Simulação do comportamento à compressão uniaxial do betão reforçado com fibras de aço

Ensaios de compressão uniaxial efectuados com provetes cilíndricos reforçados com diferente percentagem de fibras de aço são descritos no presente trabalho. Os ensaios foram realizados sob controlo de deslocamento, de forma a obter-se a resposta até ao completo esgotamento da capacidade de carga dos provetes. Os resultados obtidos são apresentados e analisados. Duas expressões tensão-extensão foram definidas com base nos resultados experimentais. Estas expressões podem ser utilizadas em modelos numéricos de simulação do comportamento não linear material destes compósitos. Uma das expressões adequa-se a modelos formulados em extensões totais, enquanto a outra expressão é apropriada a modelos elasto-plásticos. O desempenho destas expressões é avaliado

Memória descritiva e justificativa do projecto de estabilidade do edifício do Centro de Solidariedade de Braga

A presente memória descritiva e justificativa refere-se ao edifício do Centro de Solidariedade de Braga, construído no Monte do Picoto em Braga. Este complexo é constituído por um polivalente e um edifício central (ver Figuras 1 e 2). O edifício central é formado por seis blocos ligados entre si por uma laje de cobertura com duas grandes aberturas, uma oval e outra quadrada. Existem uns passadiços suspensos a ligar alguns destes blocos. O polivalente encontra-se enterrado, com uma camada de solo sobre a sua cobertura. No dimensionamento foram consideradas as recomendações da regulamentação em vigor, Nacional e Estrangeira. Os casos não devidamente tratados pela regulamentação disponível foram analisados tendo-se em conta resultados obtidos em investigação recente. O edifício é todo em betão aparente. A construção do edifício iniciou-se em 2001 e terminou em 2004

Centro de Solidariedade de Braga

O presente trabalho descreve os procedimentos que mais interessam do ponto de vista do dimensionamento estrutural de um edifício constituído por estruturas laminares, algumas delas de dimensão e geometria invulgar. A elevada complexidade estrutural de alguns dos módulos que constituem a estrutura exigiu a utilização de sofisticados modelos de cálculo, alguns deles desenvolvidos para o efeito

Modelos constitutivos para a análise não linear material de estruturas de betão incluindo efeitos diferidos

Tese de doutoramento Estruturas /Engenharia CivilIn recent years concrete technology has been improved significantly due, mainly, to the development of self-compacting concrete, ultra-fluid cement based materials, high performance fiber reinforced concrete and engineered cement composites. These developments have their main applicability in the pre-casting industry, where the earliest demoulding of the pre-cast elements is an important aim for economic reasons. Due to the relatively high cost of these advanced cement based materials, optimization of the behavior of structural elements made with these materials is a fundamental issue for their competitiveness. As a consequence, these materials are, in general, applied to relatively thin elements that require special attention in terms of shear and punching resistance. With the aim of studying these types of structures, a multi-directional fixed smeared crack model for plane shells has been developed. This model implements an innovative approach for capturing the behavior of laminar structures failing in punching, which is based on the adoption of a softening diagram to simulate the behavior of the out-of-plane shear stress components. Since most advanced cement based materials have relatively high binder content, the risk of cracking at an early age should be evaluated using models that can estimate the heat generated by the hydration of pozolanic components and the induced stress fields. For this purpose, a FEM-based heat transfer model has been developed and integrated into a mechanical model that can simulate the crack initiation and propagation in structures discretized with solid finite elements. The mechanical model is a 3D multi-directional smeared crack model with the capability of simulating the behavior of structures failing in punching and shear. Shrinkage and creep are also a concern mainly for service limit states due to crack opening limits. In the last two decades fiber reinforced polymer composite materials have also been used for the structural rehabilitation of concrete structures, mainly for the flexural and shear strengthening of reinforced concrete beams. The prediction of the behavior of the shear viii Abstract strengthened beams requires the use of crack constitutive models to simulate the decrease of the shear stress degradation with the crack opening evolution, in agreement. Two numerical approaches are proposed to simulate this phenomenon. One is based on the use of a softening crack shear stress versus crack shear strain diagram to model the fracture mode II, while in the other the total crack shear stress is obtained from the total crack shear strain adopting a crack shear modulus that decreases with the crack normal strain. Fiber reinforcement mechanisms are more effectively mobilized when support redundancy of a structure is high, since the stress redistribution capacity provides to this type of structure an ultimate load that is much higher than the load at crack initiation. However, the supporting conditions of a structure can change during the loading process, and even a loss of contact can occur. To simulate accurately these situations, linear, nonlinear and unilateral support conditions are numerically implemented. To increase the robustness of the developed numerical models, innovative numerical strategies are implemented in the stress update phase of the nonlinear finite element analysis process. Furthermore, to improve the convergence performance of the finite element nonlinear analyses an arc-length algorithm is implemented. All the numerical models are implemented in the FEMIX 4.0 FEM package, using the ANSI-C computer language.Fiber reinforcement mechanisms are more effectively mobilized when support redundancy of a structure is high, since the stress redistribution capacity provides to this type of structure an ultimate load that is much higher than the load at crack initiation. However, the supporting conditions of a structure can change during the loading process, and even a loss of contact can occur. To simulate accurately these situations, linear, nonlinear and unilateral support conditions are numerically implemented. To increase the robustness of the developed numerical models, innovative numerical strategies are implemented in the stress update phase of the nonlinear finite element analysis process. Furthermore, to improve the convergence performance of the finite element nonlinear analyses an arc-length algorithm is implemented. All the numerical models are implemented in the FEMIX 4.0 FEM package, using the ANSI-C computer language. Estes materiais avançados de matriz cimentícia têm uma quantidade de ligante relativamente elevado, pelo que a possibilidade de ocorrência de fendilhação nas primeiras idades deve ser avaliada usando modelos que permitam estimar o calor gerado pela hidratação do ligante durante o seu processo de cura, bem como a determinação das correspondentes tensões. Para o efeito, no presente trabalho é desenvolvido um modelo de transferência de calor baseado no método dos elementos finitos, o qual é integrado num modelo mecânico que permite simular o início de fendilhação e a sua propagação em estruturas discretizadas por elementos finitos de volume. Este modelo de fendilhação 3D tem a possibilidade de simular a ocorrência de múltiplas fendas fixas distribuídas, bem como o comportamento de estruturas cujo modo de ruptura é condicionado pelas componentes de corte. A retracção e a fluência também são fenómenos de relevância em estruturas constituídas por estes tipos de materiais, principalmente em estados limites de serviço por abertura de fenda, pelo que a sua modelação foi também integrada no modelo termo-mecânico. Nas últimas duas décadas, materiais de matriz polimérica reforçados com fibras contínuas têm sido utilizados na reabilitação estrutural de estruturas de betão, principalmente para o reforço à flexão e ao corte de vigas de betão armado. A previsão do comportamento das vigas reforçadas ao corte requer o uso de modelos constitutivos capazes de simular a diminuição da capacidade de transferência de tensão de corte com a evolução da abertura de fenda. Duas abordagens numéricas são propostas para este fim. Uma delas baseia-se na utilização de um diagrama de amolecimento para a modelação do modo II de fractura. A outra suporta-se numa formulação total para a relação entre a tensão e a extensão de corte na fenda, adoptando um módulo de rigidez correspondente ao modo II de fractura que diminui com a extensão normal à fenda. De forma a aumentar a robustez dos modelos numéricos desenvolvidos, foram implementadas algumas estratégias de actualização do estado de tensão no material durante o processo de análise não linear. Com o objectivo de melhorar as características de convergência dos métodos numéricos utilizados foram introduzidos algoritmos baseados na técnica arc-length. Condições de apoio com comportamento linear, não linear e unilateral também foram numericamente implementadas. Todos os modelos numéricos foram implementados no software de elementos finitos FEMIX usando a linguagem de programação ANSI-C

Shear strengthening of reinforced concrete beams with hybrid composite plates

The effectiveness of Hybrid Composite Plates (HCPs) for the shear strengthening of the Reinforced Concrete (RC) beams was assessed by an experimental program. HCP is a thin plate of Strain Hardening Cementitious Composite (SHCC) reinforced with Carbon Fiber Reinforced Polymer (CFRP) laminates. Due to the excellent bond conditions between SHCC and CFRP laminates, these reinforcements provide the necessary tensile strength capacity to the HCP. Two HCPs with different inclination of CFRP laminates (45º and 90º) were adopted for the shear strengthening of RC beams by bonding these HCPs to the lateral faces of the beam with an epoxy adhesive. The results showed that these HCPs have assured a significant increase in terms of load carrying capacity, mainly those with inclined laminates. The SHCC surrounding the CFRP laminates in the HCP has offered effective resistance to the degeneration of micro-cracks on macro-cracks, which has avoided the occurrence of premature mixed shallow semi-pyramid-plus-debonding failure modes registered currently when using the NSM-CFRP technique. Advanced numerical simulations were performed by using a FEM-based computer program, whose predictive performance was demonstrated by simulating the experimental tests carried out. In this context a parametric study was executed to evaluate the shear strengthening efficiency of the arrangement and percentage of CFRP laminates in HCPs, as well as the influence of using mechanical anchors to avoid premature detachment of the HCPs.“PrePam –Pre-fabricated thin panels by using 17 advanced materials for structural rehabilitation” with reference number of PTDC/ECM/114511/200

Assessing the applicability of a smeared crack approach for simulating the behaviour of concrete beams flexurally reinforced with GFRP bars and failing in shear

Numerical simulation of beams failing in shear is still a challenge. With the scope of verifying the applicability of smeared crack approaches to simulate the behavior of reinforced concrete (RC) beams failing in shear, a set of concrete beams reinforced with longitudinal glass fiber reinforced polymer (GFRP) bars, experimentally tested up to their failure, and comprehensibly monitored, are numerically simulated. The simulations are carried out with a multi-directional fixed smeared crack model available in the FEMIX computer program that has several options for modeling the crack shear stress transfer, which is a critical aspect when simulating RC elements failing in shear. The predictive performance of the numerical simulations is assessed in term of load vs deflection, crack pattern at failure, concrete strains in critical shear regions, and moment–curvature relationship. The influence on the predictive performance of the following modeling aspects is also investigated: finite element mesh refinement; simulation of the crack shear stress transfer by using the classical shear retention factor and a crack shear-softening diagram; bond conditions between flexural reinforcement and surrounding concrete. The simulations carried out demonstrate that small dependence of the results on the finite element mesh refinement and adequate crack patterns can be obtained with refinement levels suitable for design purposes and taking into account the actual computer performances, as long as a crack shear-softening diagram is used. However, the predictive performance of the simulations depends significantly on the values adopted for the parameters that define this diagram, as demonstrated by the performed parametric studies.The first author aims to acknowledge the support provided by FCT through the research project ICoSyTec -Innovative construction system for a new generation of high performance buildings, with reference: POCI-01-0145-FEDER-027990

Plastic-damage smeared crack model to simulate the behaviour of structures made by cement based materials

This work proposes a constitutive model to simulate nonlinear behaviour of cement based materials subjected to different loading paths. The model incorporates a multidirectional fixed smeared crack approach to simulate crack initiation and propagation, whereas the inelastic behaviour of material between cracks is treated by a numerical strategy that combines plasticity and damage theories. For capturing more realistically the shear stress transfer between the crack surfaces, a softening diagram is assumed for modelling the crack shear stress versus crack shear strain. The plastic damage model is based on the yield function, flow rule and evolution law for hardening variable, and includes an explicit isotropic damage law to simulate the stiffness degradation and the softening behaviour of cement based materials in compression. This model was implemented into the FEMIX computer program, and experimental tests at material scale were simulated to appraise the predictive performance of this constitutive model. The applicability of the model for simulating the behaviour of reinforced concrete shear wall panels submitted to biaxial loading conditions, and RC beams failing in shear is investigated.The authors wish to acknowledge the FCT financial support provided by the Portuguese Foundation for Science and Technology in the scope of the SlabSys-HFRC research project, with reference PTDC/ECM/120394/2010

A new basic creep model coupled with a thermomechanical model for the numerical simulation of the time-dependent behaviour of concrete structures

This work is dedicated to the description of a new basic creep model that was developed and integrated in a thermo-mechanical model already available in a FEM-based software – FEMIX. The basic creep model is based on the Dirichlet series expansion of the Double Power Law (DPL) approach, and is capable of predicting the aging creep behaviour of cement based materials (CBM) since early ages. Based on experimental results, the model resorts to a non-linear least square datafitting operation to various loading ages creep compliance curves, and determines a set of model defining coefficients to simulate the aging viscoelastic properties of any CBM. This model was integrated with a thermo-mechanical model capable of simulating maturation, shrinkage and cracking phenomena of CBM. The good predictive performance of the implemented model is appraised by simulating experimental tests at material and structural scale.The authors acknowledge the support provided by FEDER funds through the Operational Programme for Competitiveness and Internationalization - COMPETE and by national funds through FCT (Portuguese Foundation for Science and Technology) within the scope of the project InOlicTower, POCI-01- 0145-FEDER-016905 (PTDC/ECMEST/2635/2014). The first Author would like to acknowledge the grant SFRH/BDE/96381/2013 co-funded by CiviTest and by FCT

Nonlinear analysis of offshore wind towers in prefabricated segments of prestressed fibre reinforced concrete

This paper presents the nonlinear finite element analysis of a new concept of offshore wind tower made by prefabricated prestressed fibre reinforced concrete (FRC) segments that are assembled to form the final structure. Fibre reinforcement aims to eliminate conventional passive steel reinforcement in order to avoid corrosion concerns and decrease the thickness of the segments. The first stage of the design approach consists on an analytical model that optimizes the geometry of the eolic tower by considering the relevant loading cases, the properties of the developed FRC, the resisting stress levels of the constituent materials and the frequency and lateral deformability of the tower. By determining the thickness and radius along with the height of the tower, this model can provide the solution of minimum FRC volume for the eolic tower. In the second stage of the design approach, the optimum solution from the previous design stage is simulated by a finite element approach that considers the geometric and material nonlinear features. This paper describes the main relevant aspects of this design methodology.This work was supported by FEDER funds through the Operational Programme for Competitiveness and Internationalization - COMPETE and by national funds through FCT - Portuguese Foundation for Science and Technology within the scope of the project InOlicTower - Innovative structural system based on advanced materials for lightweight and durable Offshore Wind Towers, POCI-01-0145-FEDER-016905 (PTDC/ECM-EST/2635/2014)

Comportamento de vigas de betão armado de secção oca submetidas a flexão, corte e torção

Neste trabalho são descritos ensaios de vigas de betão armado submetidas a flexão, corte e torção. Estes ensaios fazem parte de um trabalho de investigação experimental e numérico relativo ao comportamento não linear material de estruturas de betão armado. Os resultados obtidos são apresentados e discutidos. Para determinar as principais características do betão e do aço foram também realizados ensaios de compressão uniaxial em provetes cilíndricos de betão simples, ensaios de flexão sob três pontos de carga em vigas entalhadas de betão simples e ensaios de tracção em varões de aço