Installation Considerations for IBC-Compliant Generator Sets
The International Code Council's (ICC) International Building Code (IBC) has replaced regional codes in most jurisdictions, making it the primary code document for engineers who specify and supervise the installation of generating equipment.
Currently, authorities may be using either the 2000, 2003, or 2006 edition of the IBC, with one of these editions now adopted at either the state or local level in all 50 states. Recently, the State of California adopted the 2006 edition, leading the way for that edition to become a preeminent standard across the United States.
The IBC addresses both the design and installation of building systems, with an emphasis on how those systems will perform in emergency situations—and continue to perform after such events. One big change for engineers specifying seismic-related equipment is that the United States is no longer divided into broad seismic zones. Instead, seismic requirements are based on U.S. Geological Survey (USGS) data for ground accelerations at specific ZIP codes or at a specific longitude and latitude. And, not only must critical power-generating systems be designed to the same seismic-design category as the buildings in which they are located, their location within a building—whether it's belowground, at grade level, or on a rooftop—also must be considered. Where custom enclosures or subbase fuel tanks are a part of the design, the entire system must be seismically certified. This is due to the fact that fuel tanks carry flammable substances and also because of the interrelationship clause in the code, which dictates that the failure of one component cannot cause the failure of a secondary critical component.
The building's purpose, as designated by its Occupancy Category or Seismic Use Group (depending on the version of the code used), also plays an important role in code-defined design requirements. The Seismic Use Group is based on the Occupancy Category of the building and is defined as follows:
• Occupancy Category IV—Buildings and other structures designated as essential facilities (e.g., hospitals, police stations, and fire stations)
• Occupancy Category III—Buildings and other structures that represent a substantial hazard to human life in the event of failure (e.g., schools, theaters, and jails)
• Occupancy Category II—Buildings and other structures except those listed in Occupancy Categories I, III, and IV
• Occupancy Category I—Buildings and other structures that represent a low hazard to human life in the event of failure (e.g., agricultural facilities, certain temporary facilities, and minor storage facilities).
The primary applications for emergency power systems covered by these various Occupancy Categories are for emergency lighting and life-support systems. However, engineers need to understand that other building systems also might be critical to a building's operation during and after an emergency. For example, HVAC equipment could be essential in some healthcare settings, and therefore require its own code-compliant backup power systems.
A typical emergency power system includes an automatic-transfer switch, a generator, a battery to start the generator, and associated wiring and ventilation connections. Under the IBC, each piece of equipment within the emergency power system must be seismically certified—and wind certified, in locations where that requirement applies.
Generator sets are most susceptible to earthquake-related damage at the points where components—including the engine exhaust system, fuel lines, ac power-supply wiring, load wiring, and control wiring—interconnect. These connections must be flexible (see Figure 1), so relative motion can be absorbed without damage. Without this flexibility, the building or generator set may be damaged and the generator may fail while in service. Additionally, when vibration isolation is required in a seismic application, seismically certified vibration isolators must be used to ensure the generator set isn't dislodged from the building structure during the seismic event.
It is worth mentioning here that the design force for a piece of equipment mounted on the roof is about three times greater than for that same piece of equipment mounted at grade. This means that the generator set and the attachment of the equipment to the building structure need to be capable of handling these higher design forces. The vibration isolators must be selected based on the weight and center of gravity of the equipment, the type of foundation where the equipment will be installed (concrete or steel), the location within the building where the equipment will be situated, and the ground accelerations for the location as outlined in the code.
To ensure a manufacturer's generator sets can survive a seismic event, Section 1708.5 of the IBC (all versions) outlines the criteria for testing equipment for IBC compliance, which may include shake-table testing, structural modeling, or a combination of both (see Figure 2).
Shake-table testing involves placing the generator set or component onto a shaker table where the equipment is tested in three different axes. The table creates random movement that is monitored in order to validate that frequency and forces simulate those of an earthquake. Generator sets are subjected to a range of different tests charting various performance characteristics under different potential operating conditions. The analytical method uses software to simulate the dynamic characteristics and forces of wind and seismic events, and evaluates the system's capability to withstand them. Units that pass these rigorous evaluations are considered to be seismically certified. The test standard as outlined in the IBC codes is ICC-ES-AC 156.
The IBC 2006 requires that generator sets meet the wind-load requirements specified for the installation location. The United States is divided into a number of wind zones, according to the highest wind speeds occurring in those zones due to hurricanes or other storms (to review the U.S. Wind Zone Map, see Figure 1609 of IBC 2006, pp. 294-295).
In jurisdictions following either the 2000 or 2003 edition of the IBC, manufacturers of “designated seismic systems”—including generator sets—must supply a certificate of compliance to the code's seismic requirements, along with equipment labeling that contains the name of the approved agency that performed the certification testing. In the 2006 edition, the requirement for equipment labeling was removed. Manufacturers must still supply a certificate of compliance for seismic and/or wind (when required), based on testing and/or mathematical modeling, but the 2006 edition allows for self-certification.
Ensuring that a manufacturer's generating equipment is seismically certified is only half the story when it comes to designing emergency power systems. Engineers also must make sure generator sets are installed to withstand referenced seismic and wind forces. So specifying the right equipment isn't enough; designers also should consider whether their plans for installing that equipment meet the right seismic standards.
Support and isolation are the key goals of the IBC's installation requirements, to ensure that ground and building movement don't end up dislodging the generators or disconnecting them from their fuel lines and wiring systems. Incorporating flexible connections into the design is critical for protecting the equipment from this kind of damage. But engineers need to be aware of several other installation requirements as well.
A primary factor in successful seismic design is how generating systems and their ancillary components are anchored. Anchors, and the concrete into which they are embedded, must meet strict guidelines to pass IBC requirements. The appendix, “Meeting IBC Seismic Installation Requirements” (common to all versions of the IBC), describes these guidelines for both floor- and wall-mounted equipment.
Vibration isolation is another key success factor in ensuring generator-set survivability. Dynamic loading is a major consideration in this process and can be addressed by designing a foundation that weighs at least twice—and up to 10 times—as much as the generator set. Additionally, the foundation should extend at least 12 in. beyond the skid on all sides and rise at least 1 ft above the floor, and its base should lie below the frost line, to prevent heaving. The soil-bearing load at the location should be confirmed with local officials and the building's soil-analysis report to ensure it is adequate to support the combined foundation and generator set.
A generator set's engine and alternator must be isolated from its mounting structure, regardless of how this foundation is constructed. Bolting a generator set directly to a floor or foundation will result in excessive noise and vibration, and could damage the generator, the floor, and other equipment. Vibrations transmitted through the building structure also could damage the structure itself.
Seismic isolators are used to accomplish this task (see Figure 3). While standard vibration isolators—including integral isolators built into the design of the generator set—help reduce generator-induced vibrations, seismic isolators are required to reduce the threat posed by building vibrations resulting from a seismic event. These devices ensure equipment remains anchored and doesn't break free of the structure to which w attached. They are often installed between the skid base and the structure and are available in both synthetic rubber and steel-spring fabrications.
Seismically rated vibration isolators are not “one size fits all.” Depending on the ground accelerations in a particular location and where the equipment is located in a building, the design (and consequently cost) of a seismic vibration isolator can vary tremendously. The IBC provides calculations to help engineers determine which strength isolator is required for a given project. These calculations take a number of factors into account, including the:
• Horizontal g-level the equipment needs to withstand
• Equipment's weight and center of gravity
• Location in a building where equipment is installed
• Spectral response coefficients.
In addition, because the IBC uses site-specific data rather than regional information to determine seismic hazards, these calculations require inputs based on local conditions. These seismic design values, including probabilistic hazard curves and uniform hazard-response spectra, can be found using an application available from the USGS website at www.earthquake.usgs.gov/research/hazmaps/design/index.php. Entering either the site location's ZIP code or latitude and longitude will provide the figures necessary to complete the calculations.
Finally, the installation's location within the building also must be factored into these calculations. For example, seismic forces will have greater impact on rooftops than on ground-level or belowgrade sites. As a result, rooftop generators may require stronger seismic isolators than those installed elsewhere in a building.
With the IBC now the primary U.S. building code, engineers need to be familiar with the code's strict seismic and wind provisions. In some areas, design professionals may be facing seismic-certification requirements that previous map-based standards didn't present. Now, they need to assure the equipment they specify can meet the same seismic loads as the buildings in which that equipment is located. Understanding seismic-performance requirements, and addressing those conditions in specifications and installation designs, has become a key component in any building's approval by local authorities.
As a sales application engineer with Cummins Power Generation, Natekar provides technical recommendations on installations, engineering support to customers, and training assistance to the sales force. He holds a master of science in Automotive Engineering from Lawrence Technological University (Southfield, Mich.) and a bachelor of science in Mechanical Engineering from University of Pune (India).
Meeting IBC's seismic installation requirements
The International Building Code (IBC) provides specific guidance on how generator sets must be installed to meet the code's seismic requirements. The following guidance, generic for all versions of the IBC, represents some of the most important points to consider when designing installations for seismic-related projects (see Figure 4). Be sure to reference the applicable edition of the code and any manufacturer requirements to ensure your projects are in compliance.
• Anchors that are post-installed into concrete for use with generator-set components must be prequalified for seismic applications in accordance with ACI 355.2 and documented in a report by a reputable testing agency.
• Anchors must be installed to an embedment depth as recommended in Note 1 of the prequalification test report. In jurisdictions using IBC 2000 or IBC 2003, the minimum embedment is eight times the anchor's diameter.
• Anchors must be installed in normal-weight concrete with a minimum compressive strength of 4,000 psi. Concrete aggregate must comply with ASTM C33. Installation in structural lightweight concrete is not permitted unless it has been approved by the structural engineer of record.
• Anchors must be installed to the torque specification recommended by the anchor manufacturer to obtain maximum loading.
• Anchors must be installed in the locations specified on the seismic installation drawing or in the IBC seismic-certification report.
• Wide washers must be installed at each anchor location, between the anchor head and connected equipment, for tension load distribution. These wide washers must be Series W of American National Standard Type A Plan Washers (ANSI B18.22.1-1965, R1975), and the nominal washer size should match the nominal diameter of specified anchors.
Slab, pad, and mounting requirements
• Concrete floor slabs and housekeeping pads must be designed and reinforced with rebar in accordance with ACI 318's requirements for seismic applications.
• Housekeeping-pad thickness must be designed in accordance with Note 1 of the prequalification test report, or to a minimum of 1.5 times the anchor-embedment depth, whichever is greater.
• Housekeeping pads must be dowelled into the building's structural floor slab and designed for seismic application per ACI 318, as approved by the structural engineer of record.
• Wall-mounted equipment must be installed to a rebar-reinforced structural concrete wall or a seismically designed, grout-filled concrete block wall that is approved by the engineer of record to resist the added seismic loads created by the anchored components.
• Floor-mounted equipment—with or without a housekeeping pad—must be installed to a rebar-reinforced, structural-concrete floor that is seismically designed and approved by the engineer of record to resist the added seismic loads created by the anchored equipment.
• Rebar interference must be considered whenever equipment is anchored to a floor or wall.
• Seismic-certified equipment shouldn't be attached to floors constructed of lightweight concrete over steel decking.
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