A.P. Jeary

Damping in structures

The mechanism causing energy loss, or damping, in structures is discussed, and measurements from full scale structures are used to demonstrate that the conventionally assumed model for damping is incorrect. Fracture at both a microscopic and a macroscopic scale is assumed as the dominant mechanism for energy release and measurements at a microscopic scale are presented to support the argument. The implications of the assumption of fracture as the dominant mechanism are considered and a generalised non-linear damping model is considered as a basis for prediction of damping values at the design stage.

Full scale measurements of wind effects on tall buildings

This paper describes the results obtained from the measurements of wind effects on two tall buildings with 70-storeys and 30-storeys, respectively. The field data presented are wind velocity and acceleration response measured at the top of the tall buildings over the last two years. The damping characteristics which were obtained by using the random decrement technique are demonstrated and discussed.

The description and measurement of nonlinear damping in structures

Abstract This paper addresses the question of a description of the damping characteristics inherent in real structures. A discussion of the mechanisms causing energy loss and the practicalities of measurement when the parameter is not only nonlinear but also the forcing function and the response are nonstationary is followed by some practical results obtained from very tall buildings. The random decrement signature is presented as being particularly useful for the measurement of damping characteristics under hostile loading regimes, and its use is demonstrated and described.

Damping in buildings: its neural network model and AR model

The results of full scale measurements of damping as well as other researches on damping show that damping in buildings exhibits randomness and amplitude dependent behaviour in the case of tall buildings subjected to dynamic loading. In this paper, based on full scale measurements of damping in a tall building, a time series analysis method (TSA) is employed to obtain the relationship between damping and vibration amplitude. Then, two models of damping in a tall building, the artificial neural network (ANN) model and the auto-regressive (AR) model, are established by employing ANN and AR methods, and used to predict the damping values at high amplitude level, which are difficult to obtain from field measurements. In order to get high accuracy, a genetic algorithm strategy is employed to aid in training the ANN. Comparison analysis of the neural network model and the AR model of damping is made, and the results are presented and discussed.

Evaluation of wind effects on a supertall building based on full‐scale measurements

This paper describes the results obtained from the full-scale measurements of wind effects on a 70-storey building in Hong Kong. The building which has a height of approximately 370 m is the second tallest structure in Hong Kong. The field data such as wind speed, wind direction and wind-induced acceleration responses have been measured since 1995 including the close passage of two typhoons; typhoon Sally and typhoon Kent. Detailed analysis of the field data is conducted. The full-scale measurements are compared with the wind tunnel results obtained in the Boundary Layer Wind Tunnel Laboratory at Western Ontario University. The amplitude-dependent characteristics of damping and natural frequency that were obtained by using the random decrement technique are investigated. Copyright

Random damping in buildings and its AR model

Abstract The full-scale measurements of damping show that damping in buildings exhibits randomness. The randomness of damping is examined in this paper. The factors which govern damping contributions in buildings, at different vibration amplitude levels, are investigated based on the Jearys damping model [Earthquake Eng. Struct. Dyn. 14 (1996) 733–750]. It is found that, at a high amplitude level, damping in buildings is mainly dominated by random factors. In order to investigate the amplitude-dependent characteristics and randomness of structural damping, a time series analysis method (TSA) is employed to obtain the relationship between damping and vibrating amplitude. The AR (auto-regressive) models of damping in a building have been established and used to predict the damping values at high amplitude level, which are difficult to obtain from field measurements. The predicted data of damping show that damping values, at high amplitude, fluctuate around a plateau value described by Jearys damping model.

Free vibration analysis of cantilevered tall structures under various axial loads

Many cantilevered tall structures can be treated as cantilever bars with variable cross-section for the analysis of their free vibrations. In this paper, the differential equations for free flexural vibration of bars with variable cross-section under various axial loads are reduced to Bessels equations or ordinary equations with constant coefficients by selecting suitable expressions, such as power functions and exponential functions, for the distributions of stiffness and mass as well as for the axial forces acting on the bars. The general solutions for free flexural vibration of a one-step bar with variable cross-section subjected to simple or complex axial loads, including concentrated and variably distributed axial loads are presented first in this paper. Then the general solutions of one-step bars are used to derive the eigenvalue equation of multi-step bars subjected to more complicated axial loads by using the transfer matrix method. One of the advantages of the present method is that the total number of the finite elements (segments) required could be much less than that normally used in the conventional finite element methods. The numerical example 1 demonstrates that the calculated fundamental natural frequency of a 27-storey building under the actual axial loads is closer to the measured field data than that computed without considering the axial forces. The numerical example 2 shows that the natural frequencies of a television transmission tower calculated by the proposed methods are in good agreement with those computed by Finite Element Method. It is also shown through the numerical examples that the selected expressions are suitable for describing the distributions of flexural stiffness, mass and axial loads of typical tall shear-wall buildings and high-rise structures.

Calculation of vertical dynamic characteristics of tall buildings with viscous damping

The magnitude of the vertical component of earthquake ground motion is often about one-third of the horizontal component. Thus, it is necessary to calculate vertical dynamic characteristics of tall buildings and high-rise structures in design stage for certain cases. In analysing free vibrations of tall buildings and high-rise structures, it is possible to regard such structures as a cantilever bar with variable cross-section. In this paper, the differential equations of free longitudinal vibrations (in vertical direction) of bars with variably distributed mass and stiffness considering damping effect are established. The damping coefficient of a bar is assumed to be proportional to its mass, and the general solutions of mode shapes of damped distributed parameter systems are reduced to Bessels equations by selecting suitable expressions, such as power functions and exponential functions, for the distributions of stiffness and mass. An approach to determine the natural frequencies and mode shapes in vertical direction for tall buildings with variably distributed stiffness and variably distributed mass is proposed. The presented method is also applicable to the free longitudinal vibration analysis without considering damping effect (damping coefficient in vibration equations is equal to zero). A numerical example shows that the computed values of the fundamental longitudinal natural frequency and mode shape by the proposed method are close to the full scale measured data. It is shown through the numerical example that the selected expressions are suitable for describing the distributions of stiffness and mass of typical tall buildings. A comparison between undamped structural dynamic characteristics and damped natural frequencies, mode shapes is made in this paper.

Free vibration analysis of multi-storey buildings with narrow rectangular plane configuration

In this paper, the general solutions of free vibrations of one-step cantilever shear plates with variably distributed mass and stiffness are derived by selecting suitable expressions, such as power functions and exponential functions, for the distributions of stiffness and mass along the height of the plates. Then the general solutions of one-step shear plates are used to derive the general solutions and frequency equations of multi-step cantilever shear plates by using transfer matrices. A numerical example demonstrates that the calculated dynamic characteristics of a building with narrow rectangular plane configuration (narrow building), which is considered as a cantilever shear plate with variable cross-section, are in good agreement with the corresponding experimental data. It is shown that when the stiffness of each floor of a narrow building can be treated as infinitely rigid, such a building can be considered as a cantilever shear bar which is a special case of a cantilever shear plate. Thus, the proposed methods in this paper are suitable for the calculation of free vibrations of narrow buildings and common shear-type buildings.

The wind climate of Hong Kong

The wind climate of Hong Kong is examined in this paper. Data relating to typhoons, their structure, their recurvature over the western pacific, and the winds which they induce over the territory are presented. The relevance of these observations to codification for Hong Kong is discussed.