B.J. Vickery

Across-wind vibrations of structures of circular cross-section. Part I. Development of a mathematical model for two-dimensional conditions

Abstract A model is presented for predicting the across-wind response of constant-diameter circular cylinders vibrating in a mode of uniform amplitude and subject to uniform flow. A key feature of the model is the representation of all motion-dependent phenomena by a nonlinear aerodynamic damping force. This force coexists with the fluctuating force which arises from vortex shedding on a stationary cylinder, and the two forces are assumed to be uncorrelated. The ability of the device used in representing the motion-induced force to model certain aeroelastic characteristics associated with vibrating cylinders is demonstrated. The device is shown to be capable of successfully reproducing two effects; namely, the increase of the spanwise correlation of forces with increasing amplitude, and the phenomenon of “lock in” where the shedding frequency is apparently dictated by the vibration frequency. The model is developed within the framework of random-vibration theory, and a number of simplifying assumptions are necessary to incorporate the nonlinear aerodynamic damping force and also to account for the influence of turbulence. Numerical experiments, undertaken to examine the nature of the approximations involved in the assumptions adopted, are described. The results of the numerical experiments are very encouraging and justify the simplifications made in the modelling process.

Design wind speeds in regions dominated by tropical cyclones

Abstract In recent years computer simulation methods predicting extreme wind speeds in regions dominated by tropical cyclones have been developed and shown to give reasonable estimates of long-term risk levels arising from these winds. In this paper refinements of the model are presented with the emphasis on improvements to the statistical distribution functions describing tropical cyclone characteristics used in the simulation. In particular the relationships chosen for the two parameters which influence wind speeds the most, the central pressure difference and the decay rate after landfall, represent significant changes from previous models. The application of the simulation to specific oceanic regions is discussed and the results of a case study conducted along the Gulf and Atlantic Coasts of the United States presented.

Extreme wind speeds in mixed wind climates

Abstract In this paper, a technique is discussed which enables extreme wind speed probabilities in mixed wind climates to be determined. It is shown that in such wind climates the method is likely to yield more accurate probability estimates than the traditional Gumbel analysis of annual wind speed maxima, particularly for annual probabilities of less than 2% (i.e. return periods greater than 50 years). The technique requires a separate analysis of each significant wind-producing meteorological phenomenon and practical methods of achieving this are presented. Extreme wind speed parameters obtained from each analysis may then be combined to yield a “composite extreme wind speed diagram” and several examples illustrating this procedure are shown. By means of a numerical simulation of a typical mixed wind climate, a detailed study is made of the distribution of extreme wind gusts from different meteorological phenomena. This study indicates that a Gumbel analysis of 20 annual maxima may severely underestimate low annual probability (i.e. long return period) gust speeds.

Across-wind vibrations of structure of circular cross-section. Part II. Development of a mathematical model for full-scale application

Abstract A model is presented for predicting the across-wind response of slender structures of circular cross-section to turbulent shear flow. Fundamental aspects of the model have been described in a companion paper [1], the key feature of the model being the representation of all motion-dependent phenomena by a nonlinear damping force. The development described in this paper is concerned with extending the earlier work, which was confined to two-dimensional laboratory configuration, such that the model is able to provide estimates of response for full-scale structures. Based on the work outlined above, a computer program was developed to obtain estimates of response in full-scale situations. The program was used to compute the response of a 330 m TV tower and the results were compared with the measured response. In general, the computed and measured responses are in fair agreement. The uncertainties associated with some of the input parameters are noted, and are considered a greater hindrance to further development than any theoretical difficulties.

Internal pressure dynamics with a dominant opening

Abstract The paper examines the transient and steady state behaviour of the internal pressure following the sudden formation of a windward opening in an otherwise sealed (or almost so) building. Comparatively simple predictive methods are developed for both the steady state and the transient behaviour and these are confirmed by experimental observations. Although the theory is restricted to single openings, the effect of background leakage is examined experimentally and shown to be of minor consequence if the leakage area is less than about 10% of the main opening. The results presented are applicable to the design problem of assessing the effects of the failure by overload or impact of a window or door in a low rise building or in an office of a large building if the flow connection between the office and the building has an area which is small compared to that of the failed window. While the experimental program was limited to a windward opening the theory is, for the most part, equally applicable to openings elsewhere on the structure.

Internal pressures and interactions with the building envelope

Abstract The paper is concerned primarily with the dynamics of internal pressure and the significance of dynamic action on the peak loads developed across the envelope. The two cases studied in some depth are that of a nominally sealed but leaky building with well distributed leakage paths and that of a building with a large opening and little or no background leakage. In the latter case, attention is given both to the transient behaviour following a sudden opening and the steady state behaviour. The theoretical development is supplemented by worked examples for two low rise buildings.

The response of reinforced concrete chimneys to vortex shedding

Abstract Predicted vortex induced response of circular cross-section reinforced concrete structures is compared with observed full-scale behaviour. On average the predictions agree to within 5% with observations but have a large scatter attributable to the models failure to recognize the dependence of aerodynamic parameters on free stream turbulence. The high coefficient of variation of the model is, however, not markedly different from that associated with the prediction of drag loads.

Performance of a tuned sloshing water damper

Abstract A study was carried out to investigate and evaluate the performance of a one-dimensional tuned sloshing water damper (TSWD). The study examined the performance of a 1:13 scale model of a full scale TSWD. During the course of the study, the frequency and amplitude dependent response characteristics of the TSWD were determined experimentally, an effective method of increasing the inherent damping, and therefore the performance of the TSWD, was developed and verified, a theoretical model based on the linear theory was formulated, and the effect of the TSWD on a model structure was quantified.

Simplified approaches to the evaluation of the across-wind response of chimneys

Abstract A model for the prediction of the response of chimneys to vortex shedding is outlined and the major characteristics of solutions employing the model are described. Simplified equations suitable for routine office use are derived. For modes other than the fundamental the simplified forms require a knowledge of the mode shapes and frequencies but, for the fundamental mode, it is shown that an equivalent static load can be defined with a knowledge of the frequency only. The application of the simplified forms is demonstrated with sample calculations presented for two chimneys. The results of the simplified forms are shown to be slightly conservative in relation to estimates obtained using the detailed approach.

Tuned vibration absorbers with dry friction damping

The response of a linear SDOF system subjected to harmonic excitation to which a Tuned Vibration Absorber with linear stiffness and dry friction damping is attached is considered. Based on an intuitive examination of the physical behaviour of the system, closed-form expressions for TMD optimum parameters and for the steady-state amplitudes of vibration are presented. Two examples allow the comparison between the predicted behaviour and that found by numerically integrating the equations of motion.