Andrew J Deeks

Structural damage will cause changes in structural vibration properties such as vibration frequency, mode shape and damping ratio. Therefore the structural vibration properties are commonly used in structural condition monitoring and damage identification by comparing them with the theoretical or numerical vibration properties of the undamaged structures. In practice, owing to variations in workmanship, material properties, and environmental conditions, the structural vibration properties derived from theoretical or numerical model may differ from the actual values. This may lead to the damage being undetectable if the change in vibration properties owing to Structural damage is smaller than the modelling difference. In this study, six continuously supported RC slabs having the same dimensions and support conditions were constructed in the laboratory. Four slabs were tested to failure after the concrete was cured 35 days, and another two left unloaded in the lab for more than a year before load tested to failure. Vibration tests were performed on all the slabs before and during the tests at different loading levels to extract slab vibration frequency, mode shape and damping ratio. The primary objective of the study is to demonstrate and quantify the variations of the vibration properties of the RC slab structures.

Finite element methods are well-established for modelling problems in geotechnics involving elasto-plastic constitutive models. However, meshless methods could offer some advantages for problems involving finite deformations as the mesh entanglement problem is then removed. In addition, major savings could be made in the pre-processing stage for 3D problems. Both of these features are of interest to geotechnical modellers. In this paper we review the use of meshless methods for modelling plasticity, highlighting some potential problems particular to the methods' formulations. We then demonstrate the use of a recently developed hybrid method based on the meshless local Petrov-Galerkin method for some simple examples in elasto-plasticity and show that there are important issues to be considered. Finally we comment briefly on the amendments required for modelling finite deformation, which are underway.

Ambient vibration tests have been gaining more and more popularity in structural condition monitoring because they are easy to carry out and cause only minimum disturbance to the normal operation of the tested structures. Compared to the forced vibration test, the input force is not known in ambient vibration test. The ambient excitation sources may come from wind, sea wave, traffic, and other natural sources and human activities. Common approaches to extract structural vibration properties from ambient vibration tests are based on the assumption that the ambient excitation force is a white noise. In reality, the unknown excitation forces may not be white noise, or even not a wide band process, and inevitably contain noises. In this paper, the reliability of using ambient vibration tests to extract structural vibration properties is investigated. Vibration test data from the ASCE/IASC benchmark model obtained from three excitation sources, namely impact hammer, shaker with random white noise excitations and ambient excitation (traffic and human activities) are analyzed. The extracted vibration properties (frequency, mode shape and damping) are compared. A numerical model is also created to simulate dynamic testing. Numerically simulated data is smeared with different types of noises to investigate the influence of noises on extracted structural vibration parameters. Discussions are made on the accuracy of using ambient vibration tests to extract structural vibration properties. The conditions under which the ambient vibration tests lead to accurate structural property extraction are also defined.

The scaled boundary finite-element method is a novel semi-analytical technique and has been applied to a wide variety of computational problems with great success. This paper deals with the solution of problems involving moving loads through the computational domain at a constant velocity. The problem has been formulated previously with a moving coordinate system following the applied load. As the reference frame is moving with the applied load the problem is reduced to a pseudo-static case which can be solved using conventional techniques. In the original formulation damping was ignored for simplicity, and as a result symmetric solutions were obtained. In this paper damping is introduced into the formulation, including the introduction of mode shapes compatible with the boundary condition at infinity. This results in a non-homogeneous set of equations requiring a Frobenius solution for the resulting set of differential equations.

This paper discusses work done in the pursuit of accuracy in computational mechanics, with particular application to the computation of linear elastic stress fields for problems of structural mechanics. Practical and theoretical aspects of modeling such problems are addressed, including appropriate specification of boundary conditions and the presence of stress singularities at re-entrant corners. The scaled boundary finite element method is introduced as a method of efficiently and accurately computing stress fields in the region of singularities. Various types of adaptivity are discussed, and the reasons for the current low level of use of these techniques in practice considered. The advances necessary for wider application of linear stress analysis to structural design are addressed.

Artificial Neural networks (ANN) have been proven in many studies to be able to efficiently detect damage from vibration measurements. Their capability to recognize patterns and to handle non-linear and non-unique problems provides an advantage over traditional mathematical methods in correlating the vibration data to damage location and severity. However, one shortcoming of ANN is they require enormous computational effort and sometimes prohibitive time and computer memory for training a reliable ANN model, especially when structures with many degrees of freedom are involved. Therefore, in most cases, rather large elements are used in the structure model to reduce the degrees of freedom. This makes the structural vibration properties not sensitive to small damage in a large element. As a result, direct application of ANN to detecting damage in a large civil engineering structure is not feasible. In this study, a multi-stage ANN incorporating a probability method is proposed to tackle this problem. Through this method, a structure is divided to several substructures, and each substructure is assessed independently. In each subsequent stage, only the damaged substructures are analyzed, and eventually the location and severity of small structural damage can be detected. This approach greatly reduces the computational time and the required computer memory. Moreover, a probabilistic method is also used to include the uncertainties in vibration frequencies and mode shapes in damage detection analysis. It is found that the uncertainty effect in frequencies due to duplication error in the multi-stage ANN model and the uncertainty effect in mode shapes due to the damage in other substructures can be reduced. Numerical examples demonstrate that the proposed method can detect small damage with a higher level of confidence, and the undamaged elements are less likely to be falsely detected.

Balla Balla River Bridge in the Pilbara region of WA was selected for condition assessment, with particular attention to be paid to the condition of the shear connectors. Application of local and global vibration methods to a scaled model found that the local vibration approach can detect the artificial damage successfully and consistently by directly comparing vibration responses of the RC slab and girders. A two-day field test on the bridge was carried out, and dynamic properties were extracted. Through both local and global approaches, it was found that the girders are in good condition, slabs are deteriorated and some shear connectors are more flexible than expected.

In this paper, to analyze the initial valued non-homogeneous elastic half space by the scaled boundary analysis, the infinite element approach was introduced. The free surface of the initial valued non-homogeneous elastic half space was modeled as a boundary of the circumferential direction of the scaled boundary coordinate. The infinite element was used to represent the infinite length of the free surface. The initial value of material property was considered by the position of the scaling center and the power function of the radial direction. By the using of the mapping type infinite element, the consistent elements formulation could be available. The performance and the feasibility of proposed approach are examined by a numerical example.

This study develops a two-step method, coupling the finite element method (FEM) and the scaled boundary finite element method (SBFEM), to model cohesive crack growth in quasi-brittle normal-sized structures. In the first step, the crack trajectory is fully-automatically predicted by a remeshing procedure using the SBFEM based on linear elastic fracture mechanics theories. In the second, interfacial finite elements with tension-softening constitutive laws are inserted into the crack path to model energy dissipation in the fracture process zone, while the elastic bulk material is modelled by the SBFEM. The resultant nonlinear equation system is solved by local arc-length controlled solvers. A concrete beam subjected to mixed-mode fracture is modelled using the proposed method. The numerical results demonstrate that this new method can predict both satisfactory crack trajectories and accurate load-displacement relations with a small number of degrees of freedom.

The scaled boundary finite element method (SBFEM) models the linear elastostatics problem excellently and out-performs the finite element method (FEM) when solving problems involving unbounded domains or stress singularities. This study enhances the classical energy norm based adaptive procedure by introducing new refinement criteria, based on the projection-based interpolation technique and the steepest descent method, to drive the mesh refinement. The technique is applied to p-adaptive procedure in this paper but an extension to other adaptive versions such as h- and hp-adaptivity is straightforward. The reference solution, which is the solution of the fine mesh formed by uniformly refining the current mesh, is used to represent the unknown exact solution. In the conventional adaptive approach, the optimum mesh is assumed to be obtained when each element contributes equally to the global error. In the new adaptive approach, the projection-based interpolation technique is developed for2Din the scaled boundary finite element method and the projection-based interpolants are computed from different approaches. New refinement criteria are proposed. The optimum mesh is assumed to be obtained by maximizing the decrease rate of the projection-based interpolation error appearing in the reference solution. Numerical studies showthat the newapproach out-performs the conventional approach.