Publications

Abstract

In this paper, we employ a Bayesian approach to estimate the parameters of a high cycle accumulation model for sands using experimental data. Global sensitivity analysis and Markov-Chain Monte Carlo simulation are conducted for each of the twenty-four available experimental drained triaxial test results, considering the effect of estimating soil parameters at each strain-cycle under several loading conditions. Probability distributions inferred from each data source are then combined to obtain a single distribution for model parameters. Model calibration is then validated against new observations. The accumulated strain model is calibrated through explicit computation of strain at each cycle and the strain dependence of model parameters is included through the cyclic variation of the model constants.

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Abstract

Quicumque vult salvus esse, ante omnia opus est, ut teneat catholicam fidem: Quam nisi quisque integram inviolatamque servaverit, absque dubio in aeternum peribit. Fides autem catholica haec est: ut unum Deum in Trinitate, et Trinitatem in unitate veneremur. Neque confundentes personas, neque substantiam separantes.

Alia est enim persona Patris alia Filii, alia Spiritus Sancti: Sed Patris, et Filii, et Spiritus Sancti una est divinitas, aequalis gloria, coeterna maiestas. Qualis Pater, talis Filius, talis Spiritus Sanctus. Increatus Pater, increatus Filius, increatus Spiritus Sanctus. Immensus Pater, immensus Filius, immensus Spiritus Sanctus. Aeternus Pater, aeternus Filius, aeternus Spiritus Sanctus.

Et tamen non tres aeterni, sed unus aeternus. Sicut non tres increati, nec tres immensi, sed unus increatus, et unus immensus. Similiter omnipotens Pater, omnipotens Filius, omnipotens Spiritus Sanctus. Et tamen non tres omnipotentes, sed unus omnipotens. Ita Deus Pater, Deus Filius, Deus Spiritus Sanctus. Et tamen non tres dei, sed unus est Deus. Ita Dominus Pater, Dominus Filius, Dominus Spiritus Sanctus. Et tamen non tres Domini, sed unus est Dominus.

Haec est fides catholica, quam nisi quisque fideliter firmiterque crediderit, salvus esse non poterit.

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Abstract

The need to build tall buildings has been increasing worldwide, creating new challenges in earthquake engineering and design. Many of the current analysis methods cannot be extrapolated beyond the definition under which they were established. Prior studies and existing seismic design guidelines have indicated that the current fixed-base hypothesis for evaluating the seismic response of structures is not sufficient to properly represent the boundary conditions and behavior of tall buildings with basement levels. Studies of soil-structure interaction (SSI) for tall buildings have, however, typically been inconclusive. It is not clear under which conditions consideration of soil-basement-structure interaction (SBSI) is necessary for the design of the superstructure, foundation, and basement levels and when it can safely be avoided. Given the rising demand, it is essential to evaluate the relation of global system variables such as the basement depth, structure height, and soil characteristics with the building's response via numerical and experimental modeling. Therefore, an experimental-numerical approach is presented to better understand the seismic response of tall buildings with basement levels, considering explicit SBSI modeling. Chilean tall buildings and soil conditions are used as study cases, analyzed using non-linear finite element analyses in conjunction with results from centrifuge experiments. The results show how seismic response parameters and modals characteristics, such as inter-story drifts, shear force, bending moment, natural frequencies and damping ratios, change when SBSI is appropriately incorporated. The results point to the importance of considering soil-basement-interaction effects to evaluate the seismic response of tall buildings with basement levels and avoid unsafe estimations or the need for overdesign.

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Abstract

Construction of tall buildings has recently gone through an exponential growth in major cities, creating new challenges in earthquake engineering and design. For instance, existing analytical procedures for evaluating seismic lateral earth pressures on basement walls that are connected to these buildings typically ignore the inertia and dynamic properties of the superstructure. The inertial forces from a tall superstructure may cause additional displacements and rotations in its basement that would affect the distribution and magnitude of seismic lateral earth pressures. These additional soil- basement-structure interaction (SBSI) effects are currently not well understood. Hence, the applicability and reliability of existing procedures to the basements of tall buildings remains questionable. In this paper, we use an experimental-numerical approach to provide insight on how the lateral resisting system of tall superstructures may impact the magnitude and distribution of seismic earth pressures on basement walls buried in dry sand and gravel. Numerical simulations are first validated in 3D using a prior centrifuge experiment that included a simplified model of a 42-story, highrise structure in medium-dense, dry sand. Then, the numerical tool is used to perform 156, 2D, nonlinear simulations of more realistic buildings and basements, ground motion characteristics, as well as both sandy and gravely soil profiles. Nonlinear numerical simulations are shown to successfully capture the building’s inertial and kinematic seismic interactions with the basement and an adjacent underground structure. The subsequent numerical sensitivity study showed that inertial forces from a tall superstructure increase total lateral earth pressures on the basement walls. This increase is particularly notable in the top two-thirds of the basement wall and can be approximated by a trapezoidal distribution. The superstructure’s inertia amplifies the seismic earth pressure increments at shallow depths, with an approximately inverted triangular shape. These effects and reliability of existing analytical procedures are shown to be highly sensitive to the building’s modal frequencies in relation to the frequency content of the input motion as well as the stiffness of the structure-basement system in relation to the underlying soil. The results point to the importance of considering the building’s dynamic properties and inertia in evaluation of seismic earth pressures on basement walls, in order to avoid unsafe estimations or the need for overdesign.

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Abstract

ShakerMaker is an open-source python framework which simplifies the generation of synthetic broad-band seismograms, produced by finite-fault kinematic representations of earthquake ruptures, using a 1-D layered model of the crust and the frequency-wavenumber (\(f\)–\(k\)) method. It is designed to bring closer the engineering seismology and earthquake engineering communi- ties, by catering to the earthquake simulation needs of both disciplines. One particular goal of this framework is to provide a simple way to produce high- fidelity earthquake motions for use with the domain-reduction method, sim- plifying the setup of physically accurate finite-element simulations of multi- scale seismological and earthquake engineering problems through the use of a new specialized file format. ShakerMaker’s core is composed of a high-performance Fortran imple- mentation of the \(f\)-\(k\) method, that is exposed to the user as a python frame- work. Its software architecture emphasizes simplicity, extensibility, and per- formance, allowing users to specify complex simulation scenarios with short scripts. The message passing interface is used to achieve scalability from simple single-processor machines to HPC clusters.

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Abstract

The modeling of structural damping due to the excitation of elastic-waves into the surrounding soil domain, and its effect on structural response as apparent modal damping is explored herein. Four high-fidelity, linear finite-element mod- els of building-site systems, with 20 to 50 storeys and 2 to 7 basement levels, are simulated in OpenSees to evaluate their frequency response. Radiation-damping is provided by a layer of high-damping elements, which design is explored in detail. Results show that up to 1% of apparent, low-amplitude damping can be attributed to radiation-damping depending on number of stories and depth of embedment.

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Abstract

The modeling of structural damping due to the excitation of elastic-waves into the surrounding soil domain, and its effect on structural response as apparent modal damping is explored herein. Four high-fidelity, linear finite-element mod- els of building-site systems, with 20 to 50 storeys and 2 to 7 basement levels, are simulated in OpenSees to evaluate their frequency response. Radiation-damping is provided by a layer of high-damping elements, which design is explored in detail. Results show that up to 1% of apparent, low-amplitude damping can be attributed to radiation-damping depending on number of stories and depth of embedment.

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Abstract

The present work extends the capabilities of the Particle Finite Element Method (PFEM), which allows modeling of soil-fluid-structure interaction problems, to allow the modeling of sediment transport and scouring effects. This is accomplished by implementing scouring rules on an evolving scourable-interface, i.e. the interface surface between fluid and soil. The proposed method improves upon previous proposals by jointly capturing both the temporal and spatial scales of scouring evolution, as shown in the presented validation exercise, and also because its parametrization is conforms with commonplace engineering procedures for scouring prediction. The extension preserves desirable PFEM properties such as conservation of mass, mesh-size independence, and stability of the numerical solution of the PFEM equations.

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Abstract

This work presents a critical state plasticity model for predicting the response of sands to cyclic loading. The well-known bounding surface SANISAND framework by Dafalias & Manzari (2004) is enhanced with a `memory surface' to capture micro-mechanical, fabric-related processes directly effecting cyclic sand behaviour. The resulting model, SANISAND-MS, was recently proposed by Liu et al. (2019) , and successfully applied to the simulation of drained sand ratcheting under thousands of loading cycles. Herein, novel ingredients are embedded into Liu et al. (2019) 's formulation to better capture the effects of fabric evolution history on sand stiffness and dilatancy. The new features enable remarkable accuracy in simulating undrained pore pressure build-up and cyclic mobility behaviour in medium-dense/dense sand. The performance of the upgraded SANISAND-MS is validated against experimental test results from the literature - including undrained cyclic triaxial tests at varying cyclic loading conditions and pre-cyclic consolidation histories. The proposed modelling platform will positively impact the study of relevant cyclic/dynamic problems, for instance, in the fields of earthquake and offshore geotechnics.

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Abstract

The gas−liquid mass transfer coefficient is a key parameter to the design and operation of biotrickling filters that governs the transport rate of contaminants and oxygen from the gas phase to the liquid phase, where pollutant biodegradation occurs. Mass transfer coefficients are typically estimated via experimental procedures to produce empirical correlations, which are only valid for the bioreactor configuration and range of operational conditions under investigation. In this work, a new method for the estimation of the gas−liquid mass transfer coefficient in biotrickling filters is presented. This novel methodology couples a realistic description of the packing media (polyurethane foam without a biofilm) obtained using microtomography with computational fluid dynamics. The two-dimensional analysis reported in this study allowed capturing the mechanisms of the complex processes involved in the creeping porous air and water flow in the presence of capillary effects in biotrickling filters. Model predictions matched the experimental mass transfer coefficients (±30%) under a wide range of operational conditions.

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Abstract

The modelling and simulation of cyclic sand ratcheting is tackled via a plasticity model formulated withinthe well-known critical state, bounding surface SANISAND framework. For this purpose, a third locus –termed ‘memory surface’ – is cast into the constitutive formulation, so as to phenomenologically capturemicro-mechanical, fabric-related processes directly relevant to the cyclic response. The predictive capabilityof the model under numerous loading cycles (‘high-cyclic’ loading) is explored with focus on drainedloading conditions, and validated against experimental test results from the literature – including triaxial,simple shear and oedometer cyclic loading. The model proves capable of reproducing the transition fromratcheting to shakedown response, in combination with a single set of soil parameters for different initial,boundary and loading conditions. This work contributes to the analysis of soil-structure interaction underhigh-cyclic loading events, such as those induced by environmental and/or traffic loads.

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In soil-structure interaction modeling of systems subjected to earthquake motions, it is classically assumed that the incoming wave field, produced by an earthquake, is unidimensional and vertically propagating. This work explores the validity of this assumption by performing earthquake soil-structure interaction modeling, including explicit modeling of sources, seismic wave propagation, site, and structure. The domain reduction method is used to couple seismic (near-field) simulations with local soil-structure interaction response. The response of a generic nuclear power plant model computed using full earthquake soil-structure interaction simulations is compared with the current state-of-the-art method of deconvolving in depth the (simulated) free-field motions, recorded at the site of interest, and assuming that the earthquake wave field is spatially unidimensional. Results show that the 1-D wave-field assumption does not hold in general. It is shown that the way in which full 3-D analysis results differ from those which assume a 1-D wave field is dependent on fault-to-site geometry and motion frequency content. It is argued that this is especially important for certain classes of soil-structure systems of which nuclear power plants subjected to near-field earthquakes are an example.

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Abstract

Conventional acceleration records do not properly account for the observed coseismic ground displacements, thus leading to an inaccurate definition of the seismic demand needed for the design of flexible (long-period) structures. Large coseismic displacements observed during the Feb 27, 2010, Maule earthquake, suggest that this effect should be included in the design of flexible structures by modifying the design ground-motions and spectra considered. Consequently, Green’s Functions are used herein to compute synthetic low-frequency seismograms that are consistent with the coseismic displacement field obtained from interferometry using synthetic aperture radar images. In this case, the coseismic displacement field was determined by interfering twenty SAR images of the ALOS-PALSAR satellite taken between 10/12/2007 and 05/28/2010. These images cover the region affected by the M w 8.8 2010, Maule earthquake. Synthetic broadband seismograms are built by superimposing the low-pass filtered synthetic low-frequency seismograms with high-frequency strong-motion data. The broadband seismograms generated are then consistent with the coseismic displacement field and the high-frequency content of the earthquake. A sensitivity analysis is performed using three different fault and slip parameters, the rupture velocity, the corner frequency, and the slip rise time. Results show that the optimal corner frequency of the low-pass filter \(f_c = 1/T_c\) , leads to a trade-off between acceleration and displacement accuracy. Furthermore, spectral response for long periods, say \(T \ge 8s\), is relatively insensitive to the value of \(T_c\) , while shorter periods are strongly dependent on both, the slip rise time and \(T_c\) . In general, larger displacements consistent with coseismic data are obtained using this technique instead of digitally processing the acceleration ground-motion records.

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Abstract

Tall buildings and flexible structures require a better characterization of long period ground motion spectra than the one provided by current seismic building codes. Motivated by that, a methodology is proposed and tested to improve recorded and synthetic ground motions which are consistent with the observed co-seismic displacement field obtained from interferometric synthetic aperture radar (InSAR) analysis of image data for the Tocopilla 2007 earthquake (\(M_w=7.7\)) in Northern Chile. A methodology is proposed to correct the observed motions such that, after double integration, they are coherent with the local value of the residual displacement. Synthetic records are generated by using a stochastic finite-fault model coupled with a long period pulse to capture the long period fling effect.It is observed that the proposed co-seismic correction yields records with more accurate long-period spectral components as compared with regular correction schemes such as acausal filtering. These signals provide an estimate for the velocity and displacement spectra, which are essential for tall-building design. Furthermore, hints are provided as to the shape of long-period spectra for seismic zones prone to large co-seismic displacements such as the Nazca-South American zone.

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