Ahsan Kareem

Damping in structures: its evaluation and treatment of uncertainty

Abstract This paper concerns the damping in structures with emphasis on treatment of inherent uncertainty in its prediction and estimation. Material or structural damping is addressed as well as damping due to the aerodynamic and hydrodynamic forces of the fluid surrounding the structure. The reported data base on damping information is examined in light of wind sensitive structures that rely heavily on damping for their performance under winds. The basic techniques for estimation of damping from response time histories are reviewed, and the random decrement technique is considered in some detail. The implications of the uncertainty of damping on system response are analyzed in terms of a perturbtion technique, second-moment analysis and Monte Carlo simulation. Several simple illustrative examples are provided throughout the text.

Analysis and simulation tools for wind engineering

This paper examines state-of-the-art analysis and simulation tools for applications to wind engineering, introduces improvements recently developed by the authors, and directions for future work. While the scope of application extends to a variety of environmental loads (e.g. ocean waves and earthquake motions), particular reference is made to the analysis and simulation of non-Gaussian features as they appear in wind pressure fluctuations under separated flow regions and non-stationary characteristics of wind velocity fluctuations during a gust front, a thunderstorm or a hurricane. A particular measured non-Gaussian pressure trace is used as a focal point to connect the various related topics herein. Various methods of non-linear system modeling are first considered. Techniques are then presented for modeling the probability density function of non-Gaussian processes. These include maximizing the entropy functional subject to constraints derived from moment information, Hermite transformation models, and the use of the Kac-Siegert approach based on Volterra kernels. The implications of non-Gaussian local wind loads on the prediction of fatigue damage are examined, as well as new developments concerning gust factor representation of non-Gaussian wind loads. The simulation of non-Gaussian processes is addressed in terms of correlation-distortion methods and application of higher-order spectral analysis. Also included is a discussion of preferred phasing, and concepts for conditional simulation in a non-Gaussian context. The wavelet transform is used to decompose random processes into localized orthogonal basis functions, providing a convenient format for the modeling, analysis, and simulation of non-stationary processes. The work in these areas continues to improve our understanding and modeling of complex phenomena in wind related problems. The presentation here is for introductory purposes and many topics require additional research. It is hoped that introduction of these powerful tools will aid in improving the general understanding of wind effects on structures and will lead to subsequent application in design practice.

SIMULATION OF A CLASS OF NON-NORMAL RANDOM PROCESSES

Abstract This study addresses the simulation of a class of non-normal processes based on measured samples and sample characteristics of the system input and output. The class of non-normal processes considered here concerns environmental loads, such as wind and wave loads, and associated structural responses. First, static transformation techniques are used to perform simulations of the underlying Gaussian time or autocorrelation sample. An optimization procedure is employed to overcome errors associated with a truncated Hermite polynomial transformation. This method is able to produce simulations which closely match the sample process histogram, power spectral density, and central moments through fourth order. However, it does not retain the specific structure of the phase relationship between frequency components, demonstrated by the inability to match higher order spectra. A Volterra series up to second order with analytical kernels is employed to demonstrate the bispectral matching made possible with memory models. A neural network system identification model is employed for simulation of output when measured system input is available, and also demonstrates the ability to match higher order spectral characteristics.

PRESSURE FLUCTUATIONS ON A SQUARE BUILDING MODEL IN BOUNDARY-LAYER FLOWS

Abstract Spatio-temporal measurements of a fluctuating pressure field acting on the side faces of a square prism of finite height in boundary-layer flows are presented for 0° angle of attack. Two typical neutral atmospheric flow conditions were simulated in the wind tunnel to represent open country and urban flow environments. The fluctuating pressure field data allowed computations of the mean and r.m.s. (root mean square) pressure coefficients, power spectral density, autocorrelations, co-spectra, cross-correlations, orthogonal eigenfunction expansions and statistical dependence. Increased levels of turbulence in the incident flow have a marked influence on the fluctuating pressure field, through modifications which take place in the structure of the separated shear layers. The periodic vortex-shedding process is vitiated in the presence of high levels of turbulence intensity in the incident flow, resulting in redistribution of the energy associated with pressure fluctuations over a wider frequency range.

Semi-active tuned liquid column dampers for vibration control of structures

Semi-active systems are attractive for structural control applications because they offer some of the best features of both passive and active systems. This paper examines a passive tuned liquid column damper which is converted into a variable-damping semi-active system. Different semi-active algorithms based on the clipped-optimal and fuzzy control strategies are studied using numerical examples. The main objective of this paper is to study the effectiveness of different control algorithms for semi-active tuned liquid dampers for structural control applications.

Analysis of Non-Gaussian Surge Response of Tension Leg Platforms Under Wind Loads

The nonlinearity in the wind loading expression for a complaint offshore structure, e.g., a tension leg platform (TLP), results in response statistics that deviate from the Gaussian distribution. This paper focuses on the statistical analysis of the response of these structures to random wind loads. The analysis presented here involves a nonlinear system with memory. As an improvement over the commonly used linearization approach, an equivalent statistical quadratization method is presented. The higher-order response cumulants are based on Volterra series. A direct integration scheme and Kac-Siegert technique is utilized to evaluate the response cumulants. Based on the first four cumulants, the response probability density function, crossing rates, and peak value distribution are derived. The results provide a good comparison with simulation. A nonlinear wind gust loading factor based on the derived extreme value distribution of nonlinear wind effects is formulated.

Time-frequency analysis of wind effects on structures

Abstract As many physical processes of interest to Civil Engineers manifest nonlinear and nonstationary features, their complete characterization may not be accomplished via Fourier transforms, necessitating a new analysis framework in the time-frequency domain. This paper overviews recent developments in wavelet-based analysis of a number of physical processes of relevance to the Civil Engineering community. It is shown that the dual nature of wavelet transforms, being a simultaneous transform in time and frequency, permits adaptation of a number of traditional system identification and analysis schemes. For example, the extension of wavelet transforms to the estimation of time-varying energy density permits the tracking of evolutionary characteristics in the signal using instantaneous wavelet spectra and the development of measures like wavelet-based coherence to capture intermittent correlated structures in signals. Similarly, system identification methodologies originally referenced in either the time or frequency domain can be extended into the realm of wavelets. Though the application of wavelet transforms in Civil Engineering is in its infancy, as the examples in this study demonstrate, its future shows great promise as a tool to redefine the probabilistic and statistical analysis of wind effects.

STOCHASTIC RESPONSE OF STRUCTURES WITH FLUID - CONTAINING APPENDAGES

Abstract The influence of fluid-containing appendages on the dynamic response of multi-degree-of-freedom systems subjected to stochastic environmental loads, e.g., earthquakes, waves, or winds, is investigated. The modal properties of a system comprising of a fluid-containing appendage attached to a multi-degree-of-freedom system are expressed in terms of the individual dynamic properties of the primary and secondary systems. The primary system is modeled as a lumped mass multi-degree-of-freedom system. An equivalent lumped mass model of the sloshing fluid is used to represent the secondary system. All the frequencies of the secondary system that are less than or equal to the fundamental natural frequency of the primary system are included in the dynamic analysis of the combined system. The input to the system may be a stationary or a non-stationary white or filtered white noise vector-valued excitation. In this study the structural response to an earthquake represented by a non-stationary filtered white noise is computed at any time interval by utilizing the modal impulse-response function and approximating the envelope intensity function with a staircase unit impulse function. The formulation presented here renders the computational procedure very efficient by reducing the multiplicity of the integrals. The covariance matrices of the response components of the combined system are computed by using this formulation. The peak response values at any level on the structure may be obtained by following the evolutionary distribution of the extreme values. An important feature of the combined system is that the response of the primary system is suppressed when one of the sloshing modes of the secondary fluid appendage is tuned to the fundamental mode of the primary system. A building with a water tank situated at any floor, excited by an earthquake, is used to illustrate the methodology.

Peak Factors for Non-Gaussian Load Effects Revisited

The estimation of the extreme of non-Gaussian load effects for design applications has often been treated tacitly by invoking a conventional peak factor on the basis of Gaussian processes. This assumption breaks down when the loading process exhibits non-Gaussianity, in which a conventional peak factor yields relatively nonconservative estimates because of failure to include long tail regions inherent to non-Gaussian processes. To realistically capture the salient characteristics of non-Gaussian load effects and incorporate these in the estimates of their extremes, this study examines the peak factor for non-Gaussian processes, which can be used for estimating the expected value of the positive and negative extremes of non-Gaussian load effects. The efficacy of previously introduced analytical expressions for the peak factor of non-Gaussian processes on the basis of a moment-based Hermite model is evaluated and the variance of the estimates in standard deviation is derived. In addition, some improvements ...

Nonlinear response analysis of long-span bridges under turbulent winds

Abstract This paper presents a time domain analysis framework for predicting nonlinear response of long-span bridges under turbulent winds. The nonlinear unsteady aerodynamic forces are modeled based on static force coefficients, flutter derivatives, admittance functions, and their spanwise correlations at varying angles of incidence. This analysis framework incorporates frequency dependent parameters of unsteady aerodynamic forces by utilizing a rational function approximation technique. A comparison with conventional linear approach is made through response analysis of a long-span suspension bridge. The effects of turbulence on the flutter instability are also addressed.