Earthquake Engineering Practice Research Articles

Probabilistic generation of hazard‐consistent suites of fully non‐stationary seismic records

AbstractA novel, practical, and computationally efficient probabilistic methodology for the stochastic generation of suites of fully non‐stationary artificial accelerograms is presented. The proposed methodology ensures that the produced ground motion suites match a given target spectral mean and target variability for the whole period range of interest. This is achieved by first producing an ensemble of random target spectra with the given mean and variability and then using them to generate artificial, target spectrum‐compatible, acceleration time‐histories with spectral representation techniques. Spectral correlation can also be assumed for the generated ground motion spectra. Based on the same backbone, two different formulations are proposed for generating spectrum‐compatible acceleration time‐histories of the non‐stationary kind. The distinction between these two variants lies in the techniques employed for modeling the temporal and spectral modulation, focusing on the site‐compatibility of the produced records. The first approach uses past‐recorded seismic accelerograms as seed records, and the second proposes and uses a new, probabilistic time‐frequency modulating function. The outcome of the proposed methodology is suites containing site‐compatible ground motion time‐histories whose spectral mean and variability match those obtained from any of the usually employed target spectra used in the earthquake engineering practice. An online tool implementing the proposed methodology is also freely provided.

Evaluation and updating of Ishihara’s (1985) model for liquefaction surface expression, with insights from machine and deep learning

Liquefaction surface-manifestation is a popular proxy of damage potential for infrastructure. Models for predicting it are thus commonly used, and often codified, in earthquake engineering practice. One such model is that of Ishihara (1985) who proposed empirical “H1–H2” curves considering the influence of the non-liquefied crust on surface expression. Yet, while widely used and cited, these curves were trained on just ∼300 data points from two earthquakes. Accordingly, this study evaluates and updates the Ishihara (1985) model using 14,400 data points from 24 earthquakes, while also comparing against three other manifestation models from the literature. In addition to retraining the H1–H2 model via traditional regression, new variants are developed via machine- and deep-learning. Each of the new H1–H2 models outperforms the original in unbiased testing and is suitable for application. Ultimately, however, this paper also explores the limits of H1–H2 models and the apparent inefficiency and/or insufficiency of their predictor variables. In this regard, the models developed herein may perform better than any other, yet new models are still needed to account for factors influential in producing surface manifestation in a more explicit and mechanistic manner.

Re-evaluation of New Zealand seismic hazard for geotechnical assessment and design

This paper scrutinises the seismic hazard of New Zealand (NZ) from a geotechnical engineering perspective. The two codified versions of the seismic hazard in NZS1170.5 (structural loading standard) and NZTA Bridge Manual (NZTA-BM) are shown to yield consistently different peak ground acceleration (PGA) hazards throughout NZ. Results from site-specific PSHA for 24 locations in NZ are used to examine key hazard characteristics, including earthquake magnitude and the effects of site conditions (classes) on the PGA hazard. The comparative evaluations show that for most of the locations considered, NZS1170.5 and NZTA-BM overestimate the PGA hazard. However, NZS1170.5, and NZTA-BM in particular, significantly underestimate the PGA hazard for locations that are at a short source-to-site distance from the Hikurangi Subduction Zone (HSZ), and for which HSZ significantly contributes to their hazard. Using the results from this study, an interim PGA hazard is recommended for geotechnical assessment and design in support of the NZ guidelines for geotechnical earthquake engineering practice. The recommended interim PGA hazard is applicable to all site classes without any modification or use of site amplification factors.

Estimation uncertainty for some common seismic fragility curve fitting methods

This technical note illustrates and makes available some simple procedures to assess the estimation uncertainty for the parameters of seismic fragility curves. The considered fragility fitting methods refer to the lognormal assumption and are supposed to be based on the results of multi-stripe dynamic analysis of a deterministic non-linear structural model, so that the uncertainty in the fragility parameters arises from the so-called record-to-record variability. The discussed procedures are based on the statistics approach of resampling with substitution, which is commonly referred to as bootstrap. It is also briefly discussed how the estimation uncertainty depends on the maximum value of the probability of failure given seismic intensity that is observed from structural analysis. This work may aid earthquake engineering practice because, both the curve fitting and estimation uncertainty algorithms are implemented in a major update of an application-ready software tool made available at https://www.reluis.it/it/progettazione/software/r2r-eu.html.

How much are sites affected by 2-D and 3-D site effects? A study based on single-station earthquake records and implications for ground motion modelling

SUMMARY1-D site response analysis dominates earthquake engineering practice, while local 2-D/3-D models are often required at sites where the site response is complex. For such sites, the 1-D representation of the soil column can account neither for topographic effects or dipping layers nor for locally generated horizontally propagating surface waves. It then remains a crucial task to identify whether the site response can be modelled sufficiently precisely by 1-D analysis. In this study we develop a method to classify sites according to their 1-D or 2-D/3-D nature. This classification scheme is based on the analysis of surface earthquake recordings and the evaluation of the variability and similarity of the horizontal Fourier spectra. The taxonomy is focused on capturing significant directional dependencies and interevent variabilities indicating a more probable 2-D/3-D structure around the site causing the ground motion to be more variable. While no significant correlation of the 1-D/3-D site index with environmental parameters and site proxies seems to exist, a reduction in the within-site (single-station) variability is found. The reduction is largest (up to 20 per cent) for purely 1-D sites. Although the taxonomy system is developed using surface stations of the KiK-net network in Japan as considerable additional information is available, it can also be applied to any (non-downhole array) site.

New conditional, scenario-based, and non-conditional cumulative absolute velocity models for subduction tectonic settings

The PEER NGA-Sub ground-motion intensity measure database is used to develop new conditional ground-motion models (CGMMs), a set of scenario-based models, and non-conditional models to estimate the cumulative absolute velocity ([Formula: see text]) of ground motions from subduction zone earthquakes. In the CGMMs, the median estimate of [Formula: see text] is conditioned on the estimated peak ground acceleration ([Formula: see text]), the time-averaged shear-wave velocity in the top 30 m of the soil ([Formula: see text]), the earthquake magnitude ([Formula: see text]), and the spectral acceleration at the period of 1 s ([Formula: see text]). Multiple scenario-based [Formula: see text] models are developed by combining the CGMMs with pseudo-spectral acceleration ([Formula: see text]) ground-motion models (GMMs) for [Formula: see text] and [Formula: see text] to directly estimate [Formula: see text] given an earthquake scenario and site conditions. Scenario-based [Formula: see text] models are capable of capturing the complex ground-motion effects (e.g. soil non-linearity and regionalization effects) included in their underlying [Formula: see text]/[Formula: see text] GMMs. This approach also ensures the consistency of the [Formula: see text] estimates with a [Formula: see text] design spectrum. In addition, two non-conditional [Formula: see text] GMMs are developed using Bayesian hierarchical regressions. Finally, we present comparisons between the developed models. The comparisons show that if non-conditional GMMs are properly constrained, they are consistent with scenario-based GMMs. The [Formula: see text] GMMs developed in this study advance the performance-based earthquake engineering practice in areas affected by subduction zone earthquakes.

Site-Response Analysis Using the Shear-Wave Velocity Profile Correction Approach

ABSTRACT The traditional approach used to incorporate site response into the ground-motion hazard analysis is to compute a design spectrum for a rock-site condition and then propagate the rock motion from the base of the soil model to the surface. The main limitation with this approach is that it can be inconsistent with the ground-motion models (GMMs) used to develop the input rock motion. The VS profile implicit in the GMMs is unlikely to match the site-specific VS profile (value and gradient), because the GMMs were developed for ground motions from different VS profiles over large regions and are unlikely to match the profile of any one site well. This article presents the VS profile correction method for developing surface ground motions as an alternative to the soil-over-rock approach routinely used in earthquake engineering practice. This approach is similar to the standard soil-over-rock analysis, but uses different input motions and involves performing two site response analyses—one for the generic profile associated with the GMM(s) and one for the site-specific profile—then applying the ratio of the two site response analysis results to correct the design spectrum for the reference site condition developed using the GMMs. Two example applications are included to illustrate the VS profile correction methodology as well as some of the challenges that may arise when doing so.

Influence of fines content on liquefaction from a critical state framework: the Christchurch earthquake case study

In earthquake engineering practice, the liquefaction potential of soils is commonly evaluated through simplified procedures. These approaches are suitable for sands with very low to no fines content, which have been traditionally thought to be the only liquefiable materials. However, field observations and experimental research have extensively demonstrated that low plasticity silty sands can also be highly liquefiable. Thus, this paper investigates the effect of nonplastic fines contents on the liquefaction potential of soils, taking the 2010–2011 Canterbury Earthquake Sequence as a case study. The validity of standard simplified procedures for high fines content soils is critically evaluated and compared with a finite element model based on a full solid–fluid coupled formulation. The model includes a state parameter-based constitutive law within the generalised plasticity theory, which allows the fines content to be taken into account explicitly. The standard simplified procedures are shown to be less effective in the evaluation of liquefaction potential in soils with high fines content but are still indispensable tools for evaluating the performance of soils over large urban areas. As the main conclusion, it is recommended that empirical models are complemented with an advanced numerical analysis in those cases where silty sands with high fines content are identified, as its outcomes can more realistically represent the soil behaviour during a seismic event.

Accessing European Strong-Motion Data: An Update on ORFEUS Coordinated Services

AbstractStrong ground motion records and free open access to strong-motion data repositories are fundamental inputs to seismology, engineering seismology, soil dynamics, and earthquake engineering science and practice. This article presents the current status and outlook of the Observatories and Research Facilities for European Seismology (ORFEUS) coordinated strong-motion seismology services, namely the rapid raw strong-motion (RRSM) and the engineering strong-motion (ESM) databases and associated web interfaces and webservices. We compare and discuss the role and use of these two systems using the Mw 6.5 Norcia (Central Italy) earthquake that occurred on 30 October 2016 as an example of a well-recorded earthquake that triggered major interest in the seismological and earthquake engineering communities. The RRSM is a fully automated system for rapid dissemination of earthquake shaking information, whereas the ESM provides quality-checked, manually processed waveforms and reviewed earthquake information. The RRSM uses only data from the European Integrated Waveform Data Archive, whereas the ESM also includes offline data from other sources, such as the ITalian ACcelerometric Archive (ITACA). Advanced software tools are also included in the ESM to allow users to process strong-motion data and to select ground-motion waveform sets for seismic structural analyses. The RRSM and ESM are complementary services designed for a variety of possible stakeholders, ranging from scientists to the educated general public. The RRSM and ESM are developed, organized, and reviewed by selected members of the seismological community in Europe, including strong-motion data providers and expert users. Global access and usage of the data is encouraged. The ESM is presently the reference database for harmonized seismic hazard and risk studies in Europe. ORFEUS strong-motion data are open, “Findable, Accessible, Interoperable, and Reusable,” and accompanied by licensing information. The users are encouraged to properly cite the data providers, using the digital object identifiers of the seismic networks.

Seismic behavior & risk assessment of an existing bridge considering soil-structure interaction using artificial neural networks

The Peak Ground Acceleration (PGA) is extensively used in earthquake engineering practice to describe the ground motion characteristics for establishing the seismic vulnerability curves. However, a single parameter is not enough to describe the seismic excitation and does not allow expressing the complex relationship between the structural damage and the ground movement. Motivated to overcome these shortcomings, several ANN-based models were proposed to predict the seismic structural damage using other parameters. Unfortunately, not all include soil structure interaction. This paper aims to explore the predictive power of an ANN-based approach to reproduce the nonlinear dynamic behavior taking into account the various ground motion intensities, the variability of soil, and SSI. The basic strategy is to train a neural network by a numerical database obtained from a FEM model. This numerical model is further validated by experimental results. An optimum prediction for a nonlinear dynamic response is achieved using Artificial Neural Networks. Finally, fragility curves were established considering SSI for three different soil classes. Results revealed the importance of considering SSI effects on the evaluation of seismic structural damage and risk assessment analysis.

High-performance resilient earthquake-resisting moment frames

The need for resilient or sustainable seismic design is at the forefront of structural engineering challenges worldwide. Sustainable seismic design is a relatively new concept that is rapidly gaining interest and hints at the arrival of the next generation earthquake engineering practice. In the present context ‘sustainable seismic’ refers to structural operability with a view to post-earthquake realignment and repairs. Experience has shown that it is cost inhibitive to elevate the status of conventional earthquake-resistant structures to seismic sustainability by means of traditional methods of design and construction. This paper introduces two simple analytic concepts, performance control and design led analysis, that lead to the development of sustainable seismic designs for purpose-specific archetypes. Seismic energy control, global stiffness reduction and restoring force adjustment are introduced as relatively simple methodologies that help achieve efficient post-earthquake realignment and repairs. Combinations of structures of uniform response and rigid rocking cores are used as ideal models for sustainable seismic design. Three simple technologies, the replaceable energy dissipating moment connection, the energy-dissipating grade beams and the hybrid rocking–stepping core, are also introduced. Several parametric examples are provided to demonstrate the applications of the proposed methodologies.

Discrete-analytical nonlinear analysis with improved computation accuracy for steel frame lateral response evaluation

The structural performance evaluation under the monotonic increase of a lateral load due the whole loading process in modern earthquake engineering practice can be performed with the help of different methods and software and can be dramatically represented by «Base reaction – Roof displacement» relationship, which is also known as capacity curve. This kind of relationship is used by nonlinear static Pushover-analysis for seismic performance evaluation of buildings considering any possible damage, which is acceptable by serviceability requirements of a building. However, there are many problems arising due to physically nonlinear analysis of structure, primarily associated with features of the employing finite elements analysis software. Main problems are rather long analysis time among the necessity of high performance computing, or shortcomings of nonlinear analysis methods inherent to the program used. One of the most important shortcomings of structural analysis software is the necessity of single-handed assignment of a failure mechanism for the structure by nonlinear hinges formation zones emplacement to elements of analytical model. This circumstance may lead to results substantially different from reality. These problems can be resolved with the aid of discrete-analytical methods.

A CPT-based effective stress analysis procedure for liquefaction assessment

There is a growing awareness of the need for the earthquake engineering practice to incorporate in addition to empirical approaches in evaluation of liquefaction hazards advanced methods which can more realistically represent soil behaviour during earthquakes. Currently, this implementation is hindered by a number of challenges mainly associated with the amount of data and user-experience required for such advanced methods. In this paper, we present key steps of an advanced seismic effective-stress analysis procedure, which on the one hand can be fully automated and, on the other hand, requires no additional input (at least for preliminary applications) compared to simplified cone penetration test (CPT)-based liquefaction procedures. In this way, effective-stress analysis can be routinely applied for quick, yet more robust estimations of liquefaction hazards, in a similar fashion to the simplified procedures. Important insights regarding the dynamic interactions in liquefying soils and the actual system response of a deposit can be gained from such analyses, as illustrated with the application to two sites from Christchurch, New Zealand.

An iterative linear procedure using frequency-dependent soil parameters for site response analyses

Local site amplification predictions that derive from the traditional equivalent linear (EL) procedure are deemed to be reliable when weak shaking is considered, while it is well recognized that significant discrepancies can occur for stronger earthquakes where soil nonlinearity dominates the site response. To better capture nonlinearity effects, frequency-dependent equivalent linear methods, commonly referred to as FDEL methods, were proposed by modifying the EL procedure with frequency-dependent soil parameters. However, these methods have never been widely used in earthquake engineering practice. Existing FDEL methods rely on generic spectral shape fitting parameters to approximate the variation of shear strain amplitudes in the frequency domain, which can lead to substantial inconsistencies in the model prediction when applied to a particular seismic event, with a specific signature. This study investigates a range of spectral seismic intensities to identify a proxy that could be used directly to update the shear strain spectra in accordance with the simulated ground motions. A new FDEL algorithm to compute effective shear strain as a function of frequency is proposed and tested on various scenarios. When simulating strong ground motions that exhibit higher levels of soil deformation, the frequency content of ground motions processed within the FDEL procedure is substantially improved as compared to the EL method, with results more consistent with those from NL analyses.

Areal exceedance of ground motion as a characteristic of multiple-site seismic hazard: Sensitivity analysis

Probabilistic seismic hazard assessment (PSHA) is widely used in earthquake engineering practice and seismic risk management. However, the results of classical point-wise PSHA contain no information about simultaneous ground motions during one earthquake at different sites located within a defined area. Such multiple-site estimations are very useful for assessment of damage and corresponding losses in densely populated areas and for analysis of lifeline performance. Recently Wenzel and Sokolov (2018) introduced parameter that describes probability that specified ground motion level (e.g. design ground motion) will be exceeded at least once in at least fraction of the reference area during specified period of time. The parameter was called as Fraction of Areal Exceedance (FAE). The estimations of FAE may be used both for the risk management decisions and for selection of appropriate design ground motion. This article describes the sensitivity analysis of FAE that includes consideration of various input characteristics that may influence on the results.

Experimental and numerical assessment of the seismic response of steel structures with clutched inerters

Supplemental rotational inertia devices provide an efficient means of suppressing ground-induced vibrations over a large range of structural periods. The beneficial effects of the inerter can be further enhanced by coupling it with a clutch system that prevents it from driving the structural response and ensures that its supplemental rotational inertia is only employed to resist the motion. In this paper, we examine the behaviour of single-degree-of-freedom and multi-degree-of-freedom structures equipped with twin inerter-clutch devices subjected to strong ground-motion. The influence of the clutch stiffness, gears play, viscous damping and dry friction, on the non-linear dynamics of the system are explored first, by analysing the stable periodic solutions of a structure with inerters under harmonic-sweeps. We demonstrate that, for the range of parameters typically expected in earthquake engineering practice, the influence of dry-friction and clutch damping are limited, although the clutch stiffness and gear play may need to be accounted for when large inertances or defective clutches are considered. Based on these findings, we propose a simplified numerical modelling strategy suitable for implementation in conventional Finite Element simulations. Small scale experiments on bare elastic structures as well as structures equipped with 3D-printed inerter and inerter-clutch twins are presented and employed for concept demonstration and for the validation of the numerical model proposed. Finally, a series of studies on detailed numerical models of multi-storey steel frames under idealized and real pulse-like ground-motions are used to demonstrate the vibration absorbing capabilities brought about by the twin inerter-clutch system and to highlight practical aspects related to their structural implementation.

Seismic Assessment of the Lima Cathedral Bell Towers via Kinematic and Nonlinear Static Pushover Analyses

ABSTRACTThe protection of cultural heritage against earthquake induced actions is one of the main challenges the earthquake engineering science and practice are facing. This article presents a seismic assessment study on one of the most ancient colonial buildings present in Peru, the Cathedral of Lima, focusing on its towers. A historical review highlighted how these structures, together with the whole Cathedral, suffered intense damage and partial collapse during previous earthquakes. In order to identify the structure main deficiencies, both linear kinematic analyses and nonlinear static analyses have been performed. Different nonlinear finite element models have been created to evaluate the influence of the adjacent walls. Different load distributions have been compared to evaluate how simplified patterns could provide results close to load distributions taken from a modal analysis of the complex. A simple retrofit strategy, consisting on the introduction of steel ties, has also been studied as a reference. Results show good correlation between kinematic and pushover analyses. The construction, when compared to the requirements of the national code for new buildings, results significantly vulnerable, pointing out the need to accept some structural damage even after seismic retrofit.

Performance evaluation of base-isolated structures with sliding hydromagnetic bearings

Being an efficient technique for mitigating the seismic response of structures, base isolation is widely used in earthquake engineering practice. The sliding hydromagnetic bearing is viewed as a promising isolation system for seismic protection, owing to its adaptive damping energy dissipation, mild deformation constraint, and easy assembly and maintenance. To verify the applicability and efficiency of sliding hydromagnetic bearings, a performance evaluation of base-isolated structures with a sliding hydromagnetic base-isolation system is carried out in this paper. Numerical simulations are performed to explore the dynamic behaviors of sliding hydromagnetic bearings fabricated recently, for which two models, the power function and polynomial models, are proposed. Parameter identification and model validation are carried out using the data from numerical simulations and quasi-static tests. Backward differential formulas are employed to solve the stiff differential equation invoked by the Coulomb friction associated with the models, which bypasses the computational challenges inherent in conventional integral schemes. For a real-time definition of the instantaneous frequency in the models, the Hilbert–Huang transform is utilized. Comparative studies reveal that the sliding hydromagnetic bearing has an advantage over the lead rubber bearing and the curved surface slider in the control of structural displacement, structural velocity, and structural acceleration. Furthermore, the residual displacement yielded in the sliding hydromagnetic base-isolation system merely shifts the starting position of the bearing on the next movement, and the sliding surface and deformation constraints remain approximately as the initial condition.

On the complexity of seismic waves trapped in irregular topographies

Documented observations from strong seismic events have repeatedly shown that the ground surface topography significantly affects the characteristics of seismic waves (amplitude, frequency and duration) travelling from the deeper layers of the crust compared to what the ground motion would have been on the surface of a flat homogeneous linear elastic half-space. Although numerous theoretical studies have qualitatively corroborated these observations, they systematically underestimate the absolute level of topographic amplification up to an order of magnitude or more in some cases. In this paper, we try to bridge the quantitative gap between previous theoretical studies and observations by systematically studying the role of geometry, stratigraphy, and ground motion characteristics through a series of elaborate numerical analyses. We show a collection of examples that highlight the effects of topography on seismic ground shaking, and we point out what these results suggest in the context of the current state of earthquake engineering practice. Examples range from semi-analytical solutions of wave propagation in infinite wedges to three-dimensional numerical simulations of topography effects using digital elevation map-generated models and layered geologic features. We conclude by demonstrating that topography effects vary strongly with the stratigraphy and material properties of the underlying geologic materials, and thus it cannot be accurately predicted by studying the effects of ground surface geometry alone.

Analytical expressions for determining the stability of cohesionless soil slope under generalized seismic conditions

In recent major earthquakes, the researchers have found the need for consideration of vertical seismic acceleration for the stability analysis of the man-made and natural slopes. However, in most past studies, the performance of slopes has been assessed by accounting only the horizontal seismic component of the ground motion, without giving due weightage to the effect of vertical component. In the present study, analytical expressions are derived to determine the factor of safety, yield seismic coefficient and consequently the seismic displacement of cohesionless soil slope under combined horizontal and vertical components of the ground motion. The derivation uses the Newmark’s sliding block approach, in which the soil slope with a planar failure surface within the framework of conventional pseudo-static analysis is assumed to follow the Mohr-Coulomb failure criterion. The effects of vertical seismic coefficient on the stability of cohesionless slope have been studied through a set of graphical presentations for a specific range of soil parameters. It is observed that overlooking the effect of the vertical component of the ground motion on factor of safety and the displacement while designing the slope may be detrimental, resulting in the slope failure. The general expressions presented in this paper may be highly useful in the field of earthquake geotechnical engineering practice for designing the cohesionless soil slopes under combined horizontal and vertical seismic loads.