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Step 6: Determine the location of the cooling tower

The elevation of a cooling-tower structure within a building impacts design seismic acceleration. As the installed elevation of a cooling tower increases relative to building height, ground seismic accelerations are amplified.

Equipment manufacturers stating seismic qualification using the terms "restricted" and "unrestricted" is an accepted industry practice. For cooling-tower installations, a restricted seismic qualification means the cooling tower is qualified for installation on a grade. An unrestricted seismic qualification means the tower is qualified to be installed on top of a building. In other words, for projects with restricted seismic qualification, the cooling tower must be installed on the ground. With an unrestricted seismic qualification, the cooling tower can be installed in any building location from the roof to ground level. These normally are expressed as a restricted or unrestricted S_DS.

Step 7: Select an independently certified cooling tower (seismic qualification methods and independent certification)

Seismic-design requirements for nonstructural components, including mechanical equipment, are described in Chapter 13 of ASCE/SEI Standard 7. Mechanical equipment must be qualified using one of the following methods (Table 6):

1. Analysis

   With this method, a cooling tower is evaluated mathematically to determine if it can resist code-prescribed seismic-design forces. Typically, an evaluation of this type focuses only on the anchorage or on the anchorage and main structural components, depending on the component I_p. Analysis cannot effectively address non-structural portions that affect functionality, such as drive, water-distribution, and heat-transfer systems. The analysis method also is somewhat difficult for code bodies to review and accept/reject. It takes time to examine an analysis and understand all of the assumptions made.

2. Testing
   

   TABLE 6: Applicable methods of seismic qulification for cooling towers.¹

   With this method, a full-scale cooling tower is subjected to a simulated seismic event in a test laboratory. Typically, the testing method is a shake-table test conducted in accordance with a code-recognized test procedure, such as AC 156, Acceptance Criteria for Seismic Qualification by Shake-Table Testing of Nonstructural Components and Systems, published by ICC Evaluation Service Inc. The standard is applicable to all types of equipment, including mechanical and electrical equipment. Therefore, it requires that a testing plan be developed for all pre- and post-seismic-testing-verification activities. Testing results are easier for a code body to review and accept/reject.

3. Experience data

   With this method, a cooling tower is qualified using actual earthquake performance data collected in accordance with a nationally recognized procedure. Though this method is used to some extent in the nuclear-power industry, it is not used in commercial mechanical-equipment applications because of limitations, such as:

   • Lack of a recognized data-collection procedure and a national database with widespread access.

   • Infrequency of strong-motion earthquakes.

   • Low probability that the data are applicable to the current generation of products.

   • Low probability that the actual seismic accelerations experienced by a field unit can be translated to current levels of seismic demand.

   Based on these limitations, experience data is excluded as a viable qualification method. Analysis and testing methods are not equally suitable for verification of all of the aspects of cooling-tower seismic performance. For example, mathematical analysis is well-suited for verification of anchorage resistance, but not reliable for verification of cooling-tower functionality after a seismic event. The only reliable method of verifying functionality after a seismic event is through testing.