A.G. Davenport

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.

The base balance technique for the determination of dynamic wind loads

Abstract The paper describes the construction of an ultra-sensitive 5 component balance capable of measuring the forces at the base of light structural models. The application of this system to the evaluation of static and dynamic wind loads is described. The extension into the inelastic range is discussed.

How can we simplify and generalize wind loads

Abstract The complexity of wind loading arises from their variations in space and time. The paper discusses three key spatial functions which control the magnitude of responses: the influence lines, the mode shapes and pressure distributions. The last can be efficiently described in terms of orthonormal expansions. The paper describes how the different time dependent loadings — the mean, the background and resonant responses - are related to the spatial functions. These tools provide a promising route for generalization and simplification of wind loading.

Guidelines for the calculation of wind speed-ups in complex terrain

Abstract Several wind loading codes of practice make use of simple guidelines to assess the effect of topography on design wind speeds. These guidelines generally use some measure of the hill slope to predict the speed-up, and are normally derived by considering an isolated hill. In practice, hills are unlikely to be isolated and the effects of the surrounding topography must be included. This paper considers the results of a wind tunnel study of boundary layer flow over a number of two-dimensional complex surfaces. The effect of upstream topography is quantified and the observed speed-ups are compared to those predicted by the current Canadian and UK codes of practice. Comparison is also made with speed-up values calculated by using a linear model of flow over topography.

The relationship of reliability to wind loading

Abstract The paper presents the case for the wider application of reliability principles in the treatment of wind loading. Illustrations are given of its usefulness in the treatment of a variety of uncertainties concluding those associated with the estimation of internal pressures, of damping and also the influence of wind direction. The important role full scale testing can play in reducing uncertainty is discussed.

Simple approximations for wind speed-up over hills

There is a need to properly assess design wind speeds and correctly evaluate wind loads for structures on hills. In this paper, a simple method, called the LSD approach, is proposed, based on computational and wind tunnel data obtained for various shapes of hills. Comparisons of the LSD approach with other models are also given.

The magnitude and distribution of wind-induced pressures on hip and gable roofs

Abstract Some recent post-disaster studies have suggested that in severe wind storms hip roofs survive better than gable roofs. This has stimulated the present study, which has derived aerodynamic data for otherwise similar 4:12 hip and gable roofs. These show that for this common roof geometry there is little difference in overall lift and overturning loads; however, there are dramatic differences in local pressures and in the loads applied to the primary structural elements. These differences appear to explain the improved survival of hip roofs.

A novel approach for wind tunnel modelling of transmission lines

Abstract This paper is concerned with the behaviour of transmission line cables in severe boundary layer winds. It examines the effect of the scale of turbulence on the response of a line-like structure (cable model) through wind tunnel tests and the comparison of wind tunnel tests with theoretical predictions made through the statistical method using influence lines. Consistency with theory allowed the development of a new modelling approach to conductor systems using a distorted horizontal length scale (span wise) to accommodate these systems in the wind tunnel. Transverse and oblique wind incidences were tested. From the results obtained in the experimental work, it is apparent that the new modelling approach to conductor systems in wind tunnels is a valid technique. It is necessary, however, that a correction be made in the values of the variance of the response measured in the distorted model.

The effects of high winds on transmission lines

Abstract The behaviour of transmission lines under severe winds is examined. Possible reasons to explain transmission line failures in strong winds are investigated through aeroelastic wind tunnel tests, and the experimental results are successfully compared with theoretical predictions made through the statistical method using influence lines. Cables with different characteristics are simulated and tested at transverse wind incidence. The results obtained confirm the importance of turbulence in the dynamic response and demonstrate that the aerodynamic damping plays an important role in the dynamic behaviour of the cables. They also show that, although the background response may indeed be the biggest contributor for the total fluctuating response in most typical cases, the resonant response can be important depending on the characteristics of the structure and of the wind flow, and should not be neglected in the design procedures.

Variability of low building wind loads due to surroundings

Abstract A myriad of surface obstructions form the basic roughness that generates the Earths planetary boundary layer. Among these are the class of structures termed low buildings. Ironically, attempts to determine wind loads on low buildings have been hampered by the random nature of these surface elements and researchers have been forced to simplify to deal with the complex problem. As a result, almost all low building research to date has been carried out on simple rectangular block-like buildings in homogeneous surroundings. While the aerodynamic behaviour of wind on isolated low buildings has been well researched, the effect of realistic surroundings has not been examined in detail. Whether the results from these simplified tests lead to the most appropriate design values remains an open question. With the establishment of a set of rational design values for low buildings in mind, it is proposed that a series of experiments be carried out to determine the wind loads experienced by low buildings in realistic environments. The experiments will be designed to collectively represent the entire spectrum of full-scale situations, i.e. the statistical distribution of parameters associated with geometry and surroundings will be matched between model and full scale. The wind loads measured in the wind tunnel will then be statistically representative of the wind loads expected on all low buildings. These expected wind loads and their variability will form a good basis to further examine the determination of appropriate wind loads for design. The first phase of this proposed study has been to examine the effect of the surroundings on the wind loads on flat-roofed low buildings, as well as to determine the relative significance of some of the more basic wind load parameters, such as effects of simulated upstream terrains, in relation to the variation introduced by the local effects of different realistic sites.