Joaquim Figueiras

Machine learning algorithms for damage detection under operational and environmental variability

The goal of this article is to detect structural damage in the presence of operational and environmental variations using vibration-based damage identification procedures. For this purpose, four machine learning algorithms are applied based on the auto-associative neural network, factor analysis, Mahalanobis distance, and singular value decomposition. A base-excited three-story frame structure was tested in laboratory environment to obtain time-series data from an array of accelerometers under several structural state conditions. Tests were performed with varying stiffness and mass conditions with the assumption that these sources of variability are representative of changing operational and environmental conditions. Damage is simulated through nonlinear effects introduced by a bumper mechanism that induces a repetitive, impact-type nonlinearity. This mechanism intends to simulate the cracks that open and close under dynamic loads or loose connections that rattle. The unique contribution of this study is a direct comparison of the four proposed machine learning algorithms that have been reported as reliable approaches to separate structural conditions with changes resulting from damage from changes caused by operational and environmental variations.

Finite element analysis of reinforced and prestressed concrete structures including thermal loading

Abstract This paper describes the application of finite element techniques to the solution of nonlinear concrete problems. Reinforced concrete thick plates and shells are first considered for which both a perfect and strain-hardening plasticity approach are employed to model the compressive behaviour. A dual criterion for yielding and crushing in terms of stresses and strains is considered, which is complemented with a tension cut-off representation. Degenerate thick shell elements employing a layered discretisation through the thickness are adopted and both reduced and selectively integrated 8-node serendipity and heterosis elements are considered. Thermal loading of prestressed concrete structures is also considered which necessitates the inclusion of time effects in the analysis. The technique described in this paper involves concurrently solving an uncoupled set of equations within a time interval to provide both the displacement and temperature increments. A two-level time stepping scheme is employed to predict temperature changes within a time interval and elasto-viscoplastic material analysis is performed using an explicit forward-difference scheme incorporating an equilibrium iteration procedure. The constitutive model for the concrete is essentially identical to that employed for the plate and shell analysis. Numerical examples are presented for both types of analysis and comparison is made with experimental results whenever possible. Additionally, results for thermal loading are presented which indicate that a full transient thermal-mechanical analysis is sometimes essential in order to obtain a realistic structural response.

Structural health monitoring algorithm comparisons using standard data sets

The real-world structures are subjected to operational and environmental condition changes that impose difficulties in detecting and identifying structural damage. The aim of this report is to detect damage with the presence of such operational and environmental condition changes through the application of the Los Alamos National Laboratory’s statistical pattern recognition paradigm for structural health monitoring (SHM). The test structure is a laboratory three-story building, and the damage is simulated through nonlinear effects introduced by a bumper mechanism that simulates a repetitive impact-type nonlinearity. The report reviews and illustrates various statistical principles that have had wide application in many engineering fields. The intent is to provide the reader with an introduction to feature extraction and statistical modelling for feature classification in the context of SHM. In this process, the strengths and limitations of some actual statistical techniques used to detect damage in the structures are discussed. In the hierarchical structure of damage detection, this report is only concerned with the first step of the damage detection strategy, which is the evaluation of the existence of damage in the structure. The data from this study and a detailed description of the test structure are available for download at: http://institute.lanl.gov/ei/software-and-data/.

Fatigue behavior of fiber-reinforced concrete in compression

Abstract An experimental program has been carried out to evaluate the performance of plain concrete and fiber-reinforced concrete under compressive fatigue loading. Two types of hooked-end steel fibers (30 mm length and 60 mm length) have been tested and their performance compared. The displacements and the acting load were measured during the tests so that several material parameters could be identified and assessed. Wohler diagrams were determined, cyclic creep curves were plotted and the evolution of the secant stiffness was also appraised for the tested materials. The correlation between the secondary creep rate and the fatigue life was investigated. The monotonic stress–strain envelope was compared with fatigue deformations at failure and a good agreement was found between them.

Influence of the Autoregressive Model Order on Damage Detection

Abstract: An important step for using time-series autoregressive (AR) models for structural health monitoring is the estimation of the appropriate model order. To obtain an optimal AR model order for such processes, this article presents and discusses four techniques based on Akaike information criterion, partial autocorrelation function, root mean squared error, and singular value decomposition. A unique contribution of this work is to provide a comparative study with three different AR models that is carried out to understand the influence of the model order on the damage detection process in the presence of simulated operational and environmental variability. A three-story base-excited frame structure was used as a test bed in a laboratory setting, and data sets were measured for several structural state conditions. Damage was introduced by a bumper mechanism that induces a repetitive impact-type nonlinearity. The operational and environmental effects were simulated by adding mass and by changing the stiffness properties of the columns. It was found that these four techniques do not converge to a unique solution, rather all require somewhat qualitative interpretation to define the optimal model order. The comparative study carried out on these data sets shows that the AR model order range defined by the four techniques provides robust damage detection in the presence of simulated operational and environmental variability.

Model for the analysis of steel fibre reinforced concrete slabs on grade

Abstract A constitutive model is developed for material non-linear analysis of steel fibre reinforced concrete slabs supported on soil. The energy absorption capacity provided by fibre reinforcement is taken into account in the material constitutive relationship. The theory of plasticity is used to deal with the elasto-plastic behaviour of concrete. A smeared-crack model is used for reproducing the concrete cracking behaviour. The soil non-linear behaviour is simulated by springs on orthogonal direction to the slab. The loss of contact between the slab and the soil is accounted for. The model performance is assessed using results of experimental research.

Experimental behaviour of fibre concrete slabs on soil

The cracking control of plain concrete slabs on soil foundation requires the execution of joints with mechanisms of load transfer between adjacent panels. These joints increase the construction costs and, often, are the source of local damage and loss of service performance. Slabs reinforced with steel wire mesh have been used in order to increase the load-carrying capacity and to enhance the cracking control. However, the use of this conventional reinforcement increases the costs, mainly due to labour time spent on the arrangement and positioning of the reinforcement. Fibre-reinforced concrete is a recent material well fitted for applications in industrial floors on soil foundation. The cost of fibres is compensated by a faster construction process and a reduction in the number of expansion joints. The fatigue, impact and flexural strength are significantly improved when steel fibres are added to the concrete mix. The work developed aims to contribute to the on going research effort to clarify the behaviour of fibre-reinforced concrete slabs on soil foundation. For this purpose, an experimental and numerical investigation were carried out. The present article deals basically with the experimental work developed, describing the tests performed and discussing the main results obtained.

Modelling of prestress in non-linear analysis of concrete structures

Abstract This paper describes a technique for modelling the structural action of prestressing tendons in finite element based models developed for the non-linear analysis and design of prestressed concrete structures. Prestressing steel is simulated by 1-D curved elements embedded in the finite elements employed to discretize the concrete structure. The contribution of bonded or unbonded prestressing tendons to the serviceability behaviour and to the ultimate strength capacity of prestressed structures is adequately quantified and accounted for. The numerical procedures adopted to model the prestressing operation and the associated prestressing losses are discussed. The formulation is developed to be incorporated with curved thick shell elements and the applicability of the computational code is appraised by the results of two numerical examples.

Fiber-optic-based displacement transducer to measure bridge deflections

Structural health monitoring is an emerging area of strategic engineering with a great potential for extending the service life of civil infrastructures and reducing their maintenance costs. In spite of the deflection measurement being one of the main parameters for assessing the real state of a bridge, the measurement techniques and the existing deflection transducers often do not respond to the necessities of structural monitoring. Therefore, a novel displacement transducer to measure bridge deflections based on noncontact measurement technique supported by the liquid leveling method was developed. The measurement of bridge deflections is performed using a hydrostatic level position along the structure as an absolute reference and without any external physical reference to the ground. Fiber-optic sensors based on fiber Bragg grating technology are the sensing units of this original bridge deflection transducer. The transducer performance obtained in a set of experimental tests has shown an accurate linear response and a very good behavior under the environmental conditions simulated in a climatic chamber. A field application at the Lezíria Bridge across the Tagus River, Portugal, evinces the ability of the present transducer to be applied in both temporary and permanent structural health monitoring systems with automatic and remote data acquisition.

Numerical Evaluation of the Long-Term Behavior of Precast Continuous Bridge Decks

Continuous bridge decks constructed with precast girders undergo significant stress changes caused by the concrete delayed deformations because of creep and shrinkage. These effects must be taken into account in the design of new structures. The validation of the analysis procedures should ideally be carried out through the comparison between the calculations and the results observed in real structures. However, experimental results of the construction and long-term behavior of these structures are scarce. The construction of a major bridge in Portugal has provided the opportunity to monitor one such structure. This paper presents the monitoring campaign and the analysis strategy that was developed to assess the long-term variation of strains and stresses in precast continuous bridges. The numerical analysis was validated by comparison with the results observed in the real bridge. The consequences of carrying out simplified analyses based on limited information about the concrete properties and the construction sequence are also evaluated.