277 Design and Detailing of Components

The dimensioning of the tower components should follow the requirements of the applicable codes of practice. In Ref. [5], limit states of serviceability and of failure are presented to provide sufficient safety. In the limit states of serviceability, a total safety factor concept is applied and the following load combinations are recommended:

Deformation, m

FIGURE 27.39 Load-displacement diagrams for load combination D + DT45 + 1W.

Deformation, m

FIGURE 27.39 Load-displacement diagrams for load combination D + DT45 + 1W.

the latter for checking crack widths. Here, T is taken for standard temperature service conditions. In the failure limit state design, partial safety factors are used for the following load combinations:

The structural elements of the tower should be constructed with a suitable grade of concrete following the provisions of applicable codes and standards. The design of the mixture should reflect the site conditions for placement of the concrete and the external and internal environment of the tower.

The shell wall should be of a thickness that will permit two layers of reinforcement in two perpendicular directions to be covered by a minimum of 3 cm of concrete, and should be no less than 16 cm thick [2,4,8]. The buckling considerations mentioned in Section 27.6.3 have proven to be convenient and evidently acceptable criteria for setting the minimum wall thickness, subject to a nonlinear analysis. The formula qc = 0.052E(h/R)2'3 (27.8)

where E is the modulus of elasticity has been used to estimate the critical shell buckling pressure, qc [4,12]. Then, h(z) is selected to provide a factor of safety of at least 5.0 with respect to the maximum velocity pressure along the windward meridian, q(z)(1 + g). Also, the cornice should have a minimum stiffness of

where Ix is the moment of inertia of the uncracked cross-section about the vertical axis and dH is shown in Figure 27.10 [4]. Some typical forms of the cornice cross-section are shown in Figure 27.49.

The elements of the cooling tower should be reinforced with deformed steel bars so as to provide for the tensile forces and moments arising from the controlling combination of factored loading cases. The shell walls may be proportioned as rectangular cross-sections subjected to axial forces and bending. As mentioned above, a mesh of two orthogonal layers of reinforcement should be provided in the shell walls, generally in the meridional and circumferential directions [8]. In each direction, the inner and outer layers should generally be the same, except near the edges where the bending may require an

Inner face

Outer face

Wind

Inner face

Outer face

Wind

Wind

Inner face

Outer face

Inner face

Outer face

Wind

Crack pattern at D + 2.3W, wc > 0.05 mm FIGURE 27.40 Different crack patterns for load combination D + DT45 + 1W.

unsymmetrical mesh. It is preferable to locate the circumferential reinforcement outside of the meridional reinforcement except near the lintel, where the meridional reinforcement should be on the outside to stabilize the circumferential bars [16]. A typical heavily reinforced segment of the lintel, also showing the anchorage of the column reinforcement into the shell, is depicted in Figure 27.50.

A summary of the most important minimum construction tolerance values for the shell wall reinforcement is given in Figure 27.51 [2,4,8]. The bars should not be smaller than 8 mm diameter, f, and, for meridional bars, not smaller than 10 mm. Further, a minimum of 0.35 to 0.45%, depending on the admissible cracking, should be used in each direction. The minimum cover, as mentioned previously, should be 3 cm, the maximum spacing of the bars, e, should be 20 cm, and the splices should be staggered as specified for the construction of walls in the applicable codes or standards. Particular attention should be given to splices in tensile zones.

The supporting columns should ideally be proportioned for the forces and moments computed from an analysis in which they are represented as discrete members, using the appropriate factored loading

Number of circumferential waves n FIGURE 27.42 Natural frequencies for different cornice stiffnesses for the cooling tower in Figure 27.4.

combinations [9]. If the column region has not been modeled discretely, but rather by a continuum approximation, the columns may be proportioned to resist the tributary factored forces and moments at the interface with the lintel, as computed from the shell analysis. The effective length may be taken as unity. Particular attention should be directed toward splices of the column bars when net tension is present. Since large bars will be involved, welded or mechanical splices capable of transmitting tension forces are recommended in such regions.

It is possible to add discrete circumferential stiffeners to the shell to increase the stability or to restore capacity that may have been lost due to cracking or other deterioration [3] (see Figure 27.10). Such stiffeners can generally be included in a finite element model of the shell wall and should be

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