Showing papers in "Soil Dynamics and Earthquake Engineering in 2018"

Liquefaction resistance of bio-cemented calcareous sand

Abstract: Coral reefs and other calcareous deposits may experience various types of significant dynamic loading, such as those from waves and earthquakes. When submerged and subjected to earthquake loading, the potential for liquefaction of calcareous deposits may cause a loss of human life and property; however, few studies have evaluated the liquefaction potential of calcareous sands relative to those conducted on silica sands. Accordingly, it is critical to study the cyclic resistance of calcareous sands as well as methods to mitigate their liquefaction potential. Microbial induced calcite precipitation (MICP) offers one such strategy that can be considered for improving the cyclic resistance of calcareous sands, particularly for those applications below existing infrastructure that would pose technical difficulties for traditional modes of ground improvement. This paper examines the effectiveness of MICP on the cyclic resistance of as a function of cementation solution (CS) content, effective confining pressure, and cyclic stress ratio (CSR) through a cyclic triaxial test program. The generation and accumulation of excess pore pressure and corresponding axial strains are compared across a range of treated and untreated sands. This study shows that the liquefaction resistance of clean calcareous sand may be significantly improved by the MICP treatment. Scanning electron microscope images are presented to help link the improvement in cyclic response to the microstructural features of the microbial-induced calcite and bio-cemented sand.

Soil-dependent optimum design of a new passive vibration control system combining seismic base isolation with tuned inerter damper

Abstract: The papers addresses a novel passive vibration control system combining seismic base isolation with a tuned inerter damper (TID) system. The latter, by analogy with the tuned mass damper (TMD), is a dynamic vibration absorber in which the physical mass of the TMD is partly or entirely replaced by an apparent mass, also called inertance, created by a particular arrangement of mechanical gearings—the inerter. By attaching a TID to the isolation floor, not only the displacement demand of base-isolated structures can be significantly reduced, but also the superstructure response (e.g. interstory drift, base shear) is effectively controlled. Optimum parameters of this system are found based on a simplified three degree-of-freedom model that reflects the dynamic properties of both the isolation system and the TID while accounting for the flexibility of the base-isolated superstructure. Within a probabilistic framework, the influence of soil conditions is investigated by modeling the seismic ground motion as a filtered Gaussian random process. Different filter parameters are considered that may be associated with firm, medium or soft soil conditions depending on the frequency content of the power spectral density function. A wide parametric study is performed in order to detect the optimal TID parameters depending on the soil conditions for a variety of isolation ratios, mass ratios and damping ratios of both the superstructure and the isolation system. Finally, a multi-story building equipped with the proposed passive vibration control system is examined. Effectiveness of the proposed system is assessed via the evaluation of the structural response in the time domain. Detuning effects are investigated via a sensitivity analysis. Comparison with alternative passive vibration control systems proposed in the literature and based on different arrangements of TMD and inerter-based device is discussed.

Historical development of friction-based seismic isolation systems

Abstract: Base isolation has emerged as one of the most effective high-tech strategies for protecting infrastructure under seismic loading. This review paper discusses the historical development of friction-based seismic isolation systems, focusing on systems that have successfully been deployed and used as seismic safety measures for structures located in Europe. The conception and implementation of the Friction Pendulum system, the development of low friction materials and the effects of heating, contact pressure and velocity are discussed in light of past and recent numerical and experimental evidence. The merits of multiple surface devices, namely the Double Curvature Friction Pendulum and the Triple Friction Pendulum are also discussed, along with current knowledge and research gaps. Two European case studies, the Bolu Viaduct and the C.A.S.E. Project, are presented to illustrate that sliding base isolators can be used to meet otherwise unachievable design objectives. Finally, existing problems such as the response to high vertical accelerations, the potential for bearing uplift and the relevance of residual displacement are analyzed.

Earthquake-resilient design of base isolated buildings with TMD at basement: Application to a case study

Abstract: The earthquake-resilient design of a reinforced concrete framed building is presented in this paper as a case study A non-conventional application of the base isolation in conjunction with a tuned-mass-damper (TMD) located at basement, below the isolation floor, is studied for improving the seismic performance of the building The seismic base isolation is implemented below the first story, with low-damping rubber isolators mainly placed throughout the perimeter of the building At the center of the building a large-mass TMD is inserted, which consists of a box filled with large aggregate concrete The box is located at basement and is connected to the base isolation system via an auxiliary set of lead-core rubber isolators, the latter playing the role of damper and spring elements of the TMD The TMD box is disconnected from the ground via low-friction flat sliding devices Optimal design parameters of the auxiliary TMD isolators are detected by minimizing an objective function that is based on the stochastic dynamic analysis of a simplified three-degree-of-freedom system comprising the main structure, the base isolation and the TMD Four different objective functions are investigated, including the main structure displacement relative to the ground, the interstory displacements, the total acceleration and an energy-based indicator The effectiveness of this design philosophy and of the related optimization procedure, first time applied to a real case, is demonstrated via nonlinear time-history analyses with simulated accelerograms being consistent with the response spectrum of the installation site Advantages of this structural system over both the fixed-base building and the conventional application of the base isolation are shown in terms of a variety of response indicators summarizing the seismic performance of the building, including the deformation of the isolators, the displacement demand of the structure, the base shear, the interstory drifts, and the shear forces and bending moments on the beam-column members

Seismic performance assessment of monopile-supported offshore wind turbines using unscaled natural earthquake records

Abstract: The number of offshore wind turbine farms in seismic regions has been increasing globally. The seismic performance of steel monopile-supported wind turbines, which are the most popular among viable structural systems, has not been investigated thoroughly and more studies are needed to understand the potential vulnerability of these structures during extreme seismic events and to develop more reliable design and assessment procedures. This study investigates the structural performance assessment of a typical offshore wind turbine subjected to strong ground motions. Finite element models of an offshore wind turbine are developed and subjected to unscaled natural seismic records. For the first time, the sensitivity to earthquake types (i.e. crustal, inslab, and interface) and the influence of soil deformability and modeling details are investigated through cloud-based seismic fragility analysis. It is observed that monopile-supported offshore wind turbines are particularly vulnerable to extreme crustal and interface earthquakes, and the vulnerability increases when the structure is supported by soft soils. Moreover, a refined structural modeling is generally necessary to avoid overestimation of the seismic capacity of offshore wind turbines.

Support vector machine based reliability analysis of concrete dams

Abstract: This paper presents possible combination of structural responses of concrete dams with machine learning techniques. Support vector machine (SVM) method is adopted and two broad applications are presented: one for a simplified flood reliability assessment of gravity dams and the other for detailed nonlinear seismic finite element method (FEM) based analysis. Up to seventeen random variables are considered in the former example and the results of SVM contrasted with classical reliability analyses techniques (i.e., first- and second-order reliability methods, Monte Carlo simulation, Latin Hypercube and importance sampling techniques). For the latter example, a FEM-SVM based hybrid methodology is proposed for reduction of number of nonlinear analyses. A discussion is provided on the relation between the optimal earthquake intensity measures, the damage states and the accuracy of prediction. It is found that the family of SVM (i.e. standard, least squares, multi-class and regression) is an useful and effective tool for classification, response prediction and reliability analysis of the concrete dams with reasonable accuracy.

LEAP-GWU-2015 experiment specifications, results, and comparisons

Abstract: LEAP (Liquefaction Experiments and Analysis Projects) is an effort to formalize the process and provide data needed for validation of numerical models designed to predict liquefaction phenomena. For LEAP-GWU-2015, one project within LEAP, an experiment was repeated at 6 centrifuge facilities (Cambridge University, Kyoto University, University of California Davis, National Central University, Rensselaer Polytechnic Institute, and Zhejiang University) and the results were shared and archived for the purposes of validation of numerical models. This paper describes the specifications for the LEAP-GWU-2015 experiment and compares the experimental results from the six facilities. The specified experiment was for uniform medium dense sand with a 5 degree slope in a rigid container subject to a ramped, 1 Hz sine wave base motion. The experiment was meant to be relatively simple to enable different facilities to produce comparable experiments. Although it cannot be claimed that identical experiments were precisely replicated on different centrifuges, it is argued that the results are similar enough that each experiment lends veracity to the set of results. A benefit of variability between experiments is that the variety enables a more general validation. Important lessons with regard to specification of future experiments for validation of numerical models are summarized. LEAP-GWU-2015 has demonstrated an approach that is a useful reference for future validation studies.

Removable friction dampers for low-damage steel beam-to-column joints

Abstract: Beam-to-column joints equipped with friction dampers are promising solutions to improve the performance of steel moment resisting frames due to the possibility to guarantee large dissipation capacity limiting the structural damage under severe seismic conditions. In this paper, the experimental tests and the numerical simulations of two types of joints are shown and discussed with the aim of developing pre-qualified configurations. The friction dampers are designed to be easily removable from both the lower beam flange and the column face by means of bolted connections. The devices are composed of a stack of steel plates conceived to assure symmetrical friction. The friction surface is set in vertical direction in first case and in horizontal direction in the second type. The experimental tests confirmed the effectiveness of both examined joints and the finite element analyses allowed characterizing their local response, thus providing additional insights to improve the design requirements.

Seismic fragility for high CFRDs based on deformation and damage index through incremental dynamic analysis

Abstract: In this paper, a seismic fragility analysis method based on incremental dynamic analysis (IDA) is extended to evaluate the seismic performance of high concrete face rockfill dams (CFRDs). Permanent deformation and face-slab damage index using a modified generalized plasticity model for rockfills and a plastic-damage model for face-slabs are considered to be dam damage measures (DMs) after defining a new face-slab damage index. The verification to damage index through the Zipingpu CFRD and previous research indicates that the grading standards are reasonable. Fragility curves and the probabilities are determined for each DM under different earthquake intensities. The results of fragility analysis demonstrate that this method can provide a strong scientific basis for predicting the earthquake destruction and loss of high CFRDs.

Wind, wave and earthquake responses of offshore wind turbine on monopile foundation in clay

Abstract: This paper investigates the dynamic responses of offshore wind turbine (OWT) supported on monopile foundation in clay subjected to wind, wave and earthquake actions. Based on the open-source software platform OpenSees, a three-dimensional finite model of the system is developed. The tower and monopile is modeled using beam element, the pile-soil interface behavior using nonlinear Winkler foundation approach, and the pile-water interface using hydrodynamic added mass. The wind, wave and earthquake actions are applied as loadings on the system. The effects of several parameters, such as wind velocity, induction factor, wave period, peak ground acceleration, and soil parameters on the dynamic responses of the system are studied. The results indicate that it is necessary to consider the combination of wind, wave and earthquake actions in the design of offshore wind turbine.

Safety of buried steel natural gas pipelines under earthquake-induced ground shaking: A review

Abstract: Evidence from past earthquakes suggests that damage inflicted to buried natural gas (NG) pipelines can cause long service disruptions, leading to unpredictably high socioeconomic losses in unprepared communities. In this review paper, we aim to critically revisit recent progress in the demanding field of seismic analysis, design and resilience assessment of buried steel NG pipelines. For this purpose, the existing literature and code provisions are surveyed and discussed while challenges and gaps are identified from a research, industrial and legislative perspective. It is underscored that, in contrast to common belief, transient ground deformations in non-uniform sites are not necessarily negligible and can induce undesirable deformations in the pipe, overlooked in the present standards of practice. It is further highlighted that the current seismic fragility framework is rich in empirical fragility relations but lacks analytical and experimental foundations that would permit the reliable assessment of the different parameters affecting the expected pipe damage rates. Pipeline network resilience is still in a developing stage, thus only few assessment methodologies are available whereas absent is a holistic approach to support informed decision-making towards the necessary mitigation measures. Nevertheless, there is ground for improvement by adapting existing knowledge from research on other types of lifeline networks, such as transportation networks. All above aspects are discussed and directions for future research are provided.

Measured vs. predicted site response at the Garner Valley Downhole Array considering shear wave velocity uncertainty from borehole and surface wave methods

Abstract: This paper compares measured and predicted site response at the Garner Valley Downhole Array (GVDA) using a wide range of shear wave velocity (Vs) profiles developed from both borehole methods and inversion of surface wave data. Only low amplitude ground motions, resulting in approximately linear-viscoelastic site response between the downhole accelerometer and the surface accelerometers, were considered in this study. Thus, uncertainties associated with the small-strain Vs profiles used for site response predictions play a considerable role in attempting to match the recorded site response and its associated variability. Prior to our study, two borehole Vs profiles extending into rock were available for the site. However, their predicted/theoretical transfer functions (TTFs) were quite different and in poor agreement with the measured/empirical transfer functions (ETFs). These differences provided motivation to collect and interpret an extensive set of active-source and passive-wavefield surface wave measurements in an attempt to develop deep Vs profiles for the site that might be used to more accurately model the measured site response and its associated variability. Suites of non-unique Vs profiles developed from inversion of the surface wave data visually exhibited considerable differences, yet their predicted TTFs matched the measured ETFs very well. These results provide further evidence that surface wave dispersion data and horizontal-to-vertical spectral ratio curves represent an experimental “site signature” that can be used as a quantitative means of assessing whether candidate Vs profiles are appropriate for use in site response analyses.

Probabilistic seismic vulnerability analysis of corroded reinforced concrete frames including spatial variability of pitting corrosion

Abstract: A probabilistic framework for seismic vulnerability analysis of corroded Reinforced Concrete (RC) frame structures is developed. An advanced nonlinear finite element modelling technique is used to accurately simulate the nonlinear behaviour of prototype corroded RC frames over their service life. Different sources of uncertainties including modelling uncertainties, geometrical uncertainties and spatial variability of pitting corrosion are considered through Monte Carlo simulation and using Latin Hypercube Sampling (LHS) technique. A set of new seismic damage limit states (SDLS) are defined accounting for multiple failure modes of the corrosion damaged frames by means of pushover analyses. The influence of corrosion on nonlinear dynamic behaviour of corroded RC frames is investigated through Incremental Dynamic Analysis (IDA) of proposed frame structures under 44 far-field ground motions. The impact of considering corrosion damaged SDLS, spatial variability of pitting corrosion, and record-to-record variability on seismic vulnerability of RC frames are explored and discussed in detail. It is concluded that disregarding the influence of corrosion on SDLS significantly underestimates the probability of failure of corroded RC frames. The analyses results show that spatial variability of pitting corrosion does not have a significant impact on global nonlinear behaviour and seismic vulnerability/reliability of corroded RC frames.

Seismic demand of base-isolated irregular structures subjected to pulse-type earthquakes

Abstract: Base-isolated structures may be subjected to severe seismic demand in the superstructure and/or in the isolation system at sites located near an active fault. Forward directivity effects with long-period horizontal pulses in the fault-normal velocity signals are the main cause of this behaviour. However, recent studies have identified pulses in arbitrary orientations along with false-positive classification of pulse-type ground motions. The aim of the present work is to evaluate the reliability of elastomeric (i.e. high-damping-laminated-rubber bearings, HDLRBs) and sliding (i.e. curved surface sliding bearings, CSSBs) base-isolation systems for the seismic retrofitting of in-plan irregular buildings located in the near-fault area. To this end, a five-storey reinforced concrete (r.c.) framed structure, with an asymmetric-plan and bays of different length, is chosen from benchmark structures of the Re.L.U.I.S. project. Attention is focused on the pulse-type and non-pulse-type nature of near-fault earthquakes and moderately-soft and soft subsoil conditions. First, a comparison between algorithms based on wavelet signal processing, that can identify pulses at a single (e.g. fault-normal) or arbitrary orientation in multicomponent near-fault ground motions, is carried out to classify records of recent events in central Italy and worldwide. Then, nonlinear seismic analysis of the fixed-base and base-isolated test structures is performed by using a lumped plasticity model to describe the inelastic behaviour of the r.c. frame members. Nonlinear force-displacement laws are considered for the HDLRBs and CSSBs, including coupled bi-directional motions in the horizontal directions and coupling of vertical and horizontal motions.

Influence of ground motion characteristics on the optimal single concave sliding bearing properties for base-isolated structures

Abstract: This study examines the influence of ground motion characteristics on the optimal friction properties of single concave sliding bearings employed for the seismic isolation of structural systems. The evaluation of the optimal properties is carried out by considering a non-dimensional formulation which employs the peak ground acceleration (PGA) and the peak ground acceleration-to-velocity (PGA/PGV) ratio as ground motion parameters. A two-degree-of-freedom (2dof) model is employed to describe the isolated system and two different families of records representative respectively of near fault and far field seismic inputs are considered. Following the nondimensionalization of the equation of motion for the proposed ground motion parameters, it is shown that the non-dimensional responses obtained for the two types of seismic inputs are similar. This result confirms that PGA/PGV is a good indicator of the frequency content and of other characteristics of ground motion records, helping to reduce the scatter in the response. Regression expressions are also obtained for the optimal values of the friction coefficient that minimizes the superstructure displacements relative to the base as a function of the abovementioned ground motion parameter and of the dimensionless system parameters. These expressions can be used for the preliminary estimation of the optimal properties of isolation bearings with a single concave sliding surface or double concave sliding surfaces with equal friction coefficient.

Long-term prediction of track geometry degradation in high-speed vehicle–ballastless track system due to differential subgrade settlement

Abstract: To predict long-term track degradation of ballastless track due to evolution of differential subgrade settlement in high-speed railway, an iterative approach is put forward. A detailed vehicle–track coupled dynamic model taking account of track weight and local contact loss between track and subgrade is employed to obtain the short-term behavior of the system in terms of wheel–rail interaction, vehicle acceleration and interlaminar forces of track structures. The calculated track–subgrade dynamic stresses induced by a high-speed vehicle are imported into an empirical power model for long-term subgrade settlement. The profile of the subgrade settlement is updated by a self-adaptive passing number of vehicles governed by a settlement threshold, and the dynamic responses of the coupled system are re-calculated consequently. On this basis, a demonstration case is carried out aiming at a typical Chinese high-speed vehicle–double-block ballastless track system with an initial subgrade settlement described by the cosine wave. The attained results reveal the high resistance of deformation on high-speed rail subgrade, together with the significant influence of the initial differential subgrade settlement on track geometrical evolution, in particular, the initial condition with severe unsupported areas between track and subgrade. The abnormal dynamic responses inflicted by the differential settlement are gradually alleviated during the long-term operation.

Comparison of different models for high damping rubber bearings in seismically isolated bridges

Abstract: Steel-reinforced high damping natural rubber (HDNR) bearings are widely employed in seismic isolation applications to protect structures from earthquake excitations. In multi-span simply supported bridges, the HDNR bearings are typically placed in two lines of support, eccentric with respect to the pier axis. This configuration induces a coupled horizontal-vertical response of the bearings, mainly due to the rotation of the pier caps. Although simplified and computationally efficient models are available, which neglect the coupling between the horizontal and vertical response, their accuracy has not been investigated to date. In this paper, the dynamic behaviour and seismic response of a benchmark three-span bridge are analysed by using an advanced HDNR bearing model recently developed and capable of accounting for the coupled horizontal and vertical responses, as well as for significant features of the hysteretic shear response of these isolation devices. The results of the analyses shed light on the importance of the bearing vertical stiffness and how it modifies the seismic performance of isolated bridges. Successively, the seismic response estimates obtained by using simplified bearing models, whose use is well established and also suggested by design codes, are compared against the corresponding estimates obtained by using the advanced bearing model, to evaluate their accuracy for the current design practice.

Metabarriers with multi-mass locally resonating units for broad band Rayleigh waves attenuation

Abstract: Artificial soils engineered with periodic or resonant structures, also referred as “seismic metamaterials” have been investigated for earthquake mitigation applications. In particular, an array of sub-wavelength single-mass resonators buried close the soil surface, namely the metabarrier, has been recently proposed to attenuate the ground motion induced by Rayleigh seismic waves. Here we demonstrate that the use of multi-mass resonators allows for enhanced performances of the metabarrier, in terms of amount and bandwidth of ground motion attenuation, with a smaller array of resonators. To this aim, after reviewing the single-mass metabarrier, we describe the dynamic of a multi-mass metabarrier using analytical and numerical approaches. In particular, we provide a detailed study of a metabarrier with double-mass resonators and compare its performances with those of a single-mass metabarrier with equivalent overall mass. Finally, we exploit Genetic Algorithms to design a metabarrier with multi-mass resonators with minimal mass and able to target selected frequencies. As a case study, the fundamental frequencies of two concrete-frame buildings of known dynamic properties are considered. The example shows the possibility to protect multiple buildings from Rayleigh waves with more compact metabarriers.

Effect of buried depth on seismic response of rectangular underground structures considering the influence of ground loss

Abstract: Great efforts have been made in investigating the effect of buried depth influencing the seismic response of rectangular underground structures, however, a consensus hasn’t been achieved yet. This paper presents numerical studies on seismic behaviors of rectangular underground structures at different buried depths, and aims to illustrate the rule that buried depth effects the seismic response of underground structures. Firstly, to describe the softening and over-consolidated propertied of soils, a 3D elastoplastic constitutive model was developed. Then the stress states of the underground structures and surrounding soils before earthquakes, which were one of the most important issues and generally ignored in the previous studies, were discussed detailedly. Afterwards, three-dimensional numerical models for nonlinear earthquake response simulations of underground structures were built, and the seismic responses of the rectangular structure at various buried depths and under multiple ground motions were simulated. And vertical and horizontal deformations of both the underground structures and surrounding soils were systematically studied. Consequently, a buried depth of the strongest seismic response for underground structures was proposed based on the exploration of the relations between the buried depth and the structural distortions of underground structures as well as the ground subsidence. Finally, pertinence suggestions were proposed for the seismic design of underground structures at different buried depths.

Feasibility of ambient vibration screening by periodic geofoam-filled trenches

Abstract: Single in-filled trench barriers have been widely investigated to mitigate ground vibrations. However, investigations on the application of multiple rows of in-filled trenches are very limited. This paper investigates the attenuation of surface waves by periodic geofoam-filled trenches in single-phased elastic soil deposits. First, a field test of train-induced ground vibration is carried out, from which the corresponding acceleration record and main frequency are obtained. Second, attenuation zones for surface waves in periodic geofoam-filled trenches are studied based on the periodic theory of solid-state physics. Finally, a numerical model for ambient vibration isolation is built under the conditions of plane strain, and the responses are performed both in frequency domain and in time domain by finite element method. The screening effectiveness of the proposed wave barrier is studied by conducting an extensive dimensionless parameter investigation. Results show that the proposed periodic geofoam-filled trenches can attenuate surface waves effectively, when the frequencies of surface waves are located in the attenuation zones. The present study provides a new concept for designing periodic in-filled trench barriers to mitigate ground vibrations.

SPT-based probabilistic and deterministic assessment of seismic soil liquefaction triggering hazard

Abstract: This study serves as an update to the Cetin et al. (2000, 2004) [1,2] databases and presents new liquefaction triggering curves. Compared with these studies from over a decade ago, the resulting new Standard Penetration Test (SPT)-based triggering curves have shifted to slightly higher CSR-levels for a given N1,60,CS for values of N1,60,CS greater than 15 blows/ft, but the correlation curves remain essentially unchanged at N1,60,CS values less than 15 blows/ft. This paper addresses the improved database and the methodologies used for the development of the updated triggering relationships. A companion paper addresses the principal issues that cause differences among three widely used SPT-based liquefaction triggering relationships.

Experimental and numerical investigation on the tensile fatigue properties of rocks using the cyclic flattened Brazilian disc method

Abstract: Rock engineering structures are quite susceptible to cyclic tensile loading. Accurate characterizations of the tensile fatigue properties of rocks are crucial to the long-term stability assessment of rock structures. Using the cyclic flattened Brazilian disc (FBD) testing method, this study experimentally and numerically investigates the tensile fatigue response of brittle rocks to different cyclic loading conditions, involving three loading frequencies, three maximum loads and three amplitudes. Our experimental results systematically reveal the influence of the three cyclic loading parameters on the tensile fatigue properties of FBD specimens, including the fatigue deformation behavior, the tensile fatigue life and the fatigue failure mode. Under higher loading frequency or lower maximum load and amplitude, the FBD specimen is characterized by higher irreversible deformation and higher tensile fatigue life. Nevertheless, the fatigue failure modes of the tested FBD specimens are independent of cyclic loading parameters; all the cyclically failed specimens feature a prominent tensile failure. Furthermore, the progressive fracture behavior of the FBD specimen under representative cyclic tensile loading is numerically assessed via the three-dimensional DEM Code ESyS-Particle. The numerical results reveal that the cyclic FBD testing method indeed guarantees the central crack initiation of the disc specimen, which satisfies the prerequisite for a valid Brazilian-type tensile strength test. After the crack initiation, the central cracks further propagate along the vertical diameter of the FBD specimen, eventually triggering the tensile fatigue failure.

Influence of mechanical layout of inerter systems on seismic mitigation of storage tanks

Abstract: Investigations concerning impact of the mechanical layout of two typical inerter systems on mitigation of seismic response of a base-isolated storage tank are reported. To this end, parameter assessment was first performed to determine impact of the mechanical layout of inerter systems on the sloshing height, isolation displacement, base shear force, and overturning base moment of the storage tank. Conditions favorable for the design suitable of each inerter system were identified based on results of parametric analysis performed as per design guidelines. In addition, the study proposes a demand-oriented optimum design method for determining design parameters of the bearing and inerter systems to meet target performance levels of the storage tank. The proposed method facilitates design of inerter systems with different mechanical layouts. Under the same target level of vibration mitigation in storage tanks, two typical inerter systems have been designed to further explore the impact of inerter-system mechanical layouts. Finally, frequency-domain and time-history analyses were performed on numerical cases of storage tanks with above-designed inerter systems. Analyses results demonstrate that a suitable inerter-system mechanical layout can be selected in accordance with design guidelines prior to design optimization of an inerter system. Using the proposed optimum-design method, inerter systems could be designed to realize target performance levels of the storage tank. In addition, the impact of the inerter-system mechanical layout on mitigation of seismic response of a base-isolated storage tank has been appropriately considered in the proposed demand-oriented optimum-design method.

Linear and nonlinear soil-structure interaction analysis of buildings and safety-related nuclear structures

Abstract: Soil-structure interaction (SSI) analysis is generally a required step in the calculation of seismic demands in nuclear structures, and is currently performed using linear methods in the frequency domain. Such methods should result in accurate predictions of response for low-intensity shaking, but their adequacy for extreme shaking that results in highly nonlinear soil, structure or foundation response is unproven. Nonlinear (time-domain) SSI analysis can be employed for these cases, but is rarely performed due to a lack of experience on the part of analysts, engineers and regulators. A nonlinear, time-domain SSI analysis procedure using a commercial finite-element code is described in the paper. It is benchmarked against the frequency-domain code, SASSI, for linear SSI analysis and low intensity earthquake shaking. Nonlinear analysis using the time-domain finite-element code, LS-DYNA, is described and results are compared with those from equivalent-linear analysis in SASSI for high intensity shaking. The equivalent-linear and nonlinear responses are significantly different. For intense shaking, the nonlinear effects, including gapping, sliding and uplift, are greatest in the immediate vicinity of the soil-structure boundary, and these cannot be captured using equivalent-linear techniques.

Analytical and finite element investigation on the thermo-mechanical coupled response of friction isolators under bidirectional excitation

Abstract: The hysteretic behavior of friction concave isolators is affected by the variability of the friction coefficient experienced during a seismic event. This variability is a combined function of axial load, sliding velocity and temperature rise at the sliding surface, the latter being responsible for significant friction degradation. Experimental testing and corresponding numerical models are usually focused on the monodirectional performance of the friction isolators, although multi-directional paths occur in a real earthquake scenario. In this paper, the thermo-mechanical coupled (TMC) response of friction concave isolators when subjected to bidirectional excitation is investigated in both an analytical and a numerical framework. First, a simplified phenomenological model is presented that accounts for the friction degradation due to the distance traveled via a macroscale cycling variable, based on the assumption of a uniform heat flux at the sliding interface. Then, a more sophisticated numerical investigation is performed via a TMC finite element (FE) model. A customized subroutine has been developed and implemented into the FE code to account for the local variation of the friction coefficient due to the local temperature rise and sliding velocity. The mutual interaction between mechanical and thermal response is incorporated in the proposed computational approach: the friction-induced temperature rise on the contact points and the consequent friction degradation caused by heating phenomena are analyzed as two interconnected phenomena in a recursive fashion. The friction coefficient law at the sliding interface is adjusted step-by-step and is different from node to node on the basis of the temperature distribution. Validated against experimental data, the two proposed models are used within a parametric study to scrutinize some interesting features observed in the thermo-mechanical response of friction isolators.

Liquefaction experiment and analysis projects (LEAP): Summary of observations from the planning phase

Abstract: The LEAP international collaboratory is introduced and its key objectives and main accomplishments during the planning phase of the US-LEAP (LEAP-2015) are presented. The main theme of LEAP-2015 was lateral spreading of sloping liquefiable soils. A summary of the results of the laboratory element tests performed on the selected soil (Ottawa F-65) is presented. The numerical simulations submitted by several predictors at different stages of the project are compared with the measured responses of sloping deposit specimens tested in a rigid box at six different centrifuge facilities around the world. The comparisons are presented for three rounds of simulations labeled here as types A, B, and C simulations. The type A simulations involved the response of the soil specimen to a prescribed base excitation with a maximum amplitude of 0.15g (Motion #2). Comparisons of the numerical simulations with the experimental results show that a sub-set of type A simulations were in reasonably good agreement with the responses measured in the reference centrifuge experiment. The predictors subsequently assessed the performance of their type A simulations by comparing them to the measured responses, made the necessary adjustments in their models, and conducted a type B simulation of the response of the same soil specimen subjected to an amplified base excitation with a maximum amplitude of 0.25g (Motion #4). In these type B simulations, the achieved base motions were used and the simulations showed an improved correlation with the experimental results. The predictors also conducted a type C simulation of the original test (Motion #2) using the base motions achieved on the six centrifuge facilities. The results showed very good agreement with the experimental results.

Effects of strain rate on the mechanical and fracturing behaviors of rock-like specimens containing two unparallel fissures under uniaxial compression

Abstract: Rocks containing unparallel fissures are likely to be subjected to dynamic loading resulting from earthquakes in various civil engineering structures. Since dynamic loading rate significantly affects the mechanical behaviors of fissured rocks, understanding the mechanical properties and fracture mechanism of fissured rocks under different loading rates is thus crucial in rock engineering applications. This study investigates the effects of strain rate on the mechanical and fracturing behaviors of rock-like specimens containing two unparallel fissures with varying inclination angles (α2). Our experimental results demonstrate that α2 and strain rate significantly affect the strength and deformation characteristics and the failure modes of fissured specimens. Both the strength and elastic modulus of the rock-like specimens highly depend on the strain rates, and the strain rate dependence is more sensitive for fissured specimen than that for the intact specimen. Under the same strain rate, the strength of fissured specimens decreases as α2 increases up to 60°, and then increases. The fissured specimens with greater α2 are characterized by higher elastic modulus. Furthermore, the fracture mechanism of the fissured specimen is numerically revealed, and the energy characteristics of fissured models are also analyzed on a micro-level.

Finite element model of ballasted railway with infinite boundaries considering effects of moving train loads and Rayleigh waves

Abstract: This paper proposes a three-dimensional model incorporating finite element (FE) meshes with infinite element (IE) boundaries for ballasted railways. Moving train loads are simulated with sliding motions of moving elements which have hard contact feature at the interface with supporting rails. Dynamic responses of ballasted railway under different train speeds are investigated in time domain and frequency domain to identify the predominant frequency and critical speed. Rayleigh wave (R-Wave) propagation is simulated using the combined FE-IE model to determine the velocity of R-Wave in the layered embankment model and its relationship with the critical speed of the ballasted railway. The proposed model is successfully validated against the results of Euler-Bernoulli Elastic Beam (E-BEB) model.

Optimum Tuned Mass Dampers under seismic Soil-Structure Interaction

Abstract: Tuned Mass Damper (TMD) devices are widely adopted as a valid mechanical solution for the vibration mitigation of structural systems and buildings under dynamic excitation. In the specific challenging context of seismic engineering, TMDs may represent a convenient option for both aseismic structural design and seismic retrofitting. However, the expectable efficiency rate of TMDs in that context is still debated. Besides, potential Soil-Structure Interaction (SSI) effects may become crucial in the mechanical system, and should properly be taken into account for the optimum TMD design, in order to avoid possible de-tuning. This work contributes to this framework, by investigating the effectiveness of an optimum TMD in reducing the linear structural response to strong-motion earthquakes of a given set of Multi-Degree-Of-Freedom (MDOF) low- and high-rise shear-type frame structures, by embedding SSI within the dynamic and TMD optimisation model. The TMD is seismically tuned through a dedicated two-variable optimisation procedure, for each specific case (primary structure, seismic event and soil type), therefore providing the optimum device setting for each given context. Average primary structure response indices are specifically targeted to that purpose, while maximum ones are monitored. A quite considerable range of optimisation cases is considered (eighty instances), to outline rather general considerations and average trends on TMD optimisation and effectiveness within the seismic SSI framework, for both low- and high-rise buildings. Such an investigation shall provide useful guidelines for a comprehensive tuning of TMDs in mechanical systems and specifically in the presence of seismic SSI, to be consulted in view of real-case applications.

Parametric models for 3D topographic amplification of ground motions considering subsurface soils

Abstract: Amplification of seismic waves due to surface topography and subsurface soils has often been observed to cause intensive damage in past earthquakes. Due to its complexity, topographic amplification has not yet been considered in most seismic design codes. In this study, we simulate ground-motion amplification based on 3D Spectral Element Method, using Hong Kong Island as a local testbed site. The analyses revealed that topographi c amplification of ground motions is frequency-dependent. If the site is made of homogenous rock, the amplification factor is best correlated with the curvature smoothed over a characteristic length equal to half of the wavelength in rock. Amplification of high frequency wave is correlated with small-scale features, and amplification of long-period waves is correlated with large-scale features in horizontal dimension. The maximum topography amplification generally ranges from 1.6 to 2.0 on the protruded areas. When a low-velocity subsurface soil layer is considered, the topographic amplification pattern is significantly influenced by the thickness of the soil layer, as wavelength in soil is relatively short. The characteristic length reduces as soil t h i c k n e s s increases, and the amplification pattern becomes closely correlated to smaller-scale topographic features as well as slope angles. Results also show that the effect of material damping can be decoupled from the topographic effects and modeled using a theoretical attenuation factor. The study proposed parametric models to predict 3 D topographic amplification u s i n g s i m p l e p r o x i e s considering subsurface soils, material damping and input wave frequencies, which gives accurate results with a standard deviation of residuals within 0.1–0.15.