Browsing by Author "Miranda Camus, Sebastián"

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    A nonlinear model for multilayered rubber isolators based on a co-rotational formulation
    (2017) Maureira, N.; Llera Martin, Juan Carlos de la; Oyarzo, C.; Miranda Camus, Sebastián
    This article proposes a geometrically nonlinear co-rotational model aimed to characterize the mechanical behavior of elastomeric seismic isolators. The model is able to capture the axial and lateral coupling in both axial directions, i.e. compression and tension of the isolator. Also reproduces the instability the loads in tension as well as in compression, and provides theoretical evidence of the non-symmetric behavior of the isolator in these two directions. To validate model results, a quasistatic analysis was performed on a typical isolator with many different shape factors. From the parametric analysis performed, it is observed that buckling loads are higher in tension than in compression. However, as the shape factor of the isolator increases, the behavior in compression and tension becomes symmetric. It becomes apparent that significant differences in normal stresses and strains under tensile and compressives loads are observed for axial loads smaller than 10% of the nominal buckling load. The example presented shows that lateral displacements of about ±25% of isolator radius and tension forces up to 10% of the buckling load are possible without inducing cavitation in the rubber. Accuracy of the model was also tested against finite element model results and experimental data showing satisfactory results. Furthermore, a response-history analysis of an isolated structure is presented and compared for two isolator models: the two-spring model and the model proposed herein. Finally, material nonlinearity was introduced in the dynamic analysis using a Bouc-Wen type element in parallel with the isolator. The responses are similar between models; however, significant differences occur locally in the isolator for high axial loads and/or large lateral displacements.
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    A simplified and versatile element model for elastomeric seismic isolation bearings
    (SAGE PUBLICATIONS INC, 2021) Miranda Camus, Sebastián; Miranda, Eduardo; Llera Martin, Juan Carlos de la
    A novel approach for two-dimensional modeling of elastomeric bearings using three springs in parallel is presented. This simplified element model considers as follows: (1) an elastoplastic spring with a smooth transition between branches; (2) a linear elastic spring; and (3) a non-linear elastic spring, and is fully defined by only six parameters. The main advantages of the simplified model are twofold: (1) versatility, as a single model is capable of accurately reproducing the main characteristics of the hysteretic behavior of different types of rubber-based seismic isolators, including low damping rubber bearings (LDRBs), high damping rubber bearings (HDRBs), and lead-core rubber bearings (LRBs) and (2) simplicity, as it requires fewer parameters and it is easier to calibrate from experimental cyclic test results than most currently available models. Model parameters' identification is illustrated using quasi-static cyclic and earthquake simulator tests of HDRBs and LRBs, demonstrating that the model shows a good agreement between the test-measured and model-predicted hysteretic behavior. Different objective functions are evaluated in the optimization procedure, and their effect on the identified parameters is studied and discussed. This practitioner-oriented model is particularly amenable for implementation in general-purpose structural analysis software. Its usage is strongly recommended as an initial-stage design tool to select the optimal isolation system for a specific project.
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    The effect of spectral shape on damping modification factors
    (2020) Miranda Camus, Sebastián; Miranda, Eduardo; Llera Martin, Juan Carlos de la
    The main objective of this study is to investigate the effect of spectral shape on damping modification factors η used in equivalent static and response spectrum analyses of structures with damping ratios that are different from 5% critical damping. Record-to-record variability of η is also evaluated through a statistical analysis of 5270 ground motions records from 1137 interface earthquakes recorded in Chile. The effect of spectral shape is studied using recently developed spectral shape metrics SaRatio and epsilon (ε) and evaluating their use as possible predictors for η. Similarly to previous investigations, this article also examines the effect of oscillator period, earthquake magnitude, and earthquake duration for different levels of damping ratio. Results suggest that SaRatio is an effective predictor of η, particularly for highly damped structures. However, results also indicate that for rock and firm sites, earthquake faulting mechanism and site class do not have a significant influence on η. A simple period-independent regression model for η as a function of SaRatio and damping ratio is proposed. A comparison between median η from this study and those in current Chilean seismic codes shows that code factors are unconservative.
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    Uncertainty analysis of seismically isolated structures
    (2020) Miranda Camus, Sebastián; Llera Martin, Juan Carlos de la; Pontificia Universidad Católica de Chile. Escuela de Ingeniería
    This dissertation presents an original investigation of several topics related to seismic analysis and currently-implemented code-design procedures for seismically isolated structures, focusing primarily on high-damping rubber-based isolators. A considerable effort to develop accurate mathematical models to represent rubber-based isolators' force-displacement behavior has been made in the last decades. These researchoriented models can represent complex phenomena as shear-strain hardening, scragging, and strain-rate dependence. However, engineering design procedures have not embraced these advanced modeling techniques, and the implementation of equivalent linear or bilinear models using deterministic parameters is still recommended for seismic response assessment. This approach neglects the model parameters' inherent uncertainty and ignores several characteristics of the isolators' force-displacement relationship, whose relevance should be elucidated. As the number of isolated structures increases steadily in Chile, other Latin American countries, and most of the world's seismically active regions, this research aims to close some aspects of the gap between the research-oriented modeling techniques and the simplified engineering design procedures. Particular emphasis is placed on the uncertainty quantification of the isolators' effective properties currently used in engineering design procedures. To reach this goal, this thesis is divided into three stages: (i) the development of a simplified and versatile element model for seismic isolators' response history analysis, to be implemented in engineering design practices but able to capture accurately relevant features of isolators' behavior; (ii) an uncertainty analysis of the properties used in equivalent lateral force and response spectrum procedures, quantifying the variability of the measuredby- test effective stiffness and effective damping of a vast isolator dataset; and (iii) a statistical analysis of damping modification factors used to correct the seismic demand in equivalent lateral force and response spectrum procedures, aiming to find better predictors for these damping factors based on spectral shape metrics. It is highly expected that some findings of this research can improve current design methodologies, allowing for a better estimation of interstory drifts, inertial forces, and floor accelerations on protected structures at a reasonable additional effort. The implementation of the element model presented in this thesis in general-purpose software-packages for seismic analysis is encouraged. Moreover, some findings related to effective properties uncertainties and damping modification factors could enrich current provisions in the Chilean design code for seismically isolated structures.
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    Uncertainty on measurement of elastomeric isolators effective properties
    (2021) Miranda Camus, Sebastián; Llera Martin, Juan Carlos de la; Miranda, Eduardo
    Elastomeric isolators are subjected to a series of non-destructive tests with several repeated deformation cycles. For each cycle, effective properties are calculated and afterward averaged. Despite their variability, and therefore their inherent uncertainties, these properties are treated as deterministic values by seismic design procedures. In this research, these uncertainties are quantified, based on the Guide to the expression of Uncertainty in Measurement, GUM, and Monte-Carlo simulations, considering variability between repetitions and instrumentation errors. Uncertainties were calculated for a dataset of 2,498 isolators' test results, finding that the maximum relative expanded uncertainty was 12%. The GUM and Monte-Carlo methods lead to similar results, and higher-order effects in the GUM assessment were negligible. A comprehensive analysis to evaluate the influence of the directly-measured quantities in the properties uncertainties was performed. Results showed that forces and displacements measurement errors are equally relevant in stiffness uncertainties, but force measurement errors primarily control damping uncertainties.