In the past, the seismic design of mechanical equipment primarily was focused on equipment supports and attachments. The intent of seismic-design provisions in building codes was to reduce the hazards of sliding or falling equipment during an earthquake.
Now, mechanical systems often serve vital functions in critical-building facilities, such as hospitals and communication and emergency-response centers. Mechanical systems serving these types of facilities must be operational after an event because non-functioning equipment could constitute a hazard to life. Therefore, seismic design for higher levels of earthquake safety must ensure functionality as well as position retention. The 2006 International Building Code (IBC) has helped solve this issue, incorporating functionality and position retention within structural-design requirements.
This article will discuss the basis of seismic-design requirements, define seismic variables, and discuss seismic qualification methods.
First issued in 2000, the IBC is a model code developed by the International Code Council (ICC) and available for adoption by jurisdictions internationally. It is updated triennially. The three editions (2000, 2003, and 2006) of the IBC have been adopted and are in effect at the local or state level in all 50 states and the District of Columbia. Once adopted, IBC provisions become enforceable regulations governing the design and construction of buildings and structures.
The IBC defines design requirements for buildings and structures. For seismic-load-design requirements, the IBC refers to and incorporates many provisions of ASCE/SEI Standard 7, Minimum Design Loads for Buildings and Other Structures, a consensus standard published by the American Society of Civil Engineers.
“Nonstructural components” are defined in IBC seismic-design requirements as elements of mechanical, electrical, or architectural systems within buildings. Factory-assembled cooling towers typically are considered nonstructural components attached permanently to building structures. Therefore, cooling-tower structural design falls within the scope of building codes. Structural-design provisions of the IBC include requirements for cooling towers that may be exposed to various types of environmental factors, such as wind and seismic loads.
Several key variables must be examined to determine seismic-design requirements for cooling towers. These variables are unique to each project and independent of cooling-tower type. According to the IBC, these variables should be provided in structural documents and included in cooling-tower specifications by engineers of record.
Seven steps can help determine which seismic-design requirements apply to a project and which cooling towers are appropriate and provide specifications. The sidebar, “Putting Seismic Calculations Into Practice,” presents a real-world step-by-step example.
Occupancy categories I to IV classify buildings and other structures based on occupancy level and nature of use. Occupancy Category I buildings represent a low hazard to life in the event of failure, while Occupancy Category IV buildings are considered essential facilities (Table 1).
Occupancy-category classifications are not consistent among the three IBC editions and, therefore, may vary from jurisdiction to jurisdiction, depending on which edition was adopted. It is important that design professionals identify in project specifications the IBC edition being used.
All cooling towers are assigned an importance factor (Ip) equal to 1.0 or 1.5. Towers needed for the continued operation of an essential facility (an Occupancy Category IV building) or required to function after an earthquake are assigned an Ip of 1.5. All other towers receive an Ip of 1.0.
Towers with an Ip of 1.5 are classified as “designated seismic system components” and may require certification verifying that they will function fully after a seismic event.
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Seismic-design categories (SDC) A to F classify buildings based on:
Occupancy category.
Design spectral accelerations (over short periods [SDS] and at 1 sec [SD1]). SDC are based on the most severe of SDS/SD1 calculations.
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:
SDS = 2/3 x SMS
Where SMS = the maximum earthquake spectral-response acceleration over short periods as determined by:
SMS = Fa x Ss
Combining both equations results in:
SDS = 2/3 x Fa x Ss
Where Ss = the mapped spectral acceleration over short periods as determined in Section 1613.5.1 of the IBC or by using the USGS software and Fa = the site coefficient as defined in Table 2.
Design spectral accelerations at 1 sec can be calculated using the following equations:
SD1 = 2/3 x SM1
Where SM1 = the maximum-considered earthquake spectral-response acceleration at 1 sec as determined by:
SM1 = Fv x S1
Combining both equations results in:
SD1 = 2/3 x Fv 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 Fv = the site coefficient as defined in Table 3.
By knowing the SDS, SD1, and occupancy category, the SDC can be determined using tables 4 and 5.
Cooling towers exempt from the IBC's seismic-design requirements fall in two groups:
SDC A and B.
SDC C, provided Ip is equal to 1.0.
According to the IBC, all other cooling towers require seismic certification.
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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 SDS.
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):
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 Ip. 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.
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.
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.
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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.
2006 International Building Code, Copyright 2006. Washington, DC: International Code Council. Reproduced with permission. All rights reserved. www.iccsafe.org
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.
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 (Ip), and seismic-design category (over short periods [SDS] and at 1 sec [SD1]), should be provided by the engineer of record.
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.
According to Table 1, a hospital with emergency-treatment services fits in Occupancy Category IV.
Because the hospital is an essential facility and its cooling towers are required to function after an earthquake, Ip 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 SDS, Ip, and location of the tower should be compared with the same values of the equipment selected.
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 SDS/SD1 categories. Therefore, in this example, the SDC is D.
To determine the design spectral accelerations, the mapped spectral accelerations for short periods (Ss) and at the 1-sec period (S1) and the site coefficient for short periods (Fa) and at the 1-sec period (Fv) 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 SS, Fa, S1, and Fv values and calculate the SDS and SD1. In this example:
SS = 0.387
S1 = 0.076
Fa = 1.49
Fv = 2.4
SDS = 0.385 g
SD1 = 0.122 g
Check the math with the following equations:
SDS = 2/3 x Fa x SS
=2/3 x 1.49 x 0.387 = 0.384
SD1 = 2/3 x Fv x S1
= 2/3 x 2.4 x 0.076 = 0.122
Because the SDC is D and the Ip is 1.5, the cooling tower is not exempt from the structural requirements of the IBC.
Because the cooling tower will be installed on the roof of the hospital, the SDS determined in Step 3 should be compared with the rated unrestricted SDS for the desired product.
The cooling tower should have an unrestricted SDS of at least 0.384 based on an Ip of 1.5 or higher.