The influence of internal pressure on wind loading under tropical cyclone conditions

Abstract

Tropical cyclones are intense storms that have in recent times incurred up to billions of dollars worth of damage in a number of developed regions of the world. The smaller island economies are also regularly devastated from frequent occurrences of such storms. Post-cyclone investigations reveal that a significant proportion of damage is commonly suffered to low-rise buildings. While the issue of quality in construction dominates the prospects for mitigation of damage to low-rise buildings, a number of technical issues however confront the scientific community. In the present study, the issues dealing with the tropical cyclone wind characteristics and some aspects of building wind loads, particularly where "gaps" exist in current knowledge, are investigated. Since wind loads are determined primarily by the wind, an investigation of the structure of the tropical cyclone wind is firstly made from a review of past measurements. This shows conclusively that convective activity in such storms makes the wind unstable, causing turbulence intensities to be enhanced and velocity profiles to be flatter than those obtained in strong winds under neutral stability conditions. While the Australian wind loading code, AS1170.2-1989, makes special provisions for wind characteristics in cyclone prone regions, it is shown that there is however some scope for improvement. In particular, increases in turbulence intensities are recommended and some suggestions are made with regards mean and gust wind profiles, and the form of the turbulence energy spectrum. Some unexplained phenomena regarding surface pressure fluctuations, such as the shear layer instabilities on the roof that cause extreme suctions, is revealed by further development of the quasi-steady theory to be caused by Reynolds stresses. Drawing on measured values of the gradient of surface pressure with wind azimuth and wind elevation, the theory suggests that Reynolds stresses may also suppress fluctuations in surface pressure quite significantly. This immediately explains the apparent loss of mid-frequency response in roof pressures observed in full-scale measurements. In addition to the effects of distortion and blockage of turbulent eddies, it is shown that the high frequency attenuation in fluctuating surface pressures is effected by the deflection of eddies with the mean flow around bluff-bodies. Furthermore, area pressure fluctuations that drive fluctuations in internal pressure through dominant openings, are shown to be additionally effected by the filtering effects of area-averaging that leads to further attenuation. Empirical equations are proposed for windward wall pressure admittance functions that account for the high-frequency attenuation. The study on internal pressure begins with the application of computational and experimental modelling techniques in the investigation of the transient response behaviour following a sudden opening. This enables the determination of the previously ill-defined parameters in the governing equation for internal pressure. It is shown that computational modelling provides an accurate prediction of the Helmholtz frequency, and the damping characteristics are estimated with fair accuracy. Computational flow visualisation is used to show that the unsteady flow through the opening forms a vena-contracta which may persist through most sections of the oscillating flow cycle. The Helmholtz resonance phenomenon in buildings with a dominant opening makes the quasi-steady assumption invalid for internal pressures, since fluctuations are determined by the frequency response. Model-scale experiments in the present study show that the quasi-steady internal pressure coefficients (ie. coefficient of peak internal pressure to peak gust pressure) are usually larger than the mean coefficients as a result of Helmholtz resonance. A gust-factor method is thus developed for the calculation of peak internal loads. At oblique wind angles for wall openings, experiments are used to demonstrate the importance of the previously unfamiliar "Helmholtz resonance under oblique flow" phenomenon that results in a very severe response, so that peak loads may approach those obtained under normal flow. Furthermore, predictions based upon analytical models developed separately for the inclusion of the effects of internal partitioning and building flexibility on internal pressure and the net load developed across the envelope, are shown to agree well with experimental results.

While a study on the characteristics of internal pressure is important in itself, the manner in which it combines with the envelope external pressure to produce the net load is of greater significance in the design process. The characteristics of the net load developed across the roof with wall openings, and across the wall with roof openings is therefore determined through model-scale experiments. Correlation coefficients between internal and external pressures measured to range from 0.4 to 0.8, can be applied with the gust-factor equations in the calculation of the peak load across the envelope. An interesting outcome of this part of the investigation is that the worst-case situation in terms of the net roof or wall load always occurs at oblique wind angles and not under normal flow as has been assumed in past studies.

The effects of enhanced turbulence levels and internal pressure on the fatigue characteristics of wind loads are studied using a design cyclone approach. A set of loading sequences are proposed for wall and roof pressures, and a method is proposed for calculating the accumulation of fatigue over several tropical cyclones. The present results indicate that the loading sequences in the Australian wind loading code might not be conservative; a result that is in agreement with other recent studies in Australia.

Finally, a set of conclusions and recommendations are drawn from the study both in terms of fundamental interest, as well as for improvements in the relevant wind loading codes. These include increased turbulence intensities in tropical cyclones, a gust-factor method for the calculation of peak internal pressure and net envelope loads in the presence of dominant openings, the effect of a number of variables on these loads, and fatigue loading cycles for net envelope loads. The need for further investigation is highlighted, particularly on some aspects of tropical cyclone winds, remaining issues on internal pressure resulting mainly from some questions that have arisen, and fatigue characteristics of overall loads acting on the envelope.