Seismic vulnerability of offshore wind turbines to pulse and non-pulse records

The increasing number of wind turbines in active tectonic regions has attracted scientific interest to evaluate the seismic vulnerability of offshore wind turbines (OWTs). This study aims at assessing the deformation and collapse susceptibility of 2MW and 5MW OWTs subjected to shallow-crustal pulse-like ground motions, which has not been particularly addressed to date. A cloud-based fragility assessment is performed to quantify the seismic response for a given intensity measure and to assess the failure probabilities for pulse-like and non-pulse-like ground motions. The first-mode spectral acceleration S_a(T₁) is found to be an efficient response predictor for OWTs, exhibiting prominent higher-mode behavior, at the serviceability and ultimate conditions. Regardless of earthquake type, it is shown that records with strong vertical components may induce nonlinearity in the supporting tower, leading to potential failure by buckling in three different patterns: (i) at tower base near platform level, (ii) close to tower top, and (iii) between the upper half of the main tower and its top. Type and extent of the damage are related to the coupled excitation of vertical and lateral higher modes, for which tower top acceleration response spectra S_a,i(Top) is an effective identifier. It is also observed that tower's slenderness ratio (l/d), the diameter-to-thickness ratio (d/t), and the rotor-nacelle-assembly mass (m_RNA) are precursors for evaluating the damage mode and vulnerability of OWTs under both pulse-like and non-pulse-like ground motion records.     

Evaluation of seismic response of bridges under asynchronous excitation and comparisons with Eurocode 8-2 provisions

The paper aims to evaluate the way Eurocode 8 treats the consideration of asynchronous earthquake ground motion during the seismic design of bridges, and to discuss alternative solutions for cases wherein existing provisions do not lead to satisfactory results. The evaluation of EC8-2 new provisions and simplified methods is performed through comparison with a more refined approach whereas an effort is made to quantitatively assess the relative importance of various design and analysis assumptions that have to be made when spatial variability of ground motion is taken into consideration, based on the study of the dynamic response of 27 different bridges. It is concluded that, despite the complexity of the problem, there are specific cases where EC8 provisions can be safely and easily applied in practice, while in other cases ignoring the effect of asynchronous excitation or performing simplified calculations can significantly underestimate the actual seismic demand.     

EC8-based earthquake record selection procedure evaluation: Validation study based on observed damage of an irregular R/C building

This study investigates the applicability and limitations of the Eurocode 8 earthquake ground motion selection framework for the assessment of both elastic and inelastic structural response of multi-storey, irregular R/C buildings subjected to bi-directional loading. In order to minimize modelling uncertainties inherent in the quantification of structural damage and the consideration of the supporting soil–foundation system for complex structural systems, an existing building damaged by the 2003 Lefkada earthquake was adopted as case study. This selection has an advantage in that ground excitation, soil profile and damage observations are all available, thus permitting calibration of the finite element model with the observed response, especially in terms of use of appropriate plasticity models and damage indices, plus the assessment of soil–structure interaction effects. After establishing a reliable finite element model of the structure under study, extensive parametric analyses for different EC8 compliant sets of records were conducted, permitting quantification of the discrepancy of the structural response due to record-to-record and set-to-set variability (i.e., intra-set and inter-set scatter, respectively). The results confirm that many of the observations found in the literature regarding the effect of ground motion selection on the predicted seismic performance of SDOF systems are also valid for bi-directionally excited, multi-storey, irregular buildings. Finally, the results also highlight specific limitations of the EC8 provisions that may lead to erroneous results in many practical cases.     

Influence of frequency‐dependent soil–structure interaction on the fragility of R/C bridges

Bridge performance under earthquake loading can be significantly influenced by the interaction between the structure and the supporting soil. Even though the frequency dependence of the interaction mentioned in this study has long been documented, the simplifying assumption that the dynamic stiffness is dominated by the mean or predominant excitation frequency is still commonly made, primarily as a result of the associated numerical difficulties when the analysis has to be performed in the time domain. This study makes use of the advanced lumped parameter models recently developed 1 in order to quantify the impact of the assumption on the predicted fragility of bridges mentioned in this study. This is achieved by comparing the predicted vulnerability for the case of a reference, well studied, actual bridge using both conventional, frequency‐independent, Kelvin–Voigt models and the aforementioned lumped parameter formulation. Analysis results demonstrate that the more refined consideration of frequency dependence of soil–structure interaction at the piers and the abutments of a bridge not only leads to different probabilities of failure for given intensity measures but also leads to different hierarchy and distribution of damage within the structure for the same set of earthquake ground motions even if the overall probability of exceeding a given damage state is the same. The paper concludes with the comparative assessment of the effect for different soil conditions, foundation configurations, and ground motion characteristics mentioned in this study along with the relevant analysis and design recommendations. Copyright © 2016 John Wiley & Sons, Ltd.