A model for the nonlinear, time-dependent and strengthening analysis of shear critical frame concrete structures

Nonlinear fibre beam models, due to its intrinsic simplicity and computational efficiency, are often an adequate alternative to the complex nonlinear plane and solid FE models for the assessment of entire frame structures. Nevertheless, simulations of structural concrete members undergoing relevant shear stresses cannot be performed by these models, as nonlinear shear effects and shear-bending interaction are neglected. In turn, the presence of shear stresses in cracked reinforced concrete (RC) elements leads to a rather complex resistant mechanism which numerical modelling is neither straightforward nor clearly established. Within this problematic, the formulation proposed in this thesis is an upgrade version of an existent flexural fibre beam model for the time-dependent analysis of segmentally constructed RC frames by taking into account the shear effects. The model is devised for the analysis of 2D RC and prestressed frame elements under combined axial, bending and shear forces. Shear-bending interaction is taken into account by means of a hybrid kinematic/force-based sectional approach. The key characteristics of the proposed model are: (i) at the material level RC is simulated through a smeared cracked approach with rotating cracks; (ii) at the fibre level an iterative procedure guarantees equilibrium between concrete and transversal reinforcement, allowing to compute the biaxial stress-strain state of each fibre; (iii) at the section level a uniform shear stress flow is assumed in order to estimate the internal shear stress-strain distribution and (iv) at the element level, the Timoshenko beam theory takes into account the deformation due to shear. As a result, the relevant attributes of the proposed formulation can be resumed as: (i) its capability for considering shear effects in both service and ultimate levels; (ii) the time step-by-step solution procedure enables taking into account the time-dependent response due to creep and shrinkage of concrete, temperature variations and relaxation of prestressing steel considering the multiaxial stress-strain state of the fibres and; (iii) the sequential type of analysis allows capturing the strengthening effects, accounting for the state of the structure prior to the intervention. The model is validated through experimental tests available in the literature, as well as through an experimental campaign carried out by the author. Accordingly, the capacity of the model to efficiently reproduce the behaviour of shear critical beams is demonstrated. The importance of including shear-bending interaction in the numerical analysis is underlined by comparing the results with the ones provided by the pure flexural basis model. The influence of transversal stresses on the time-dependent response of shear and bending dominant beams is also studied with the proposed model. Considering shear effects in modelling the time-dependent response of diagonally cracked RC and prestressed beams is found to be relevant. The proposed model is successfully used to predict the experimental results of a shear damaged and subsequently strengthened RC beam, available in the literature. An alternative strengthening solution for the damaged beam based on post-tensioned stirrups is numerically analysed. This technique showed to be effective to avoid brittle shear failure allowing for the development of all the flexural capacity of the repaired beam. The importance of considering previous damage in the numerical assessment of strengthened RC beams is revealed. Finally, the response of a dismantled prestressed concrete bridge, with deficient shear resistance, submitted to full-scale tests is successfully simulated with the proposed model. In addition, different strengthening proposals based on post-tensioning measures are studied for this bridge. In this manner, the capacity of the model to determine the safety of existent structures and to analyse the performance of strengthening measures is demonstrated.Los modelos de vigas, debido a su simplicidad inherente y eficiencia computacional, pueden ser alternativas adecuadas a complejos modelos de elementos finitos planos y sólidos. Sin embargo, el comportamiento de elementos estructurales de hormigón sometidos a fuertes esfuerzos cortantes, no pueden ser correctamente simulado mediante estos modelos, ya que no consideran los efectos no lineales del cortante y la interacción cortante-flexión. A su vez, la presencia del cortante en el hormigón armado fisurado da lugar a un complejo mecanismo resistente cuya modelización no está aún claramente definida. En esta tesis si propone un modelo de vigas que considera la no linealidad y el comportamiento paso-a-paso en el tiempo de estructuras porticadas de hormigón construidas evolutivamente teniendo en cuenta los efectos del cortante. El modelo se basa en una formulación existente de flexión y está concebido para el análisis de estructuras planas porticadas de hormigón armado (HA) y pretensado sometido a la acción combinada del esfuerzo axil, flexión y cortante. La interacción cortante-flexión si consigue mediante una formulación seccional basada en suposiciones mixtas de cinemática y fuerza. Las características clave del modelo son: (i) a nivel del material el HA se simula mediante una aproximación de fisuración distribuida rotacional; (ii) a nivel de la fibra un procedimiento iterativo garantiza el equilibrio entre el hormigón y la armadura transversal, permitiendo calcular el estado biaxial de tensiones y deformaciones en cada fibra; (iii) a nivel de la sección un patrón de tensiones tangenciales constantes estima la distribución interna de tensiones y deformaciones de cortante y (iv) a nivel del elemento se aplicada la formulación del elemento de viga Timoshenko. De este modo, los aspectos relevantes de la formulación propuesta se resumen en: (i) su capacidad para considerar los efectos del cortante en estados de servicio y últimos; (ii) el procedimiento paso-a-paso en el tiempo permite tener en cuenta la respuesta diferida debido a fluencia y retracción del hormigón, variaciones de temperatura y relajación del pretensado considerando el estado multiaxial de tensiones y deformaciones en las fibras y; (iii) el análisis secuencial permite evaluar los efectos del refuerzo teniendo en cuenta el estado de la estructura antes de la intervención El modelo se valida mediante ensayos experimentales disponibles en la literatura, así como, a través de una campaña experimental realizada por la autora, demostrando su capacidad para reproducir la respuesta de vigas críticas a cortante. La importancia de incluir la interacción cortante-flexión en el análisis numérico es destacada por medio de la comparación de los resultados con los proporcionados por el modelo básico de flexión. Con el modelo propuesto se estudia la influencia del cortante en la respuesta diferida de vigas dominadas por cortante y flexión. La consideración de los efectos del cortante es relevante en la modelación de la respuesta diferida de vigas de HA con fisuras diagonales y en vigas pretensadas. El modelo propuesto se compara con éxito con los resultados experimentales de una viga dañada a cortante y posteriormente reforzada mediante un recrecido de hormigón y armadura transversal. Adicionalmente, se analiza numéricamente otra solución alternativa de refuerzo basada en estribos pretensados. La importancia de considerar el daño previo de la viga reforzada queda demostrada en la evaluación numérica. Finalmente, se simula la respuesta de un puente pretensado desmantelado con deficiente resistencia a cortante y sometido a ensayos de carga a larga-escala. Son también estudiadas diferentes propuestas de refuerzo basadas en soluciones de pos-tensado. De este modo, se demuestra la capacidad del modelo para determinar la seguridad de estructuras existentes y analizar la eficiencia de las medidas de refuerzo

Identificação e modelação do comportamento de estruturas de betão nas primeiras idades

Tese de mestrado. Estruturas de Engenharia Civil. Faculdade de Engenharia. Universidade do Porto. 200

Influence of time-dependent restrained strains in the shear response of RC frames

The final publication is available at Springer via http://dx.doi.org/10.1617/s11527-016-0875-8Time-dependent strains, when restrained, can lead to important tensile forces and damage, affecting, among other aspects, the shear response and ultimate load carrying capacity of shear-critical RC frames. This paper presents a detailed study of this problematic by means of an extension of a shear-sensitive fibre beam model to time dependent behaviour of concrete. The model is firstly validated with experimental tests on diagonally pre-cracked beams under sustained loads. From these analyses, the contributions of shear distortions and bending curvatures to the total long-term deflection of the beams are discerned. Afterwards, the model is applied to study the influence of restraining strains due to long-term creep and shrinkage in the service and ultimate shear response of frames. In contrast with flexural resistant mechanisms, delayed strains may influence the latter shear resistance of integral structures by reducing the concrete contribution to shear resistance and leading to a sooner activation of the transversal reinforcement. These aspects can be relevant in assessing existing structures and this model, due to its relative simplicity, can be advantageous for practical applications.Peer ReviewedPostprint (author's final draft

Analysis of FRP shear strengthening solutions for reinforced concrete beams considering debonding failure

In this paper, a fiber beam model previously developed by the authors for the nonlinear analysis of strengthened elements, including the effects of shear, is used to predict the response of reinforced concrete (RC) beams strengthened in shear with fiber reinforced polymers (FRP) sheets. In the previous version of the model, debonding failure of FRP was not included; hence, its application was limited to the simulation of wrapped configurations. The model is now extended to account for debonding failure in order to allow for its application to beams strengthened with U-shaped and side-bonded configurations. Existing experimental tests on RC beams strengthened in shear by FRP sheets in both wrapped and U-shaped configurations were numerically simulated. The model reproduces, with reasonable accuracy, the experimental failure loads, the load-deflection behavior, and the strains in FRP and stirrups with increasing load. The advantages of this proposal are related with the simplicity and straightforwardness of the beam models to be applied in practical engineering problems.Peer ReviewedPostprint (author's final draft

Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries

Abstract Background Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres. Methods This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries. Results In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia. Conclusion This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries

Code-division multiple-access (CDMA) techniques in optical fibre local area networks

SIGLEAvailable from British Library Document Supply Centre- DSC:D92527 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Passenger ships - Class IV and Class V Categorisation of waters

SIGLEGBUnited Kingdo