No_1_January_1969 — Page 33

WIND SPEED IN MILES/HOUR

70

503 FT.

60

50

30

20

10

2

210 FT.

40 FT.

TIME IN MINUTES

3

4

8

Fig. 1. Record of wind speed at three heights on a 500ft. mast

How typhoons affect tall buildings-HK study

THOSE who live in the top floors of slender skyscrapers in exposed loca- tions in Hong Kong, have experienced, perhaps with alarm, the swaying move- ments to which these buildings are subjected during the onset of typhoon winds.

It is now an established fact that under wind action such buildings un- dergo two distinct types of movement. These arise from the nature of the wind itself.

The anemograms in Fig. 1 (due to Davenport) show that wind velocity is made up of the two components: a steady, time-averaged velocity com- ponent, given by the mean of the anemogram readings over a specific time interval; and a fluctuating com- ponent shown by the rapidly changing readings, which are caused by the gus- tiness of the wind. In this particular record it will be noticed that the steady velocity component increases with height above ground level, whereas the fluctuating component is, more or less, independent of the mean velo- city value.

Due to the steady wind velocity component the top of a building will deflect laterally, but because of the

Far East BUILDER, January 1969.

fluctuating component it will also gyrate horizontally about the deflect- ed position. It is this swaying move- ment, which is most upsetting to hu- man occupants and it is the frequency of oscillation more than the amplitude which gives rise to the greatest degree of discomfort.

The human frame is quite sensitive to oscillatory movements and ampli- tudes of no more than a few milli- metres, when occurring at frequencies of around 1 cycle/second, have been known to give rise to marked feelings of sea-sickness and giddiness. Such oscillations lie well within the range of movement of many tall buildings in Hong Kong, and apart from the human discomfort caused which is of vital concern to the real-estate de- veloper - they can, if prolonged, lead to serious distress of the structural carcass of the building.

In multi-storey framed structures where substantial shear-walls are pre- cluded in order to meet the needs of horizontal access, or due to founda- tion problems, the provision of ad- equate wind resistance almost invari- ably governs the structural design. On this count alone it is important that

by Professor S. Mackey

designers should have a thorough knowledge of wind action and its ef- fects on buildings, if economical design consistent with safety requirements is to be achieved.

Conventional design of buildings to resist wind loading has been based on static wind pressures derived from steady flow conditions with a gust factor to take account of peak-loading. This has the merit of being simple and easy to apply but there is no real evi- dence to show that the steady stream of air flow in a wind tunnel exhibits the same characteristics as natural wind. Indeed, it takes but a single glance at Fig. 1 to show the fluctuating character of natural wind, which may be likened to the voltage or current fluctuation in an A-C circuit, except that the latter is usually a periodic function of time.

This implies that wind velocity is a non-periodic function or, as the Gospel of St. John 3.8. tells us “The wind bloweth where it listeth and thou hearest the sound thereof but canst not tell whence it cometh or whither it goeth". In other words, it is a random function of time, which is a direct consequence of the com-

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