Evangelia Garini

Effects of Near-Fault Ground Shaking on Sliding Systems

A numerical study is presented for a rigid block supported through a frictional contact surface on a horizontal or an inclined plane, and subjected to horizontal or slope-parallel excitation. The latter is described with idealized pulses and near-fault seismic records strongly influenced by forward-directivity or fling-step effects (from Northridge, Kobe, Kocaeli, Chi-Chi, Aegion). In addition to the well known dependence of the resulting block slippage on variables such as the peak base velocity, the peak base acceleration, and the critical acceleration ratio, our study has consistently and repeatedly revealed a profound sensitivity of both maximum and residual slippage: (1) on the sequence and even the details of the pulses contained in the excitation and (2) on the direction (+ or - ) in which the shaking of the inclined plane is imposed. By contrast, the slippage is not affected to any measurable degree by even the strongest vertical components of the accelerograms. Moreover, the slippage from a specific record may often be poorly correlated with its Arias intensity. These findings may contradict some of the prevailing beliefs that emanate from statistical correlation studies. The upper-bound sliding displacements from near-fault excitations may substantially exceed the values obtained from some of the currently available design charts.

International Benchmark on Numerical Simulations for 1D, Nonlinear Site Response (PRENOLIN): Verification Phase Based on Canonical Cases

PREdiction of NOn‐LINear soil behavior (PRENOLIN) is an international benchmark aiming to test multiple numerical simulation codes that are capable of predicting nonlinear seismic site response with various constitutive models. One of the objectives of this project is the assessment of the uncertainties associated with nonlinear simulation of 1D site effects. A first verification phase (i.e., comparison between numerical codes on simple idealistic cases) will be followed by a validation phase, comparing the predictions of such numerical estimations with actual strong‐motion recordings obtained at well‐known sites. The benchmark presently involves 21 teams and 23 different computational codes. We present here the main results of the verification phase dealing with simple cases. Three different idealized soil profiles were tested over a wide range of shear strains with different input motions and different boundary conditions at the sediment/bedrock interface. A first iteration focusing on the elastic and viscoelastic cases was proved to be useful to ensure a common understanding and to identify numerical issues before pursuing the nonlinear modeling. Besides minor mistakes in the implementation of input parameters and output units, the initial discrepancies between the numerical results can be attributed to (1) different understanding of the expression “input motion” in different communities, and (2) different implementations of material damping and possible numerical energy dissipation. The second round of computations thus allowed a convergence of all teams to the Haskell–Thomson analytical solution in elastic and viscoelastic cases. For nonlinear computations, we investigate the epistemic uncertainties related only to wave propagation modeling using different nonlinear constitutive models. Such epistemic uncertainties are shown to increase with the strain level and to reach values around 0.2 (log_(10) scale) for a peak ground acceleration of 5  m/s^2 at the base of the soil column, which may be reduced by almost 50% when the various constitutive models used the same shear strength and damping implementation.

Damage potential of near-fault records: sliding displacement against conventional “Intensity Measures”

The potential of a particular ground accelerogram to inflict damage to asymmetric strongly-inelastic systems is studied in the paper. An idealised analogue, the rigid block with frictional contact on an inclined base, is adopted as the generic representation of such systems. The inclined base (of (a sufficiently steep) angle) is shaken with numerous strong records bearing the effects of forward-directivity and/or fling-step. The accumulated slippage, D, of the block caused by each record is taken as the induced “damage” to the system. The relevance of a variety of ‘Intensity Measures’ of each accelerogram (ranging from PGA and PGV to Housner’s and Arias’ Intensities) in predicting this damage, is investigated statistically. It is shown that only a few of these ‘Intensity Measures’ are reasonably successful and their use could therefore be recommended, but only for statistical inference. A detailed deterministic analysis presented in the paper for one of these successful measures, Arias Intensity, reveals the unacceptably poor predictive power of this measure. Upper-bound curves of slippage provided in closed-form expressions, are an improvement over the state-of-practice Makdisi & Seed diagrams.

Evaluation of seismic hazard for the assessment of historical elements at risk: description of input and selection of intensity measures

The assessment of historical elements at risk from earthquake loading presents a number of differences from the seismic evaluation of modern structures, for design or retrofitting purposes, which is covered by existing building codes, and for the development of fragility curves, procedures for which have been extensively developed in the past decade. This article briefly discusses: the hazard framework for historical assets, including a consideration of the appropriate return period to be used for such elements at risk; the intensity measures that could be used to describe earthquake shaking for the analysis of historical assets; and available approaches for their assessment. We then discuss various unique aspects of historical assets that mean the characterisation of earthquake loading must be different from that for modern structures. For example, historical buildings are often composed of heterogeneous materials (e.g., old masonry) and they are sometimes located where strong local site effects occur due to: steep topography (e.g., hilltops), basin effects or foundations built on the remains of previous structures. Standard seismic hazard assessment undertaken for modern structures and the majority of sites is generally not appropriate. Within the PERPETUATE project performance-based assessments, using nonlinear static and dynamic analyses for the evaluation of structural response of historical assets, were undertaken. The steps outlined in this article are important for input to these assessments.

Elastic and inelastic systems under near-fault seismic shaking: acceleration records versus optimally-fitted wavelets

Four idealised dynamic systems, which are used as analogues in earthquake and geotechnical engineering, are studied: an elastic single-degree-of-freedom (sdof) oscillator; an elastic–perfectly-plastic sdof oscillator; a rigid block resting in simple frictional contact on a horizontal base; and a rigid block resting on a sloping plane. They are subjected to several near-fault-recorded ground motions bearing the effects of ‘forward-rupture directivity’ and fault surface dislocation (‘fling-step’) phenomena—long-period acceleration pulses and large velocity or displacement steps. Two types of idealized wavelets (the Mavroeidis & Papageorgiou and the Ricker wavelets) are optimally-fitted to each record, applying the matching procedure presented by Vassiliou and Makris (Bull Seismol Soc Am 101(2):596–618, 2011). Extensive comparisons between the accelerogram response and the corresponding-fitted wavelets response show if and when the destructive pulse-like part of the records is indeed their most deleterious component, and if and when this destructiveness can be captured with the particular fitted wavelets. For the two purely inelastic systems, in particular, the comparison elucidates the role of the contained pulses in the size of sliding displacements. The results reveal that while the response of elastic and elasto-plastic sdof systems to the wavelets is usually reasonably similar with the response to the actual records, this is not usually the case for the two purely inelastic (sliding) systems. The unpredictable consequences of seismic shaking on such systems, even if the shaking intensity and frequency content were precisely known, is best demonstrated with the sensitivity of the size of sliding displacement to the polarity (+ or

Seismic response of slender rigid structures with foundation uplifting

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Sliding and overturning potential of Christchurch 2011 earthquake records

-), the sequence and number of cycles, and even the details of the excitation.

Geotechnical design with apparent seismic safety factors well-bellow 1

This article presents the main results of the validation phase of the PRENOLIN project. PRENOLIN is an international benchmark on 1D nonlinear (NL) site‐response analysis. This project involved 19 teams with 23 different codes tested. It was divided into two phases; with the first phase verifying the numerical solution of these codes on idealized soil profiles using simple signals and real seismic records. The second phase described in this article referred to code validation for the analysis of real instrumented sites. This validation phase was performed on two sites (KSRH10 and Sendai) of the Japanese strong‐motion networks KiK‐net and Port and Airport Research Institute (PARI), respectively, with a pair of accelerometers at surface and depth. Extensive additional site characterizations were performed at both sites involving in situ and laboratory measurements of the soil properties. At each site, sets of input motions were selected to represent different peak ground acceleration (PGA) and frequency content. It was found that the code‐to‐code variability given by the standard deviation of the computed surface‐response spectra is around 0.1 (in log10 scale) regardless of the site and input motions. This indicates a quite large influence of the numerical methods on site‐effect assessment and more generally on seismic hazard. Besides, it was observed that site‐specific measurements are of primary importance for defining the input data in site‐response analysis. The NL parameters obtained from the laboratory measurements should be compared with curves coming from the literature. Finally, the lessons learned from this exercise are synthesized, resulting also in a few recommendations for future benchmarking studies, and the use of 1D NL, total stress site‐response analysis.