Probabilistic seismic demand models of RC bridge columns retrofitted with shape memory alloy spirals

The breakdown of critical bridge infrastructure in past earthquakes has often been attributed to the failure of reinforced concrete (RC) bridge columns due to lack of flexural ductility. Despite the revision of structural design standards to accommodate the lessons learned from these earthquakes, a significant number of RC bridge columns, built prior to these revisions, are vulnerable to failure under moderate and high intensity earthquakes. Recent studies have shown that retrofitting these vulnerable RC bridge columns by applying lateral active confinement using shape memory alloy (SMA) spirals can significantly improve their ductility, resulting in enhanced seismic performance. The research work done till date in this area is limited to exploring experimentally the efficacy of this new retrofit technique on a material and component level. In order to aid the implementation of this retrofit technique in actual construction practice, this thesis initiates the development of a performance-based-design framework for SMA retrofitted columns by creating seismic demand models which relate the intensity measure (IM) of an earthquake with the demand imposed by the earthquake on SMA retrofitted RC columns, which is quantified in terms of demand measures (DM). An array of vulnerable RC bridge columns, susceptible to flexural failure due to inadequate lateral confinement, is created using Latin hypercube sampling and 6 columns with varying time periods and reinforcement ratios are chosen. These columns are retrofitted with SMA spirals in their plastic hinge region and subjected to a suite of bi-directional ground motion records. The performance of the retrofitted columns is assessed using 4 DM including maximum drift, residual drift, an energy-based concrete damage index and a steel damage index based on low-cycle fatigue. The suitability of 8 IMs for the development of probabilistic demand models to predict the DMs is explored. The optimal IM, which predicts the DM with least uncertainty, is found to be a function of the fundamental period of the retrofitted columns. The final demand models, developed using the optimal IM, are presented and compared to understand the effect of lateral active confinement. The results indicate that increasing the confinement reduces the damage in concrete substantially while the damage associated with low-cycle fatigue of steel is also reduced. Higher levels of active confinement are also seen to be effective in reducing the residual drifts of long period columns

Hygrothermal effects on the free vibration and buckling of laminated composites with cutouts

The effect of moisture concentration and the thermal gradient on the free flexural vibration and buckling of laminated composite plates are investigated. The effect of a centrally located cutout on the global response is also studied. The analysis is carried out within the framework of the extended finite element method. A Heaviside function is used to capture the jump in the displacement and an enriched shear flexible 4-noded quadrilateral element is used for the spatial discretization. The formulation takes into account the transverse shear deformation and accounts for the lamina material properties at elevated moisture concentrations and temperature. The influence of the plate geometry, the geometry of the cutout, the moisture concentration, the thermal gradient and the boundary conditions on the free flexural vibration is numerically studied. (C) 2013 Elsevier Ltd. All rights reserved

Homogenized elastic response of random fiber networks based on strain gradient continuum models

International audienceThe purpose of this work is to develop anisotropic strain gradient linear elastic continuum models for two-dimensional random fiber networks. The constitutive moduli of the strain gradient equivalent continuum are assessed based on the response of the explicit network representation in so-called windows of analysis, in which each fiber is modeled as a beam and the fibers are connected at crossing points with welded joints. The principle of strain energy equivalence based on the extension to the strain gradient of the Hill–Mandel macro homogeneity condition is employed to identify the classical and strain gradient moduli, based on the application of a sequential set of polynomial displacements on windows of analysis of different sizes. The scaling of the first- and second-order moduli with network parameters, such as network density and the ratio of fiber bending to axial stiffness, is determined. We observe a similar dependency of classical and strain gradient moduli on the same network parameters. The internal length scales associated with the gradient coefficients of the constitutive equation are also defined in terms of the network parameters. The strain gradient moduli prove to be size-independent in the affine regime, and they converge toward a size-independent value in the non-affine deformation regime after a rescaling of physical dimensions by the window size. The obtained results show that the strain gradient moduli scale uniformly with the square of the magnitude of the strain gradients applied to the window of analysis