Acceleration and Deflection Analysis for Class C Edge Protection Systems in Construction Work

The requirements for edge protection systems on most sloped work surfaces (class C, according to EN 13374-2013 code) in construction works are studied in this paper. Maximum deceleration suffered by a falling body and maximum deflection of the protection system were analyzed through finite-element models and confirmed through full-scale experiments. The aim of this work is to determine which value for deflection system entails a safe deceleration for the human body. This value is compared with the requirements given by the current version of EN 13374-2013. An additional series of experiments were done to determine the acceleration linked to minimum deflection required by code (200 mm) during the retention process. According to the obtained results, a modification of this value is recommended. Additionally, a simple design formula for this falling protection system is proposed as a quick tool for the initial steps of design.

Barandillas de protección personal con solicitación dinámica

Las barandillas de seguridad con solicitación dinámica, tipos B y C según UNE-EN 13374, son el conjunto de componentes destinados a la protección colectiva de las personas contra las caídas a un nivel inferior y a retener materiales. Frecuentemente incorporan una red de seguridad. En este artículo se analizan aspectos fundamentales como su diseño, sus requisitos mecánicos y la solución a la problemática que suponen los soportes o puntos duros capaces de producir lesiones al accidentado. Para este fin, se ha estudiado el fenómeno de la caída de un lastre sobre tales redes con distintos modelos numéricos de elementos finitos en régimen dinámico y mediante consideraciones analíticas cinemáticas, obteniendo conclusiones útiles como la forma geométrica necesaria, la escuadría de los perfiles que constituyen el bastidor de la red, y los factores de impacto que sufren los accidentados. Ello permite sugerir mejoras al texto normativo.

Redes de seguridad para cierre vertical de fachadas

Entre otros medios de protección colectiva, en los últimos años se vienen utilizando en España redes verticales de cierre de fachadas, a paño completo entre forjados en fase de estructura, y también para huecos menores como ventanas o puertas, en fases posteriores. Este tipo de protección no cuenta hasta el momento con ningún tipo de regulación oficial, si bien en el grupo de trabajo AEN/CTN81/SC2/GT7 se desarrolla actualmente el borrador de la norma Pr UNE 81651 a tal fin. Desde el Dtº. de Ing. de la Construcción de la Universidad de Alicante, varios miembros del grupo de trabajo han desarrollado, contando con la experiencia en otros sistemas de protección, algunos modelos numéricos con elementos finitos para valorar y cuantificar variables mecánicas asociadas al episodio de una eventual caída de una persona sobre este tipo de redes, caracterizado por una menor energía cinética que para otros tipos de ellas. En el artículo se exponen los resultados obtenidos para ambos tipos de paños, grandes y pequeños, donde se han valorado la separación de anclajes de la red, las aberturas generadas en su perímetro durante la retención, los esfuerzos en anclajes y elementos textiles, el factor de impacto que sufriría el accidentado y la repercusión de la disposición geométrica de la red, al cuadro o al rombo.

A finite element model of perforated panel absorbers including viscothermal effects

Most of the analytical models devoted to determine the acoustic properties of a rigid perforated panel consider the acoustic impedance of a single hole and then use the porosity to determine the impedance for the whole panel. However, in the case of not homogeneous hole distribution or more complex configurations this approach is no longer valid. This work explores some of these limitations and proposes a finite element methodology that implements the linearized Navier Stokes equations in the frequency domain to analyse the acoustic performance under normal incidence of perforated panel absorbers. Some preliminary results for a homogenous perforated panel show that the sound absorption coefficient derived from the Maa analytical model does not match those from the simulations. These differences are mainly attributed to the finite geometry effect and to the spatial distribution of the perforations for the numerical case. In order to confirm these statements, the acoustic field in the vicinities of the perforations is analysed for a more complex configuration of perforated panel. Additionally, experimental studies are carried out in an impedance tube for the same configuration and then compared to previous methods. The proposed methodology is shown to be in better agreement with the laboratorial measurements than the analytical approach.

Regional subsidence modelling in Murcia city (SE Spain) using 1-D vertical finite element analysis and 2-D interpolation of ground surface displacements

Subsidence is a hazard that may have natural or anthropogenic origin causing important economic losses. The area of Murcia city (SE Spain) has been affected by subsidence due to groundwater overexploitation since the year 1992. The main observed historical piezometric level declines occurred in the periods 1982–1984, 1992–1995 and 2004–2008 and showed a close correlation with the temporal evolution of ground displacements. Since 2008, the pressure recovery in the aquifer has led to an uplift of the ground surface that has been detected by the extensometers. In the present work an elastic hydro-mechanical finite element code has been used to compute the subsidence time series for 24 geotechnical boreholes, prescribing the measured groundwater table evolution. The achieved results have been compared with the displacements estimated through an advanced DInSAR technique and measured by the extensometers. These spatio-temporal comparisons have showed that, in spite of the limited geomechanical data available, the model has turned out to satisfactorily reproduce the subsidence phenomenon affecting Murcia City. The model will allow the prediction of future induced deformations and the consequences of any piezometric level variation in the study area.

On the forbidden gap of finite graphene nanoribbons

The electronic structure of isolated finite graphene nanoribbons is investigated by solving, at the Hartree-Fock (HF) level, the Pariser, Parr and Pople (PPP) many-body Hamiltonian. The study is mainly focused on 7-AGNR and 13-AGNR (Armchair Graphene Nano-Ribbons), whose electronic structures have been recently experimentally investigated. Only paramagnetic solutions are considered. The characteristics of the forbidden gap are studied as a function of the ribbon length. For a 7-AGNR, the gap monotonically decreases from a maximum value of ~6.5 eV for short nanoribbons to a very small value of ~0.12 eV for the longer calculated systems. Gap edges are defined by molecular orbitals that are spatially localized near the nanoribbon extremes, that is, near both zig-zag edges. On the other hand, two delocalized orbitals define a much larger gap of about 5 eV. Conductance measurements report a somewhat smaller gap of ~3 eV. The small real gap lies in the middle of the one given by extended states and has been observed by STM and reproduced by DFT calculations. On the other hand, the length dependence of the gap is not monotonous for a 13-AGNR. It decreases initially but sharply increases for lengths beyond 30 Å remaining almost constant thereafter at a value of ~2.1 eV. Two additional states localized at the nanoribbon extremes show up at energies 0.31 eV below the HOMO (Highest Occupied Molecular Orbital) and above the LUMO (Lowest Unoccupied Molecular Orbital). These numbers compare favorably with those recently obtained by means of STS for a 13-AGNR sustained by a gold surface, namely 1.4 eV for the energy gap and 0.4 eV for the position of localized band edges. We show that the important differences between 7- and 13-AGNR should be ascribed to the charge rearrangement near the zig-zag edges obtained in our calculations for ribbons longer than 30 Å, a feature that does not show up for a 7-AGNR no matter its length.

Quantum theory of spin waves in finite chiral spin chains

We calculate the effect of spin waves on the properties of finite-size spin chains with a chiral spin ground state observed on biatomic Fe chains deposited on iridium(001). The system is described with a Heisenberg model supplemented with a Dzyaloshinskii-Moriya coupling and a uniaxial single ion anisotropy that presents a chiral spin ground state. Spin waves are studied using the Holstein-Primakoff boson representation of spin operators. Both the renormalized ground state and the elementary excitations are found by means of Bogoliubov transformation, as a function of the two variables that can be controlled experimentally, the applied magnetic field and the chain length. Three main results are found. First, because of the noncollinear nature of the classical ground state, there is a significant zero-point reduction of the ground-state magnetization of the spin spiral. Second, there is a critical external field from which the ground state changes from chiral spin ground state to collinear ferromagnetic order. The character of the two lowest-energy spin waves changes from edge modes to confined bulk modes over this critical field. Third, in the spin-spiral state, the spin-wave spectrum exhibits oscillatory behavior as function of the chain length with the same period of the spin helix.

A Finite Element Numerical Algorithm for Modelling and Data Fitting in Complex Systems

Numerical modelling methodologies are important by their application to engineering and scientific problems, because there are processes where analytical mathematical expressions cannot be obtained to model them. When the only available information is a set of experimental values for the variables that determine the state of the system, the modelling problem is equivalent to determining the hyper-surface that best fits the data. This paper presents a methodology based on the Galerkin formulation of the finite elements method to obtain representations of relationships that are defined a priori, between a set of variables: y = z(x1, x2,...., xd). These representations are generated from the values of the variables in the experimental data. The approximation, piecewise, is an element of a Sobolev space and has derivatives defined in a general sense into this space. The using of this approach results in the need of inverting a linear system with a structure that allows a fast solver algorithm. The algorithm can be used in a variety of fields, being a multidisciplinary tool. The validity of the methodology is studied considering two real applications: a problem in hydrodynamics and a problem of engineering related to fluids, heat and transport in an energy generation plant. Also a test of the predictive capacity of the methodology is performed using a cross-validation method.