Identifying Networks in Social Media: The case of #Grexit

Networks and Spatial Economics - We examine the intensity of ‘#Grexit’ usage in Twitter during a period of economic and financial turbulence. Using a frequency-analysis technique, we...     

Offshore infrastructure planning using a vine copula approach for environmental conditions: an application for replacement maintenance of tidal energy infrastructure

Installation and maintenance operations of offshore assets are impacted by local environmental conditions such as wave height and period, wind speed and current velocity. These parameters are substantially of influence for the asset planning (time and costs) given the uncertainty of operational windows. In this article, a method is proposed to construct realistic time series of the aforementioned dependent conditions using a vine copulas approach. This method makes it possible to obtain a large number of realizations of these conditions at a certain location. It is shown that the operational windows remain persistent with the original limited dataset. Moreover, this method enables the incorporation of environmental uncertainties in the operational planning processes. To illustrate the value of this method, an application regarding replacement maintenance of a tidal energy infrastructure is examined. For this purpose, the maintenance activities are represented as a semi-Markov decision process. For every synthetic environmental time series, the algorithm finds the optimal set of decisions and the corresponding maintenance plans, including replacement costs and revenue losses. It is shown that the proposed method is effective in replacement maintenance decision making, while taking into account the environmental uncertainties.     

The effect of geology on the suitability of the Olynthios river dam site,Northern Greece

The paper draws attention to the adverse geological and tectonic setting for a proposed dam on the Olynthios river in Northern Greece. The preliminary investigation showed the foundation area to be at the boundary of two dissimilar units in tectonic contact: Mesozoic metamorphic strata juxtaposed with Neogene sedimentary strata. It discusses the significance of the conglomerates in the Neogene deposits and the importance of the clay lenses and clay matrix for the stability and potential settlement of the dam. Although numerous faults are present, the major faults dip steeply at right angles to the dam axis. The study has indicated that there is little loose disturbed material associated with the faults. In this particular environment, in addition to selective grouting, it is recommended that a clay blanket is placed upstream to lengthen the flow path. The feasibility study has shown that despite the theoretically unfavorable conditions the site can be considered suitable.      

Buckling strength of multi-story sway, non-sway and partially-sway frames with semi-rigid connections

The objective of this paper is to propose a simplified approach to the evaluation of the critical buckling load of multi-story frames with semi-rigid connections. To that effect, analytical expressions and corresponding graphs accounting for the boundary conditions of the column under investigation are proposed for the calculation of the effective buckling length coefficient for different levels of frame sway ability. In addition, a complete set of rotational stiffness coefficients is derived, which is then used for the replacement of members converging at the bottom and top ends of the column in question by equivalent springs. All possible rotational and translational boundary conditions at the far end of these members, featuring semi-rigid connection at their near end as well as the eventual presence of axial force, are considered. Examples of sway, non-sway and partially-sway frames with semi-rigid connections are presented, where the proposed approach is found to be in excellent agreement with the finite element results, while the application of codes such as Eurocode 3 and LRFD leads to significant inaccuracies.     

Ecosystem Management: A Review of a New Concept and Methodology

Ecosystem Management (EM) is a new holistic approach to the management of natural resources. It integrates the scientificknowledge on ecological interrelationships and the complex socioeconomic and political frame of values, aiming at the long-term sustainability of a region, i.e., the ecosystem. Itincludes the human, biological and natural dimensions, and canregulate the internal structures and functions of the ecosystem,and the inputs and outputs from it, so that solutions to development and protection problems can be achieved and be socially well-accepted. It has been given attention since 1991,mainly in the U.S.A., where the existing legal framework formeda basis for EM. The former experience of programs based on the traditional methods of management, the goal of sustainable development stated by many governmental, public or private agencies, and the advent of scientific knowledge, permitted the development of a theoretical basis for EM. Today, a significant number of EM programs have been established and are in progressin the U.S.A. Here the concept, general principles and methodology of EM are reviewed, and the EM program in the Stateof Florida in the U.S.A. is presented, which is considered a successful case study. In addition, the examples of managementof two very important ecosystems in Northern Greece are critically presented for comparison, in order to evaluate if they have EM characteristics.     

Novel additively manufactured bio-inspired 3D structures for impact energy damping

Additive manufacturing enables the design of complicated structures that would not be feasible to manufacture otherwise. Novel bio-inspired 3D structures were digitally produced employing Voronoi tessellation techniques and manufactured by selective laser sintering from polyamide. Impact tests were performed to analyze the impact absorption capacity of these structures, while FEM modeling offered insights on the fracture mechanisms and the prediction of the material response, which proved to be highly sensitive to strain rate. The results show a considerable impact attenuation in the case of 80% porosity and strut radius of 0.5 mm, encountering small transmitted peak force and smooth deceleration.