Loading...
13 results
Search Results
Now showing 1 - 10 of 13
- Experimental measurement of bridge deflection using Digital Image CorrelationPublication . Sousa, Pedro J.; Barros, Francisco; Lobo, Paulo; Tavares, Paulo J.; Moreira, Pedro M. G. P.Civil engineering bridges usually face a multitude of loads, ranging from strong winds to intense traffic. Thus, it is important to understand their behaviour for adequate maintenance design and prevention of catastrophic failure. The present work reports an image sensing methodology for deflection measurement that was successfully employed on the Entre-Águas bridge in Caniçal, Madeira, Portugal. For the application of two-dimensional Digital Image Correlation, it was necessary to build and deploy a patterned target onto the bridge in the monitored area and a camera setup in stable ground on a plane orthogonal to the target. Two different 2D DIC software packages were used for calculations, one commercial and one self-developed, enabling their comparison and increasing the confidence in the experimental results. The developed system was applied in order to analyze the effects of the passage of 30-ton trucks in the bridge, with promising results.
- Shape memory alloys behaviour: a reviewPublication . Lobo, Paulo Silva; Almeida, João; Guerreiro, LuísShape memory alloys are used in a variety of fields, such as medical or aeronautical. Other fields of knowledge have been researching these materials, attracted by their capacity to dissipate energy through high-strain hysteretic cycles without significant residual strains. Because of these interesting properties for seismic protection, an example of the possible beneficiaries of these materials are civil engineering structures. This paper reports a bibliographic review on the characteristics and uniaxial macroscale constitutive models for shape memory alloys, of interest for a significant number of applications, most often based on wires and bars. The constitutive model assessment focuses on mechanical and kinetic laws, as well as on the energy balance law, of relevance for dynamic loadings. Some characteristics of these materials are still not sufficiently well known, especially those related to ageing. With regard to behaviour prediction, the most frequently used uniaxial constitutive models result in similar responses.
- Accuracy of models of concrete in circular columns using different proposals for the prediction of failure of the confining FRPPublication . Lobo, Paulo Silva; Jesus, MarianaConfinement of concrete columns with fiber reinforced polymers results in an increase of strength and ductility. For this reason, the use of aramid, carbon and glass-based composites for confinement of reinforced concrete columns has significantly increased over the last decades. Nevertheless, few models adequately predict the failure strain of the fiber reinforced polymer, which has a determinant influence on the computed results. In this paper the accuracy of existing models of confined concrete using different proposals for the prediction of the failure strain of the confining composite is assessed. This is based on the comparison of analytical results with experimental test results of concrete columns with circular cross-section reported in the literature. The comparison focusses on different parameters such as strength, maximum strain and strain energy density.
- Influence of the SMA constitutive model on the longitudinal seismic response of RC bridgesPublication . Nunes, Pedro; Lobo, Paulo SilvaSeveral studies indicate that the re-centring capacity presented by superelastic Shape Memory Alloys (SMA) can reduce the displacements of structures subjected to earthquakes. To quantify the errors of numerical predictions with different constitutive models, peak, relative and residual displacements of four two-framed reinforced concrete (RC) bridges, linked by SMA bars, subjected to seismic actions, were numerically assessed. Different ratios of the elastic natural period of vibration of the frames of the bridges were adopted. Lumped plasticity models were used to simulate the behaviour of the reinforced concrete columns. Five SMA uniaxial models were considered, combining linear, nonlinear, isothermal and nonisothermal conditions. The seismic action was modelled by real accelerograms. The results show that the relative displacements are more sensitive to the adopted SMA model than the peak and residual displacements. Significant relative differences, up to 70%, were obtained.
- Simplified assessment of the effects of columns shortening on the response of tall concrete buildingsPublication . Correia, Ruben; Lobo, Paulo SilvaThe constructive process as well as the time-dependent effects must be considered in the assessment of the response of complex concrete structures. For tall buildings, the adequate prediction of vertical elements shortening is required to determine its effects on other structural and nonstructural elements, usually overestimated by linear elastic analysis. Thus, simple numerical methods which make it possible to consider the most relevant aspects of the structural behaviour may be useful in the early stages of a project. In the research presented herein a simplified method, which considers the viscoelasticity of concrete as well as the construction sequence, was used. Its adequacy was assessed by comparison of the results for a tall concrete building with those obtained with a commercial software which incorporates a nonlinear staged construction analysis package. The good correlation between the obtained results indicates that the simplified method used may be applied to help make appropriate design choices.
- Conceptual design of transfer structuresPublication . Ribeiro, Gonçalo; Almeida, João; Lobo, Paulo SilvaA transfer structure (TS) is a structure that alters the load path of the gravity loads, shifting the line of thrust laterally to a different vertical alignment. Transfer structures are introduced in buildings that feature discontinuities in some columns or walls and where a direct load path to the foundations is not possible. They usually represent major elements of the structure and their impact on building cost and construction time can be substantial. This paper presents an overview of existing transfer systems and provides guidance on their design and construction. Extensive research on buildings with transfer structures all over the world was carried out (more than 100 examples were analyzed) with the aim of developing a rational typology of these structures. The results can be broken down into five main types: BEAM, TRUSS, INCLINED STRUT, PLATE, and ARCH & TIE.
- Analysis and construction of transfer structures: a case studyPublication . Ribeiro, Gonçalo; Almeida, João; Lobo, PauloA transfer structure of a building is a structure that alters the load path of the gravity loads, shifting the line of thrust laterally to a different vertical alignment. They usually represent major elements of a structure and their impact on building cost and construction time can be substantial. The design for deflection control is frequently the primary consideration when defining the geometry of a transfer element, given their usual long spans and the high magnitude of the forces involved. Considerations of robustness and disproportionate collapse may also be key for the design of transfer structures as these are often regarded as critical elements for the overall stability of the building. Furthermore, the sequence of construction should be modelled through a construction-staged analysis, as it greatly affects deflections as well as final forces distribution. This paper provides guidance and highlights the key aspects that are likely to determine the structural design of transfer structures, as well as typical construction methods and challenges. It also presents a comparative study between two different types of transfer structures for the same building: an alternative composite scheme for the post-tensioned concrete transfer grid of the St. Gabriel Tower (Lisbon) was designed and the most relevant factors are highlighted.
- Accuracy of models of confined concrete in rectangular columns using different proposals for the prediction of failure of the FRPPublication . Jesus, Mariana; Lobo, Paulo SilvaConfinement with externally applied fiber reinforced polymers (FRP), such as carbon, glass and aramid-based composites, results in notorious improvement of ductility and strength. Several constitutive models, regarding stress–strain relationship, have been proposed. However, few models exist for square and rectangular columns confined with FRP when compared with the number of models for circular concrete columns, and even fewer models satisfactorily predict the failure strain of FRP. In this paper, the accuracy of existing models for the prediction of the failure strain of the FRP is evaluated. Comparison of analytical results with experimental test results of concrete columns reported in the literature is presented, focusing different parameters such as strength, maximum strain and strain energy density.
- Bond-slip effect in the assessment of RC structures subjected to seismic actionsPublication . Lobo, Paulo Silva; Almeida, João; Guerreiro, LuísThe effect of bond-slip in the assessment of reinforced concrete structures under static or dynamic cyclic loads with numerical models may be significant. Its relevance is discussed in this paper, by analysis of the correlation of experimental and analytical results. The latter were obtained with a perfect bond-based model and with a fiber-section beam-column model which makes it possible to consider the effect of bond-slip in the vicinity of exterior as well of interior joints of reinforced concrete frame structures. The experimental results consist of a shaking table test of a reinforced concrete frame, with predominant flexural condition, in agreement with the premisses of this research. The model with bond-slip was capable of predicting the structural behaviour in a very satisfactory manner. Furthermore, the results of the assessment including bond-slip are significantly more accurate than those assuming perfect bond.
- Influence of the SMA Constitutive Model on the Response of StructuresPublication . Nunes, Pedro; Lobo, Paulo SilvaStrong earthquakes may impose significant displacements in structures, which can result in excessive displacements at structural joints. Previous numerical studies have shown that the recentring capability of shape memory alloys (SMA) can be applied to limit joint openings and maximum longitudinal displacements. However, these studies do not focus on the influence of the SMA constitutive model adopted on the estimated displacements. A sensitivity analysis was performed using simplified two-degree-of-freedom models, which represent two-frame reinforced concrete bridges with various ratios of natural periods of vibration, connected by SMA bars. These models were implemented in a MATLAB based program for nonlinear dynamic analysis. The obtained results show that the relative displacements are more sensitive to the SMA model than the absolute displacements.