Modal identification of bridge systems using state-space methods

Arrays of large numbers of sensors and accompanying complex system identification (SI) methods have been recently used in engineering applications on structures ranging from complex real space trusses to simple experimental beams. However, practical application to strong motion data recorded on large civil engineering systems is limited. In this study, state-space identification methods are used for modal identification from earthquake records with further investigation into the effectiveness of the methods from the viewpoint of sensor layout configuration. The study presented in this paper adopts a deterministic approach, which is complemented with statistical evaluation in another paper. The used SI methods include eigen realization, system realization with information matrix and subspace methods. The application of the methods on instrumented bridge systems in California is included and the performance of these methods is compared in terms of success and feasibility. Subsequently, viability of the methods on arrays of different numbers of sensors on the bridge systems is investigated. This is motivated by the fact that sensor arrays with wireless communication may be conveniently installed on civil engineering structures in the future. Computational costs and limits of applicability are examined using simulated ground motion analysis with detailed finite element models for a bridge system after validation using recorded data from real ground shaking. Moreover, the effect of the configuration of the sensor arrays is considered, accounting for different noise levels in the data to reflect more realistic situations. Copyright 0 (c) 2005 John Wiley & Sons, Ltd

System identification of instrumented bridge systems

Several recorded motions for seven bridge systems in California during recent earthquakes were analysed using parametric and non-parametric system identification (SI) methods. The bridges were selected considering the availability of an adequate array of accelerometers and accounting for different structural systems, materials, geometry and soil types. The results of the application of SI methods included identification of modal frequencies and damping ratios. Excellent fits of the recorded motion in the time domain were obtained using parametric methods. The multi-input/single-output SI method was a suitable approach considering the instrumentation layout for these bridges. Use of the constructed linear filters for prediction purposes was also demonstrated for three bridge systems. Reasonable prediction results were obtained considering the various limitations of the procedure. Finally, the study was concluded by identifying the change of the modal frequencies and damping of a particular bridge system in time using recursive filters. Copyright (C) 2003 John Wiley Sons, Ltd

Modal analysis of a densely instrumented building using strong motion data

A densely instrumented residential building was tested using seismic simulator (shake table) for a series of strong events. These events included several runs of 200% Izmit Turkey earthquake 1999, which caused over 20,000 casualties. The building was brought to near failure condition through these experiments sequentially. The instrumentation on the system consisted of commonly used piezoresistive and wireless micro-electromechanical systems (MEMS) accelerometers along with conventional strain and displacement measurements. Part, of the ground story was heavily damaged due to these experiments. System identification studies using multiple-input/multiple-output procedure were performed to obtain modal properties of the building. The change of the modal properties of the system was monitored and documented. The obtained natural frequencies and modes were compared with results from shaker tests. The statistical proper-ties of the estimated modal proper-ties were determined using Monte Carlo, simulations. Comparison of conventional piezoresistive accelerometers and new wireless MEMS sensors is also given in the paper for the purpose of future extensions of the presented study

Statistical significance of modal parameters of bridge systems identified from strong motion data

Modal parameters of structural systems have commonly been determined using system identification (SI) methods for damage detection and health monitoring. For determining the deterioration of the integrity of structural systems correctly, modal parameters of a healthy structure have to be obtained with adequate certainty so that these parameters can be used as reliable references for the healthy system to compare with those of the damaged system. In this study, the statistical significance of modal parameters identified using strong motion time histories recorded on two bridge structures is assessed. The confidence intervals of identified modal frequencies and damping ratios are obtained using Monte Carlo simulations and sensitivity analyses in conjunction with eigenrealization algorithm. The dependence of the statistical bounds on model parameters is examined. The effect of using different number of sensors on the statistical significance is evaluated using simulated time history data from a validated finite element model of a bridge. Copyright (c) 2005 John Wiley & Sons, Ltd

Theoretical evaluation of hybrid simulation applied to continuous plate structures

Hybrid simulation is a popular experimental technique whereby only part of a system is physically realized and the remainder is modeled in a computer with a set of actuators and sensors to connect the two subsystems. While the methodology is common, it lacks a theoretical structure that ensures users are getting valid simulation results of the entire system. Further, little attention has been paid to distributed mass systems and those that do not have a beam/column like topology. This work examines three basic issues: (1) an abstract geometric scheme is proposed by which one can reason about hybrid simulation systems and their underlying errors; (2) systems with distributed mass are explicitly considered; and (3) the model system utilized in this study has a distinctly nonbeam/column like system, namely, a Kirchhoff-Love plate with a continuous one-dimensional hybrid system interface. It is demonstrated that such systems are generally viable only below the first fundamental frequency of the system. Furthermore, it is shown that there is a tendency to accumulate global errors, relative to the classical solution, at the slightest introduction of any interface matching error but that these errors are mostly insensitive to further increase in mismatch. Finally, it is found that the different substructures of the systems are subject to resonant excitation at their own independent natural frequencies in addition to those of the complete hybrid system

Hybrid simulation theory for continuous beams

Hybrid simulation is an experimental technique involving the integration of a physical system and a computational system with the use of actuators and sensors. This method has a long history in the experimental community and has been used for nearly 40 years. However, there is a distinct lack of theoretical research on the performance of this method. Hybrid simulation experiments are performed with the implicit assumption of an accurate result as long as sensor and actuator errors are minimized. However, no theoretical results confirm this intuition nor is it understood how minimal the error should be and what the essential controlling factors are. To address this deficit in knowledge, this study considers the problem as one of tracking the trajectory of a dynamical system in a suitably defined configuration space. To make progress, the study strictly considers a theoretical hybrid system. This allows for precise definitions of errors during hybrid simulation. As a model system, the study looks at an elastic beam as well as a viscoelastic beam. In both cases, systems with a continuous distribution of mass are considered as occur in real physical systems. Errors in the system are then tracked during harmonic excitation using space-time L2-norms defined over the system's configuration space. A parametric study is then presented of how magnitude and phase errors in the control system relate to the performance of hybrid simulation. It is seen that there are sharp sensitivities to control system errors. Further, the existence of unacceptably high errors whenever the excitations exceed the system's fundamental frequency is shown to be present in hybrid simulation

Theoretical evaluation of hybrid simulation for classical problems in continuum mechanics

The primary notion of hybrid simulation is to only test part of a system physically and to simulate the rest in a computer. While this basic idea is simple to understand, there is surprisingly little theoretical work targeted towards understanding the behavior of the concept, and in particular, its theoretical limitations. In this work we present an initial investigation of the theoretical limitations of hybrid testing in the context of two canonical settings: a beam and a plate. In each case, we mathematically split the physical system into two pieces whose motion we derive in closed-form. At the splitting interface we introduce theoretical models associated with tracking and phase error of the boundary motions and forces. We are able to demonstrate that such systems are generally viable only below the first fundamental frequency of the system. Furthermore, we show there is a tendency to accumulate global errors, relative to the classical solutions, at the slightest introduction of any interface matching error but that these errors are mostly insensitive to further increase in mismatch. Finally, it is found that the different substructures of the systems are subject to excitation at their own independent natural frequencies in addition to those of the hybrid system