Office of the Dean (CENG) Scholarship

Discovery of Design Methodologies

Cirrus Shakeri et al. — Thu, 14 Aug 2008 11:22:13 PDT

In this paper we present an AI-based approach for the discovery of design methodologies for multi-disciplinary design situations. The approach is based on simulating the design process using a multi-agent system that mimics the behavior of the design team. The system activates the pieces of design knowledge when they become applicable. The use of knowledge by agents is recorded by tracing the steps that the agents have taken during a design project. Many traces are generated by solving a large number of design projects that differ in their requirements. A set of design methodologies is constructed by using clustering techniques to generalize the traces. These methodologies can be used to guide design teams through design projects.

Improvement Of Parameter Estimation for Non-Linear Hysteretic Systems With Slip By A Fast Bayesian Bootstrap Filter

S. J. Li et al. — Thu, 14 Aug 2008 11:22:12 PDT

Modeling and identification of non-linear hysteretic systems are widely encountered in the structural dynamics field, especially for the hysteresis with slip. A model, called SL model, which can describe the pinching of most practical hysteresis loops perfectly was proposed by Baber and Noori (J. Eng. Mech. 111 (1985) 1010). A method of estimating the parameters of SL model on the basis of input–output data based on bootstrap filter was proposed by the writers. Bootstrap filter is a filtering method based on Bayesian state estimation and Monte Carlo method, which has the great advantage of being able to handle any functional non-linearity and system and/or measurement noise of any distribution. The standard bootstrap filter, however, is not time efficient, i.e., it is very time consuming and is not suitable for real-time applications. In this paper, previous work by the writers is extended to do the parameter estimation of SL model by a fast Bayesian bootstrap filtering technique. Simulation results are presented to demonstrate the performance of the algorithm.

Identification of Hysteretic Systems With Slip Using Bootstrap Filter

S. J. Li et al. — Thu, 14 Aug 2008 11:22:11 PDT

Hysteretic models with slip are frequently used to predict the non-linear behaviour of many structural systems, for example wood buildings and reinforced concrete structures. A model, called SL model, which can describe the pinching of most practical hysteresis loops perfectly was proposed by Baber and Noori. This model is characterised by control parameters that have to be identified from observed experimental data. A method of estimating the parameters of SL model on the basis of input–output data based on Bayesian state estimation and bootstrap filter is suggested in this paper, which has the great advantages of being able to handle any functional non-linearity and system and measurement noise of any distribution. A numerical simulation shows its suitability and effective for the system even in the case of very severe material non-linearity.

Application of an Itô-based approximation method to random vibration of a pinching hysteretic system

Mohammad N. Noori et al. — Fri, 01 Aug 2008 14:53:57 PDT

In this paper, an extension of the Cumulant-Neglect closure scheme is utilized for the random vibration analysis of a single degree of freedom system with a general pinching hysteresis restoring force. The hysteresis element used in the system model can simulate commonly observed forms of stiffness, strength and pinching degradations. The second order statistics of the system response to a stationary Gaussian white noise input are derived using an Itô-based approximation technique. The validity of these response statistics are then verified by comparing them to Monte Carlo simulation results. The numerical studies performed for different combinations of degradation parameters and excitation levels show that the response estimates obtained by this solution method are in good agreement with Monte Carlo simulation. These studies also indicate the applicability of this technique for response analysis of complicated forms of non-linearities.

Direct Stochastic Equivalent Linearization of SDOF Smart Mechanical System

L. Duval et al. — Fri, 01 Aug 2008 14:52:38 PDT

The first and second moments of response variables for SDOF system with pseudoelastic material are obtained by a direct linearization procedure. This procedure is an adaptation of well-known statistical linearization methods, and provides concise, model-independent linearization coefficients. The method can be applied to systems that incorporate any SMA hysteresis model having a differential constitutive equation, and can be used for zero and non-zero mean random vibration. This implementation eliminates the effort of deriving linearization coefficients for new SMA hysteresis model. In this paper the complete statistical response of SDOF system containing a mass and a bar made of SMA is obtained via direct linearization procedure. The model considered is modification of phenomenological one-dimensional constitutive model originally proposed by Graesser and Cozzarelli, which provides the capability to model both the martensitic twinning hysteresis and martensitic-austenite pseudoelastic behavior, typical of shape memory alloys. Response statistics for zero mean random vibration are obtained. Furthermore, non-zero mean analysis of the system is carried out and comparisons are made with Monte Carlo simulation.

Discovering Methodologies for Integrated Product Design

Cirrus Shakeri et al. — Fri, 01 Aug 2008 14:51:25 PDT

The current methodologies for multi-disciplinary product design are based on compromising between different disciplines rather than integrating them. These methodologies do not use a systematic and holistic approach to the problem of multi-disciplinary design and thus are piecemeal rather than comprehensive. This paper presents a new approach to producing design methodologies for integration of the different disciplines in the design process. A multi-agent system has been developed that designs a 2-DOF robot arm by incorporating five proposed strategies for integration between disciplines. Design methodologies are extracted by tracking the system and generalizing the traces that are produced. The results show that the trace of the system provides invaluable information on how to improve the design process.

Estimation of Wind Load on Structures

Soheil Saadat et al. — Fri, 01 Aug 2008 14:50:10 PDT

Accurate spatial and temporal estimation of wind loads on structures plays an important role in the design and construction of buildings in coastal regions and open terrains. The common approach to this problem is using codes and standards obtained from wind-tunnel tests on isolated structures. The use of artificial neural networks for finding specific patterns in data obtained from wind-tunnel and field tests has been reported in the literature. In this study localized radial basis functions neural networks are proposed and successfully used for estimation of wind loads on a three-story shear building using a state-space model of the structure.

Letters to the Editor: Filtering Property of a Lightly Damped System, Subjected to Transient Excitation

M. F. Dimentberg et al. — Fri, 01 Aug 2008 14:50:09 PDT

An intelligent parameter varying (IPV) approach for non-linear system identification of base excited structures

Soheil Saadat et al. — Fri, 01 Aug 2008 14:47:58 PDT

Health monitoring and damage detection strategies for base-excited structures typically rely on accurate models of the system dynamics. Restoring forces in these structures can exhibit highly non-linear characteristics, thus accurate non-linear system identification is critical. Parametric system identification approaches are commonly used, but require a priori knowledge of restoring force characteristics. Non-parametric approaches do not require this a priori information, but they typically lack direct associations between the model and the system dynamics, providing limited utility for health monitoring and damage detection. In this paper a novel system identification approach, the intelligent parameter varying (IPV) method, is used to identify constitutive non-linearities in structures subject to seismic excitations. IPV overcomes the limitations of traditional parametric and non-parametric approaches, while preserving the unique benefits of each. It uses embedded radial basis function networks to estimate the constitutive characteristics of inelastic and hysteretic restoring forces in a multi-degree-of-freedom structure. Simulation results are compared to those of a traditional parametric approach, the prediction error method. These results demonstrate the effectiveness of IPV in identifying highly non-linear restoring forces, without a priori information, while preserving a direct association with the structural dynamics.

A Model For General Periodic Excitation With Random Disturbance and its Application

Y. K. Zhou et al. — Fri, 01 Aug 2008 14:46:46 PDT

Many vibration problems involve a general periodic excitation such as those of a triangular or rectangular waveform. In practice, the periodic excitation may become disordered due to uncertainties. This paper presents a stochastic model for general periodic excitations with random disturbances which is constructed by introducing random amplitude and phase disturbances to individual terms in the Fourier series of the corresponding deterministic periodic function. Mean square convergence of the random Fourier series are discussed. Monte Carlo simulation of disordered sawtooth, triangular, and quadratic wave forms are illustrated. An application of the excitation is demonstrated by vibration analysis of a single-degree-of-freedom (SDOF) hydraulic valve system subjected to a disordered periodic fluid pressure. In the present study only the phase disturbance is considered. Effects of the intensity of phase modulation on up to fourth order moment response and the convergence rate of the random Fourier series are studied by numerical results. It is found that a small random disturbance in a general periodic excitation may significantly change the response moment.

Operator Split Technique Applied to Random Vibration of Smart SDOF Mechanical Systems

L. Duval et al. — Thu, 31 Jul 2008 14:35:50 PDT

A significant amount of research in the last few years has greatly advanced the field of structural control through the use of smart materials. Smart materials have the ability to change shape, stiffness, natural frequency, damping and other mechanical characteristics in response to environmental condition changes. One of he new class of materials with promising applications in structural and mechanical systems are Shape Memory Alloys (SMAs). However, there are few studies where the random response of mechanical systems containing shape memory components has been conducted. Such study is important to verify the feasibility on the use of SMAs in structural components. The present contribution reports on the operator split technique applied to random vibration of single-degree-of-freedom (SDOF) mechanical system with a shape memory helical spring. A constitutive theory originally developed by Fremond with internal variables that must satisfy internal constrains is used to model the restoring force provided by the spring. The operator split technique has been used on the solution of well-posed nonlinear dynamic problem and its basic idea is to promote a partition of state space in sub-spaces that may be solved separately. Results in terms of root-mean-square for both zero and non-zero mean random vibration is presented for SDOF shape memory oscillator. Numerical simulations of the system under a wide range of white noise excitation are presented.

Wavelet-Based Approach for Structural Damage Detection

Z. Hou et al. — Tue, 29 Jul 2008 15:31:02 PDT

A wavelet-based approach is proposed for structural damage detection and health monitoring. Characteristics of representative vibration signals under the wavelet transformation are examined. The methodology is then applied to simulation data generated from a simple structural model subjected to a harmonic excitation. The model consists of multiple breakable springs, some of which may suffer irreversible damage when the response exceeds a threshold value or the number of cycles of motion is accumulated beyond their fatigue life. In cases of either abrupt or accumulative damages, occurrence of damage and the moment when it occurs can be clearly determined in the details of the wavelet decomposition of these data. Similar results are observed for the real acceleration data of the seismic response recorded on the roof of a building during the 1971 San Fernando earthquake. Effects of noise intensity and damage severity are investigated and presented by a detectability map. Results show the great promise of the wavelet approach for damage detection and structural health monitoring.

System Identification of Base-Isolated Building using Seismic Response Data

T. Furukawa et al. — Tue, 29 Jul 2008 15:31:00 PDT

Due to the complex nature of the excitation, and the inherent dynamics characteristics of restoring force of the base isolation systems, the response of base-isolated structures subject to strong earthquakes often experiences excursion into the inelastic range. Therefore, in designing base-isolated structures, the nonlinear hysteretic restoring force model of the base isolation system is frequently used to predict structural response and to evaluate structural safety. In this paper, the prediction error method system identification technique is used in conjunction with nonlinear state-space models for identification of a base-isolated structure. Using a variety of nonlinear restoring force models and bidirectional recorded seismic responses, several identification runs are conducted to evaluate the accuracy of the selected models. Several nonlinear restoring force models are utilized for the base-isolation system, including a multiple shear spring (MSS) model. Among all models used, results indicate that the trilinear hysteretic MSS model closely matches the actual hysteretic restoring force profile and time histories obtained directly from the observed data.

Structural health monitoring and damage detection using an intelligent parameter varying (IPV) technique

Soheil Saadat et al. — Tue, 29 Jul 2008 15:29:47 PDT

Most structural health monitoring and damage detection strategies utilize dynamic response information to identify the existence, location, and magnitude of damage. Traditional model-based techniques seek to identify parametric changes in a linear dynamic model, while non-model-based techniques focus on changes in the temporal and frequency characteristics of the system response. Because restoring forces in base-excited structures can exhibit highly non-linear characteristics, non-linear model-based approaches may be better suited for reliable health monitoring and damage detection. This paper presents the application of a novel intelligent parameter varying (IPV) modeling and system identification technique, developed by the authors, to detect damage in base-excited structures. This IPV technique overcomes specific limitations of traditional model-based and non-model-based approaches, as demonstrated through comparative simulations with wavelet analysis methods. These simulations confirm the effectiveness of the IPV technique, and show that performance is not compromised by the introduction of realistic structural non-linearities and ground excitation characteristics.

Stability and Performance of Feedback Control Systems with Time Delays

M. S. Ali et al. — Tue, 29 Jul 2008 15:29:45 PDT

This paper investigates the time delay effects on the stability and performance of active feedback control systems for engineering structures. A computer algorithm is developed for stability analysis of a SDOF system with unequal delay time pair in the velocity and displacement feedback loops. It is found that there may exist multiple stable regions in the plane of the time delay pair, which contain time delays greater than the maximum allowable values obtained by previous studies. The size, shape and location of these stable and unstable regions depend on the system parameters and the feedback control gains. For systems with multiple stable regions, the boundaries between the stable and unstable regions in the plane of the time delay pair are explicitly obtained. The delay time pairs that forms these boundaries are called the critical delay time pairs at which the steady-state response becomes unbounded. The conclusions are valid for both large and small delay times. For any system with multiple stable regions, preliminary guidelines obtained from an explicit formula are given to find the desirable delay time pair(s). When used, these desirable delay time pair(s) not only stabilize an unstable system with inherent time delays, but also significantly reduce the system response and control force. For any system with multiple stable regions, these desirable delay time pair(s) are above the maximum allowable delay times obtained by previous studies. Numerical results, for both steady-state and transient analysis, are given to investigate the performance of delayed feedback control systems subjected to both harmonic and real earthquake ground motion excitations.

Random vibration studies of an SDOF system with shape memory restoring force

L. Duval et al. — Tue, 29 Jul 2008 15:29:42 PDT

Intelligent and adaptive material systems and structures have become very important in engineering applications. The basic characteristic of these systems is the ability to adapt to the environmental conditions. A new class of materials with promising applications in structural and mechanical systems is shape memory alloy (SMA). The mechanical behavior of shape memory alloys in particular shows a strong dependence on temperature. This property provides opportunities for the utilization of SMAs in actuators or energy dissipation devices. However, the behavior of systems containing shape memory components under random excitation has not yet been addressed in the literature. Such a study is important to verify the feasibility of using SMAs in structural systems. In this work a nondeterministic study of the dynamic behavior of a single-degree-of-freedom (SDOF) mechanical system, having a Nitinol spring as a restoring force element is presented. The SMA spring is characterized using a one-dimensional phenomenological constitutive model based on the classical Devonshire theory. Response statistics for zero mean random vibration of the SDOF under a wide range of temperature is obtained. Furthermore, nonzero mean analysis of these systems is carried out.

Random Response to Periodic Excitation with Correlated Disturbances

Zhikun Hou et al. — Tue, 29 Jul 2008 15:29:39 PDT

The paper addresses non-Gaussian stationary response of linear single-degree-of-freedom (SnOF) systems subject to a periodic excitation with correlated random amplitude and phase disturbances that are modeled as correlated Gaussian white noise processes. Correlation between amplitude and phase modulation is specified by the cross-correlation coefficient. Numerical results for the second and fourth moment responses are presented. The probability density function of the response is calculated based on the cumulant-neglect closure method. Non-Gaussian nature of the response is discussed in terms of the excess factor. The results show that the moment responses generally increase with larger random amplitude disturbance and may decrease with larger random phase modulation for a lightly damped system at resonance. The cross correlation between amplitude and phase disturbances plays an important role in the system moment response. Larger system damping results in smaller system moment responses. The moment response may approach a limiting value, depending on the intensity of the amplitude disturbance, as the relative detuning or phase modulation increases. For the case of the phase modulation alone, the response may become Gaussian in the sense of up to the fourth-order moment for sufficiently large relative detuning or random phase disturbances.