275 Loading

Hyperbolic cooling towers may be subjected to a variety of loading conditions. Most commonly, these are dead load (D), wind load (W), earthquake load (E), temperature variations (T), construction loads (C), and settlement (S). For the proportioning of the elements of the cooling tower, the effects of the various loading conditions should be factored and combined in accordance with the applicable codes or standards. If no other codes or standards specifically apply, the factors and combinations given in ASCE 7 [7] are appropriate.

Dead load consists of the self-weight of the shell wall and the ribs and the superimposed load from attachments and equipment. Wind loading is extremely important in cooling tower design for several reasons. First, the amount of reinforcement, beyond a prescribed minimum level, is often controlled by the net difference between the tension due to wind loading and the dead load compression, and is therefore especially sensitive to variations in the wind-produced tension. Second, the quasistatic velocity pressure on the shell wall is sensitive to the vertical variation of the wind, as it is for most structures, and also to the circumferential variation of the wind around the tower, which is peculiar to cylindrical bodies. While the vertical variation is largely a function of the regional climatic conditions and the ground surface irregularities, the circumferential variation is strongly dependent on the roughness properties of the shell wall surface. There are also additional wind effects such as internal suction, dynamic amplification, and group configuration.

The external wind pressure acting at any point on the shell surface is computed as [5,8]

where q(z) is the effective velocity pressure at a height z (Figure 27.10) above the ground level, H(0) is the coefficient for circumferential distribution of external wind pressure, and 1 + g is the gust response factor, and g is the peak factor. As mentioned above, q(z) should be obtained from applicable codes or standards such as Ref. [7]. The circumferential distribution of the wind pressure is denoted by H(0) and is shown in Figure 27.11. The key regions are the windward meridian, 0 = 0°, the maximum side suction, 0 « 70°, and the back suction, 0 > 90°. These curves were determined by laboratory and field measurements as a function of

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Roughness parameter

Pressure

Pressure

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