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Step 3: Determine the seismic-design category

Seismic-design categories (SDC) A to F classify buildings based on:

• Occupancy category.

• Design spectral accelerations (over short periods [S_DS] and at 1 sec [S_D1]). SDC are based on the most severe of S_DS/S_D1 calculations.

Step 4a: Calculate the S_DS and S_D1

Design spectral accelerations are dependent on site class and the maximum ground-shaking intensity at a given location. Site class is based on a site's soil properties, which can range from hard rock (Site Class A) to peat and clays (Site Class F). According to the IBC, "If the soil properties are not known in sufficient detail, Site Class D shall be used."

Ground-shaking intensity can be obtained from probabilistic seismic-hazard maps provided in the IBC. However, because of the fine gradation of acceleration values in some regions, such as on the West Coast, it is more expedient and accurate to use software tools provided by the ICC or U.S. Geological Survey (USGS). (The USGS Earthquake Ground Motion Parameter Java Application can be found at http://earthquake.usgs.gov/research/hazmaps/design.) Input values for the software can include the map coordinates or ZIP code of a project site.

Design spectral accelerations for short periods (0.2 sec) can be calculated using the following equations:

S_DS = 2/3 x S_MS

Where S_MS = the maximum earthquake spectral-response acceleration over short periods as determined by:

S_MS = F_a x S_s

Combining both equations results in:

S_DS = 2/3 x F_a x S_s

Where S_s = the mapped spectral acceleration over short periods as determined in Section 1613.5.1 of the IBC or by using the USGS software and F_a = the site coefficient as defined in Table 2.

Design spectral accelerations at 1 sec can be calculated using the following equations:

S_D1 = 2/3 x S_M1

Where S_M1 = the maximum-considered earthquake spectral-response acceleration at 1 sec as determined by:

S_M1 = F_v x S₁

Combining both equations results in:

S_D1 = 2/3 x F_v xS₁

Where S₁ = the mapped spectral acceleration at 1 sec as determined in Section 1613.5.1 of the IBC or by using the USGS software and F_v = the site coefficient as defined in Table 3.

Step 4b: Determine the assigned SDC

By knowing the S_DS, S_D1, and occupancy category, the SDC can be determined using tables 4 and 5.

Step 5: Determine if the cooling tower is exempt from IBC seismic requirements

Cooling towers exempt from the IBC's seismic-design requirements fall in two groups:

• 

  TABLE 4: SDC based on short-period response accelerations¹.

  SDC A and B.

• SDC C, provided I_p is equal to 1.0.

According to the IBC, all other cooling towers require seismic certification.

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