Modern Testing Techniques for Structural Systems

System identification is the process of obtaining descriptions of dynamical systems based on the analysis of experimental observations. This chapter presents, in sufficient detail to allow a numerical implementation by the interested reader, an algorithm for identifying a state space description of a time invariant system that behaves linearly. The chapter begins by clarifying the connection between the discrete time models that are identified from sampled data and the underlying continuous time physical systems. The (formal) connection is shown to be strictly related to the assumed parameterization of the excitation in the inter-sample and it is noted that the commonly used zero order hold premise (constant load within the sampling time), ubiquitous in control applications, is usually suboptimal in off-line identification. Specifically, it is contended that accuracy in the transfer from discrete to continuous time can be promoted by operating on the premise that the input is a band limited function. The notions of controllability and observability, which relate the sensor deployment to the modes that can be identified from observations, are discussed following the discrete to continuous transfer section.

The central part of the chapter examines the extraction of pulse response functions for multiple input multiple output testing setups and the use of these functions in the formulation of a discrete time state space model by means of the Eigensystem Realization Algorithm. The chapter continues by illustrating the connection between the realization matrices and the modal properties of the system, i.e., modal frequencies, damping ratios and mode shapes. The need to separate system modes from modes that appear in the computations due to inevitable approximations and sensor noise is also discussed and some specific guidelines to discriminate between the two are given. The identification part of the chapter closes with an introduction to the identification of systems where the excitation signals are not deterministically known. An appendix presenting a technique to localize damage in structural or mechanical systems from changes in realization results concludes the chapter.

Non-linear dynamic analysis is recognized as the more accurate tool for seismic evaluation of structures in the case of both probabilistic assessment and design. The key issue in performing this kind of analysis is the selection of appropriate seismic input (e.g. ground motion signals), which should allow for a correct and accurate estimation of the seismic performance on the basis of the hazard at the site where the structure is located. To this aim several procedures have been proposed, they require specific characterization of real ground motion records via the so called ground motion intensity measures (mainly related with elastic spectral features of the record) proven to be generally efficient in the estimation of the structural performance. This kind of approach requires specific skills as well as detailed probabilistic evaluation of the seismic threat to be available to the engineers. For this and other reasons codes worldwide, in many cases, try to acknowledge these procedures in an approximate fashion.

In this paper recent and advanced literature on the topic is presented and discussed. The current best practice in record selection is reviewed for the case of probabilistic seismic risk analysis and for code-based seismic assessment and design with speical attention to the prescriptions of Eurocode 8 for both buildings and bridges. Finally, some light is briefly shaded on the effects of time scaling of records and its use in shake-table structural testing.

This chapter reflects the first of the four lectures that the author has presented at Udine, Italy, at CISM-International Centre for Mechanical Sciences, in June 2007, in the aim of the course “New Approaches to Analysis and Testing of Mechanical and Structural Systems”. He is being managing experimental activities in seismic engineering since 1993, when LNEC (the Portuguese National Laboratory for Civil Engineering) has started the construction of a large triaxial shaking table at the new earthquake testing hall of its Structures Department. Following a presentation of the main objectives of the Seismic Engineering experimentation, a brief state-of-the-art is made concerning its main laboratory methodologies, with a more clear focus on the shaking table tests. The leading problems related to the main testing methodologies (static, pseudo-dynamic, and shaking table) are identified and discussed. The advantages and disadvantages of each one are also highlighted. Moreover, a brief presentation of the centrifuge tests is introduced and, finally, the future developments needed are identified.

This work presents the development and implementation of a domain decomposition approach for solving dynamics problems involving continuous testing with analytical substructuring in the non-linear range. This approach implements an inter-field parallel time integration procedure suitable to work with an explicit experimental synchronous process using small time steps, such as the one running continuous PsD testing, and an implicit, possibly iterative, analytical process using large time steps. The characteristics and the robustness of the proposed schemes are illustrated by means of a parametric numerical study. The results of the first validation under test conditions are also presented. For completness, the presentation includes a short presentation of the continuous PsD testing and its implementation at the ELSA laboratory and some general comments regarding substructuring.

Real-time hybrid testing combines the efficiency of numerical simulation with the realism of experimental analysis. This chapter presents an overview of real-time hybrid test methods from a general perspective, and discusses schemes of structural partitioning and issues related to numerical computation, system dynamics, and system performance. Different approaches of hybrid testing are presented under a unified framework. A specific implementation, including the hardware configuration, numerical integration scheme, and delay compensation methods, is discussed in detail. Results of experimental validation and numerical simulation study are presented to illustrate the effectiveness of several delay compensation schemes and the subtleties of real-time testing. The possibility of conducting reliable real-time hybrid tests using an unconditionally stable implicit integration scheme is demonstrated.