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CONCLUSION

The IBC sets forth criteria to identify facilities that are critical for the protection of human life during and immediately following a seismic event and prescribes structural-design requirements to ensure the safe and continued operation of such facilities.

Mechanical systems often serve vital functions in critical facilities. Following an earthquake, the continued operation of these facilities could be dependent on the ability of mechanical systems to remain operable. Equipment failure in these applications could constitute a hazard to life. The most reliable method of ensuring post-event equipment functionality is shake-table testing performed in accordance with AC 156.

NOTE

1. 2006 International Building Code, Copyright 2006. Washington, DC: International Code Council. Reproduced with permission. All rights reserved. www.iccsafe.org

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Manager of product marketing for Baltimore Aircoil Co., Kavita A. Vallabhaneni has worked for the company for 14 years. She is the author of the October 2006 HPAC Engineering article "Minimizing Energy Costs With Free Cooling." Manager of engineering services, Panos G. Papavizas, PE, has worked for Baltimore Aircoil Co. for 19 years.

Putting Seismic Calculations Into Practice

The following example illustrates how to determine whether a facility requires a seismic-resistant cooling tower and how to select the right one for a particular application. As outlined in the article, there are seven steps to determining the seismic requirements that must be included in a cooling-tower specification. In execution, seismic-design criteria, including occupancy category, importance factor (I_p), and seismic-design category (over short periods [S_DS] and at 1 sec [S_D1]), should be provided by the engineer of record.

Example

A 400-ton cooling tower is required for a five-story hospital with emergency-treatment facilities in Glenrock, Wyo. The cooling tower will be installed on the roof of the hospital.

Step 1: Determine the occupancy category of the hospital

According to Table 1, a hospital with emergency-treatment services fits in Occupancy Category IV.

Step 2: Determine the I_p

Because the hospital is an essential facility and its cooling towers are required to function after an earthquake, I_p is equal to 1.5. Also, the hospital's cooling towers need to be operational after an event, and the best method to qualify them is through independently certified shake-table testing. Therefore the S_DS, I_p, and location of the tower should be compared with the same values of the equipment selected.

Step 3: Determine the seismic-design category

According to Table 4, the hospital falls into Seismic Design Category (SDC) D. According to Table 5, the hospital also falls into SDC C. According to the IBC, the SDC is based on the most severe of the S_DS/S_D1 categories. Therefore, in this example, the SDC is D.

Steps 4a and b: Determine the design spectral acceleration (S_DS and S_D1)

To determine the design spectral accelerations, the mapped spectral accelerations for short periods (S_s) and at the 1-sec period (S₁) and the site coefficient for short periods (F_a) and at the 1-sec period (F_v) are required.

As mentioned in the article, these values can be found using the U.S. Geological Survey Earthquake Ground Motion Parameter Java Application. The software can return the S_S, F_a, S₁, and F_v values and calculate the S_DS and S_D1. In this example:

S_S = 0.387

S₁ = 0.076

F_a = 1.49

F_v = 2.4

S_DS = 0.385 g

S_D1 = 0.122 g

Check the math with the following equations:

S_DS = 2/3 x F_a x S_S

=2/3 x 1.49 x 0.387 = 0.384

S_D1 = 2/3 x F_v x S₁

= 2/3 x 2.4 x 0.076 = 0.122

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

Because the SDC is D and the I_p is 1.5, the cooling tower is not exempt from the structural requirements of the IBC.

Step 6: Determine the location of the cooling tower

Because the cooling tower will be installed on the roof of the hospital, the S_DS determined in Step 3 should be compared with the rated unrestricted S_DS for the desired product.

Step 7: Select the cooling tower

The cooling tower should have an unrestricted S_DS of at least 0.384 based on an I_p of 1.5 or higher.

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