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.

BASIS OF SEISMIC-DESIGN REQUIREMENTS

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.

SEISMIC-DESIGN REQUIREMENTS FOR FACTORY-ASSEMBLED COOLING TOWERS

“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.

DETERMINING WHETHER A SEISMIC-RESISTANT COOLING TOWER IS NEEDED

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.

Step 1: Determine the occupancy category of the building

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.

Step 2: Determine the importance factor

All cooling towers are assigned an importance factor (I_p) 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 I_p of 1.5. All other towers receive an I_p of 1.0.

Towers with an I_p 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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