Comportamiento estructural de vigas de acero inoxidable ferrítico frente a cargas concentradas

Los elementos de acero conformados en frío son muy útiles dado su elevado ratio de resistencia/peso en comparación con otros materiales estructurales. Generalmente son elementos con valores elevados en ratios como altura/espesor y anchura/espesor, por lo que debe tenerse un especial cuidado en lo que se refiere a los fenómenos de inestabilidad, entre los que se encuentra el web crippling o abolladura del alma frente a cargas transversales localizadas. Por otra parte, los aceros inoxidables presentan, además de buenas características mecánicas, una excelente resistencia a la corrosión en distintos tipos de ambientes. De entre todos ellos, el acero inoxidable ferrítico cuenta con un precio estable en el mercado, debido a su bajo contenido en níquel, manteniendo unas magníficas prestaciones mecánicas. Por ello, los elementos conformados en frío de acero inoxidable ferrítico se presentan como elementos con un gran futuro como alternativa a los aceros inoxidables austeníticos. Dado que las especificaciones para elementos conformados en frío de acero inoxidable coinciden con las de los aceros al carbono y no cubren los aceros ferríticos, se están desarrollando proyectos para la elaboración de nuevas guías de diseño que permitan incluir los aceros inoxidables ferríticos y poder desarrollar técnicas de proyección eficientes. Este trabajo ha formado parte de uno de estos proyectos, estudiando el fenómeno de web crippling en aceros inoxidables en general, y en ferríticos en particular, para comprobar la aplicabilidad de las especificaciones existentes y la propuesta de nuevas expresiones

Expressive Policy-Based Access Control for Resource-Constrained Devices

Upcoming smart scenarios enabled by the Internet of Things envision smart objects that expose services that can adapt to user behavior or be managed with the goal of achieving higher productivity, often in multi-stakeholder applications. In such environments, smart things are cheap sensors (and actuators) and, therefore, constrained devices. However, they are also critical components because of the importance of the provided information. Therefore, strong security is a must. Nevertheless, existing feasible approaches do not cope well with the principle of least privilege; they lack both expressiveness and the ability to update the policy to be enforced in the sensors. In this paper, we propose an access control model that comprises a policy language that provides dynamic fine-grained policy enforcement in the sensors based on local context conditions. This dynamic policy cycle requires a secure, efficient, and traceable message exchange protocol. For that purpose, a security protocol called Hidra is also proposed. A security and performance evaluation demonstrates the feasibility and adequacy of the proposed protocol and access control model.This work was supported in part by the Training and Research Unit through UPV/EHU under Grant UFI11/16 and in part by the Department of Economic Development and Competitiveness of the Basque Government through the Security Technologies SEKUTEK Collaborative Research Projec

New approach for efficient design of stainless steel RHS and SHS elements

Premi extraordinari doctorat UPC curs 2015-2016, àmbit d’Enginyeria CivilThis thesis investigates the cross-section and member behaviour of cold-formed stainless steel RHS and SHS elements and proposes an alternative and more efficient design approach. Combined with aesthetic appeal, exceptional mechanical properties and excellent corrosion and fire resistances, efficient design methods present stainless steel as an attractive alternative to the usual carbon steel for structural applications. Exhaustive studies of the nonlinear stress-strain behaviour and the analytical modelling of the response are presented for different stainless steel alloys. The study was based on 600 experimental stress-strain curves obtained from the literature and complemented with 42 tensile coupon tests. Although the material model currently included in Annex C of EN1993-1-4 (2006) was found to accurately represent the measured stress-strain curves for the different stainless steel grades and material types, revised equations were proposed for the strain hardening parameters n and m and for the ultimate tensile stress and strain for ferritics. A comprehensive experimental programme on five cross-sections of ferritic stainless steel grade 1.4003 tubular elements is also described. The actual geometry and initial geometric imperfections were carefully measured and the material response of flat and corner regions of each section were characterized by conducting 20 tensile tests on coupons extracted from the cross-sections. The cross-sectional behaviour was investigated through 10 stub column tests under pure compression and 16 subjected to combined loading conditions, while 8 beams were tested under four-point bending configuration and 4 subjected to three-point bending loading conditions. At member level, the bending moment redistribution capacity of ferritic continuous beams was investigated by conducting 9 five-point bending tests. Finally, 12 tests were conducted on ferritic stainless steel columns to determine the behaviour of members subjected to concentric and eccentric compression loads. Additional data on austenitic, ferritic and duplex stainless steel elements was generated from parametric studies based on finite element models validated from the conducted experiments. The assessment of the codified design expressions was derived by comparing experimental and numerical strengths with the calculated resistance predictions for stainless steel cross-sections and members subjected to different loading conditions. Results demonstrated that predictions are noticeably conservative for stocky and slender cross-sections since enhanced material properties are not considered and the susceptibility of cross-sections to local buckling is underestimated. Thus, a full slenderness range Direct Strength Method (DSM) approach was proposed for stainless steel RHS and SHS cross-sections and members based on the same strength curve for all loading conditions. The proposed approach was found to be more accurate for cross-sections, columns and beam-columns since strain hardening effects are incorporated and due to the fact that the actual stress distribution of the cross-section is considered when determining the slenderness. The reliability of the approach was demonstrated by statistical analyses, enabling its use in structural design standards. Finally, the applicability of design approaches based on plastic analysis to stainless steel continuous beams was assessed. The analysis of continuous beam strengths demonstrated that capacity predictions based on the first hinge formation result in a considerable overconservatism and that traditional plastic design can be safely applied with the Class 1 cross-section limit provided in EN1993-1-4 (2006). However, it was also statistically demonstrated that the best capacity predictions are obtained for design methods including both bending moment redistribution and strain hardening effects, such as the Continuous Strength Method for indeterminate structures or the proposed DSM-based approach.Esta tesis trata sobre el comportamiento de perfiles tubulares conformados en fío de acero inoxidable y propone expresiones de diseño alternativas y más eficientes. Un diseño eficiente de las estructuras de acero inoxidable, junto con sus adecuadas propiedades mecánicas y resistencia a la corrosión, hacen de este material una atractiva alternativa frente al acero al carbono para aplicaciones estructurales. Se ha realizado un estudio exhaustivo del comportamiento tensodeformacional de diferentes tipos de acero inoxidable, así como de los modelos analíticos que caracterizan su comportamiento. El estudio se basa en más de 600 curvas experimentales de material extraídas de la literatura y complementadas con 42 ensayos. Los resultados ponen en evidencia que el modelo material actualmente recogido en el Anejo C de la EN1993-1-4 (2006) representa adecuadamente el comportamiento tensodeformacional de diferentes grados y tipos de material. No obstante, se han propuesto nuevas expresiones para la determinación de los coeficientes de no linealidad n y m, así como para la predicción de la resistencia y deformación últimas de los aceros inoxidables ferríticos. La tesis también describe una campaña experimental en elementos tubulares de acero inoxidable ferrítico EN1.4003. Tras la medición de la geometría e imperfecciones iniciales, se procedió a la caracterización del comportamiento de las regiones planas y de esquina mediante 20 ensayos de material. El comportamiento seccional se ha estudiado mediante 10 ensayos de elementos cortos a compresión y 16 sometidos a flexocompresión, realizando también 8 ensayos sobre vigas biapoyadas sometidas a flexión pura y 4 sometidas a gradientes de flexión. El comportamiento a nivel de elemento se ha caracterizado mediante 12 ensayos de pilares sometidos a compresión y flexocompresión y el estudio sobre la capacidad de redistribución de estructuras de acero inoxidable ferrítico se ha basado en 9 vigas continuas. El comportamiento de los grados austeníticos, ferríticos y duplex se ha estudiado también mediante los modelos de elementos finitos previamente validados a partir de los ensayos realizados. La valoración de las expresiones de diseño actualmente recogidas en las diferentes normativas se ha llevado a cabo mediante la comparación de los resultados numéricos y experimentales con las cargas predichas para diferentes estados de carga. Dichos resultados han puesto de manifiesto que las resistencias predichas por las normativas son considerablemente conservadoras tanto para secciones robustas como esbeltas, puesto que no se tienen en cuenta los efectos de endurecimiento por deformación y el comportamiento de abolladura no está correctamente caracterizado. Por consiguiente, se ha propuesto un nuevo método de diseño basado en el Direct Strength Method (DSM) para secciones y elementos de acero inoxidable, aplicable en todo el rango de esbelteces y que emplea una misma curva de resistencia para los distintos casos de carga. Se ha demostrado que la nueva propuesta proporciona mejores resultados tanto a nivel seccional como en elementos y su fiabilidad se ha demostrado mediante el correspondiente análisis estadístico, permitiendo su inclusión en las normativas de diseño. Por último, se ha estudiado la aplicabilidad de los métodos de diseño basados en el cálculo global plástico en vigas continuas de acero inoxidable. El estudio ha demostrado que pese a que la consideración de métodos basados en cálculos elásticos proporciona resultados notablemente conservadores, los métodos tradicionales de diseño plástico pueden ser utilizados junto con la definición de Clase 1 actualmente recogida en EN1993-1-4 (2006). No obstante, también se ha demostrado que la mejor predicción de resistencia viene dada por aquellos métodos que incorporan tanto la redistribución de esfuerzos como los efectos de endurecimiento por deformación, como son el Continuous Strength Method o el método propuesto basado en el DSMAward-winningPostprint (published version

An architecture for dynamic QoS management at Layer 2 for DOCSIS access networks using OpenFlow

Over the last few years, Software-Defined Networking (SDN) has emerged as one of the most disruptive and profitable novelties in networking. SDN was originally conceived to improve performance and reduce costs in Ethernet-based networks and it has been widely adopted in data center and campus networks. Similarly, thanks to the introduction of SDN concepts, access networks will benefit from the higher control, the lower maintenance costs and the better remote access to devices of SDN. However, its application to access networks is not straightforward and imposes great challenges to vendors and network operators, since current SDN technologies are not prepared to handle the provisioning of user equipment, specific port management or QoS requirements of common access networks. Most recent trends dealing with the SDN-ization of access networks advocate for the use of simple devices at the customer premises and the virtualization of the networking functionalities, requiring the provisioning of Layer 2 services in many cases. In such a scenario, this paper presents an architecture that brings SDN to common access networks using legacy equipment. In a nutshell, the architecture is based on the abstraction of the access network as a wide area OpenFlow switch where QoS-enabled pipes are dynamically created leveraging the high granularity of the OpenFlow protocol for packet classification. Furthermore, the OpenFlow protocol itself has been extended in order to support the advanced QoS requirements that are common to most access networks. The architecture has been implemented for DOCSIS access networks and it has been validated and evaluated using a real testbed deployed at our laboratory. The obtained results show that the architecture remains compliant with the ITU-T QoS recommendations and that the cost of introducing the elements required by the architecture in terms of service performance is negligible.European Commission, Seventh Framework Programme, through the ALIEN (317880) project Spanish Ministry of Economy and Competitiveness under the Secure deployment of services over SDN and NFV based networks project S&NSEC TEC2013-47960-C4-3-

Influence of the imperfection direction on the ultimate response of steel frames in advanced analysis

Initial geometric imperfections are unavoidable in steel members and frames due to erection and manufacturing tolerances. These include frame out-of-plumbness, member out-of-straightness and cross-sectional imperfections, and can have a significant influence on the response and resistance of steel structures. Thus, they need to be accounted for in the analysis and design of steel structures, especially when advanced design procedures are adopted. One of the easiest approaches to introduce geometric imperfections in structural finite element models is through the linear superposition of scaled eigenmodes, which are obtained from a priori elastic buckling analysis. Although the shape and magnitude of frame and member imperfections are specified in international standards, the rules for the combination of different types and directions of imperfections are unclear or impractical, and often require designers to consider many possible combinations to find the critical, or “worst case”, shape of the imperfection including the direction of each eigenmode. This paper investigates the influence of the direction of modes contributing to the imperfection on the ultimate load (i.e., resistance) of steel frames when using advanced analysis. Ultimate loads are estimated from advanced finite element simulations for 20 regular and irregular unbraced frames featuring steel and austenitic stainless steel compact sections, in which initial imperfections are modelled as linear superpositions of six scaled buckling modes considering all possible combinations of direction. The results show that the influence of the imperfection direction on the ultimate frame load is small, and that assuming a combination of all buckling modes with positive amplitudes provides a simple and accurate estimation of the critical imperfection combination.The project leading to this research has received funding from the European Union's Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant Agreement No. 842395.Peer ReviewedPostprint (published version

Experimental study on ferritic stainless steel simply supported and continuous beams

Development of efficient design guidance for stainless steel structures is key for the increased use of this corrosion-resistant material by considering both nonlinear behaviour and strain hardening into resistance prediction expressions, together with the moment redistribution in indeterminate structures. With the aim of analysing the bending moment redistribution capacity of ferritic stainless steel beams, a comprehensive experimental programme on continuous beams is presented. These tests contribute to the assessment of EN1993-1-4 specifications, where no plastic design is allowed, and the classical and new plastic design methods available in the literature for indeterminate stainless steel structures. Four three-point and eight four-point bending tests are also reported for the assessment of current codified and revised cross-sectional classification limits, analysing the different methods for the prediction of the ultimate bending capacities of ferritic hollow sections. Additional test results reported by other authors in different stainless steel grades and carbon steel are also studied and presented. The analysis indicates that Class 1 cross-sectional classification limits are too optimistic for ferritic stainless steels and further research is needed for the extension of plastic design to these grades, although promising predictions of ultimate loads are obtained for austenitic and lean duplex stainless steels.Peer ReviewedPostprint (author's final draft

Reliability of stainless steel frames designed using the Direct Design Method in serviceability limit states

Steel structures can be consistently and efficiently designed using system-based design-by-analysis approaches such as the Direct Design Method. However, since direct design approaches lead to potentially lighter structural configurations, they can also result in larger deformations under service loads. Thus, greater attention may be required to serviceability limit states in structures designed using design-by-analysis approaches than for structures designed elastically at their ultimate limit state following current two-stage approaches, especially for materials showing highly nonlinear stress vs strain responses such as stainless steel alloys. With the aim of investigating the influence of allowing larger deformations in the ultimate limit state design of stainless steel structures, this paper presents an explicit analysis framework for assessing serviceability reliability at system level. Using this framework, the paper investigates the serviceability reliability of cold-formed stainless steel portal frames designed using the Direct Design Method for different load cases, including the gravity load and the combined gravity plus wind load combinations. The study considers six baseline frames covering the most common stainless steel families and international design frameworks (i.e., Eurocode, US and Australian frameworks), for which the reliability of vertical deflection and lateral drift serviceability limit states is investigated using advanced numerical simulations and First-Order Reliability Methods. From the comparison of the calculated average annual reliability indices and the relevant target reliabilities for the different design frameworks, it was found that the reliability of stainless steel frames appears to be adequate for the serviceability limit states investigated for the Eurocode, US and Australian frameworks.Peer ReviewedPostprint (published version

CloudOps: Towards the Operationalization of the Cloud Continuum: Concepts, Challenges and a Reference Framework

The current trend of developing highly distributed, context aware, heterogeneous computing intense and data-sensitive applications is changing the boundaries of cloud computing. Encouraged by the growing IoT paradigm and with flexible edge devices available, an ecosystem of a combination of resources, ranging from high density compute and storage to very lightweight embedded computers running on batteries or solar power, is available for DevOps teams from what is known as the Cloud Continuum. In this dynamic context, manageability is key, as well as controlled operations and resources monitoring for handling anomalies. Unfortunately, the operation and management of such heterogeneous computing environments (including edge, cloud and network services) is complex and operators face challenges such as the continuous optimization and autonomous (re-)deployment of context-aware stateless and stateful applications where, however, they must ensure service continuity while anticipating potential failures in the underlying infrastructure. In this paper, we propose a novel CloudOps workflow (extending the traditional DevOps pipeline), proposing techniques and methods for applications’ operators to fully embrace the possibilities of the Cloud Continuum. Our approach will support DevOps teams in the operationalization of the Cloud Continuum. Secondly, we provide an extensive explanation of the scope, possibilities and future of the CloudOps.This research was funded by the European project PIACERE (Horizon 2020 Research and Innovation Programme, under grant agreement No. 101000162)

Influence of geometric and material nonlinearities on the behaviour and design of stainless steel frames

Material nonlinearity affects the stiffness and consequently the distribution of internal forces and moments in indeterminate structures. This has a direct impact on their behaviour and design, particularly in the case of stainless steel, where material nonlinearity initiates at relatively low stress levels. A method for accounting for the influence of material nonlinearity in stainless steel frames, including making due allowance for the resulting amplified second order effects, is presented herein. Proposals have been developed for austenitic, duplex and ferritic stainless steels. The method was derived based on benchmark results calculated through second order inelastic analysis with strain limits, defined by the Continuous Strength Method, using beam finite element models. A comprehensive set of frames was modelled and the proposed assessment of second order effects in the plastic regime was also verified against the results of four full-scale stainless steel frame tests. The proposed method is due to be included in the upcoming revision to Eurocode 3 Part 1.