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Measuring stiffness of soils in situ
Published in Fusao Oka, Akira Murakami, Ryosuke Uzuoka, Sayuri Kimoto, Computer Methods and Recent Advances in Geomechanics, 2014
Fusao Oka, Akira Murakami, Ryosuke Uzuoka, Sayuri Kimoto
Seismic site response analysis is required to understand local site effects for reliable seismic design of structures such as buildings, bridges, retaining walls, and geo-structures such as embankments and slopes. In all these cases it is imperative to realistically simulate soil response subjected to earthquake loading. In practice, 1D equivalent linear analysis (e.g. SHAKE) is widely used to simulate soil nonlinearity by means of shear modulus reduction and damping ratio variation with an equivalent level of shear strain. The procedure is iterative in a sense that shear modulus and damping ratio are adjusted to the equivalent level of shear strain and with the updated parameters computations are iterated until a specified tolerance between new and updated parameters is achieved. Main shortcomings of this method are applicability only to 1D site response analysis and prediction of larger peak accelerations (Yoshida et al. 1994, Yoshida and Iai 1998). With the advances in nonlinear constitutive models for soils, a true nonlinear seismic site response analysis can be performed. Once robustly implemented into finite difference or finite elements code, a constitutive model can be used to solve boundary value problems of any complexity.
Influence of Local Site Condition on Vertical-to-Horizontal Spectrum Ratio – Insight from Site Response Analysis
Published in Journal of Earthquake Engineering, 2022
Ground motion characterization is a dominant source of uncertainty in seismic site response estimation (Rathje, Kottke, and Trent 2010). To consider such uncertainty in nature, two sets of ground motion on hard rock (NEHRP site class B/C, Vs~760 m/s) are selected from the PEER NGA-WEST2 database (PEER) as input motions. The first set of 15 motions represents the near-source conditions (R ~ 10 km), and the second set of 15 motions represents the far-field conditions (R ~ 70 km). These motions are selected to match the target spectrum calculated by the ground motion prediction model (Abrahamson, Silva, and Kamai 2013) for a scenario (Mw = 7.0, Vs = 760 m/s). The mean V/H ratio of selected motions is shown in Fig. 3 for comparison, and the horizontal component is calculated as the geometry mean of the NS and EW components. The near-fault V/H is greater than 2/3, especially at the short period ranges, and the far-field V/H is mostly less than 2/3. The mean V/H spectrum of selected input motions generally agrees with the empirical spectrum by SBSA (Stewart et al. 2016) but higher than that of BC (Bozorgnia and Campbell 2016a).
Effects of Soil Characterization on the Seismic Input
Published in Journal of Earthquake Engineering, 2019
Stefania Viti, Marco Tanganelli, Vittorio D’intinosante, Massimo Baglione
Moreover, some soil types are not covered by any class type, i.e., the NTC 2008 classification, in fact, provides a strict description of each soil class, not covering many widespread soil profiles. In all cases, the selection of records can be done for the BR, performing subsequently a seismic site response analysis (SRA) through the proper soil profile models to obtain the surface ground motion ensemble compatible for the proper soil bedding. The selection of an ensemble compatible with the BR spectrum, in turn, is not simple, since not for all the areas, there are records compatible with the A-soil spectrum. In this work, the selection of the records representing the seismic input at the BR has been done according to the Geological Task of Tuscany experience [Rainone et al., 2004; Rota et al., 2012], who arranged a specific selection process set on the Tuscan areas.
Influence of Time Step of Ground Motions on Site Effect and Structural Response Analyses for Long-Duration Earthquakes
Published in Journal of Earthquake Engineering, 2018
The objective of this paper is to quantitatively study the effect of ground motion time step Δt for long-duration ground motions in seismic site response and structural analysis. Two major issues are addressed in this study. First, one-dimensional (1-D) equivalent-linear (EQL) and nonlinear (NL) site effect analyses are performed using a set of selected ground motions with varying time steps (Δt). Two sites representing stiff and soft soil conditions are used for this computation. Second, the selected rock motions and the computed surface motions by the site response analysis with varying Δt are regarded as input motions for nonlinear analysis of structures. The structural response analysis is performed using two numerical finite element models (FEMs), namely, a 6-story and a 12-story reinforced concrete (RC) frames. The maximum inter-story drift ratio (MIDR) is considered here to quantify the relative difference caused by varying the ground motion time steps. The influence of Δt in seismic numerical analysis is discussed. Based on these case studies, some recommendations are also presented to choose appropriate Δt for various applications.