Standby power system enclosures must withstand loads produced by hurricanes and windstorms. These enclosures must endure wind loads that are determined by many complex factors. The International Building Code is among the standards that have been created to establish common methodology for design and analysis to minimize losses due to wind events. U.S. building standards have also evolved, along with codes for electrical and mechanical systems. The latest edition of building standards is embodied in the International Building Code (IBC 2000, 2003, 2006, and 2009), which sets requirements for structures and ancillary systems, including standby power systems. Building owners and power system specifiers should be familiar with the wind-load compliance provisions of the IBC with respect to power system equipment.
International Building Code (IBC)
In 2000, the International Code Council (ICC) issued its first version of the IBC. While most of the IBC deals with life-safety and fire protection of buildings and structures, it also addresses wind-load design requirements for both buildings and components attached to them. The IBC has been updated every three years, and each edition references standards from a variety of sources, such as the design requirements originally promulgated by the American Society of Civil Engineers (ASCE 7-05) in its Minimum Design Loads for Buildings and Other Structures.
While the IBC has an international label, currently it only refers to building standards in the U.S. All states and many local authorities have adopted a version of the IBC, either the 2000, 2003, 2006, or 2009 edition. Most states have adopted the code at the state level and other local governments have adopted versions of the code at the municipal or county level. The vast majority of states have adopted the 2006 version, while fewer states have adopted the 2003 edition and several are still referencing the 2000 version. Several U.S. territories have adopted the 2009 version, which is now available for adoption by the states.
While the government does not mandate IBC adoption, its adoption has been encouraged-and in some cases required-to ensure funding coverage by the Federal Emergency Management Administration (FEMA). Generally speaking, the requirements for wind-load design are very similar regardless of which version of the code a state has adopted. The following link provides information on the IBC adoption status for each state: http://www.iccsafe.org/gr/Documents/stateadoptions.pdf.
The U.S. wind-speed map provides information on basic wind speed in miles per hour in geographic zones. The first step to identifying wind load requirements for a standby power system is to determine the installation location’s basic wind rating speed. While most of the U.S. has a basic wind rating speed of 90 miles per hour, special regions, particularly along the Atlantic and Gulf coasts, have ratings of up to 150 miles per hour. Figure 1 shows basic wind speed versus geographic regions in the United States.
Qualifying the Product
Manufacturers have three options to qualify their product: wind-tunnel testing, analytical calculation, or a combination of both. Wind-tunnel testing is often not practical due to size and wind speed constraints. Even a small standby power set, such as a 20-kilowatt (kW) unit, would be too large for the vast majority of wind tunnels. Also, huge power requirements for blower fans and massive tunnel size make testing larger sets virtually impossible. Since wind-tunnel testing is not practical, qualification is most often done using the analytical method. Using IBC 2009 and applying the proper conditions, analysis can be done to qualify sets for 2000, 2003, and 2006 versions simultaneously. Specifically, IBC 2006 and 2009 specify application of methods in ASCE 7-05 section 6.5. These methods are detailed and rigorous.
Parameters Determine Wind Loads
The analytical calculation method uses formulae identified in ASCE 7-05, Minimum Design Codes for Buildings and Other Structures, an industry-wide standard. The first step is to calculate the wind-velocity pressure at the structure, which depends on geography, local terrain, topography, the direction factor, and occupancy of the structure. The analysis begins by converting anticipated wind speed to wind pressure. The challenge is to take the complex set of many variables for each installation and simplify it to an equation with standard parameters. Per section 6.5.10 of ASCE 7-05, wind-velocity pressure at the structure is defined as:
qz = 0.00256KzKztKdV2Ip (lb/ft2)
(ASCE 7-05 equation 6-15)
The five critical parameters used to establish the wind load are:
V –The basic wind speed is defined at 33 feet above ground level and dependent on the geographic location. See figure 1.
Kz – The exposure factor depends on the height of the installation above ground and local terrain. As elevation increases, so does the wind speed; hence an amplification factor is needed as installation height increases. This factor is as high as 1.89.
Kzt – The topographic factor depends on the gross terrain. As wind speed increases with elevation of a building, so to, wind speed increases with height up a hill. For flat terrain, this value is approximately 1.0. Conversely, the factor can approach 3 on a hill.
Kd – The direction factor ranges from 0.85 to 0.95. It depends on the type and portion of the structure. For rectangular structures, which include generator set enclosures, the value is 0.85.
Ip – The “importance” factor is used to reduce or amplify the basic wind speed; it depends on the occupancy factor of the structure. This factor ranges from 0.77 for uninhabited buildings to as high as 1.15 for buildings of critical importance, such as hospitals, fire stations, and the like.
Once wind-velocity pressure has been determined, the value is applied with adjustment factors per the standard to the sides and roof of the structure. The windward side receives a positive pressure, while the remaining walls and roof receive a negative pressure. Negative pressure attempts to suck the sides off the building, while positive pressure tends to compress the building. The combination of internal and external pressures establishes the actual wind design pressures used for analysis on respective walls and roof. The combined pressures are defined by the following formula, which is also part of ASCE 7-05, section 6.5.10:
(ASCE 7-05 equation 6-17)
qz – Wind-velocity pressure as calculated above.
G – The gust effect factor and as applied to generator set enclosures, is 0.85 by definition of ASCE 7-05, section 6.5.8.
Cp – The external pressure coefficient. This value depends upon the specific geometry and proportion of the enclosure and is determined by comparison to predefined geometric proportions relative to charted Cp values in figure 6.6 of the ASCE 7-05.
Cpi – The internal pressure coefficient. It is predefined by ASCE 7-05 and depends upon the proportion of the area of the openings of the enclosure to the overall enclosure area. Openings in a building envelope create holes for airflow, which creates internal pressure. Thus qGCp is the external for pressure value. It is necessary to calculate this for each external surface. See figure 2 for a typical distribution of pressures on the surfaces of a standby power system. The term qi(GCpi) is for pressure developed on the inside of the structure. Cracks and gaps in structure allow air to penetrate it and internally pressurize the enclosure. Term qz calculated in equation 6-15 is substituted for q and qi in equation 6-17. With P established for each surface, the pressure can then be applied to the respective surfaces and stresses and loads evaluated for adequacy.
At this point, stresses are checked to make sure materials do not fail, the structure is evaluated to make sure it does not buckle or collapse, and fasteners are evaluated against calculated loads to make sure they do not break. As a way to evaluate the enclosure to assure compliance, engineers can develop a deflection plot that visually confirms the computer numerical analysis of the impact of pressure on the enclosure. Figure 3 is a deflection plot of an enclosure subjected to a 150 mile per hour basic wind speed.
Installation and Mounting
Installation and mounting of the enclosure is of equal importance to the design of the power system to ensure that the product remains connected to the foundation through a storm. The installing contractor is responsible for proper specification and installation of all anchors and mounting hardware.
Anchors used for wind load installation must be designed and rated to resist wind loading in accordance with ACI (American Concrete Institute) 355.2-04 and documented in a report by a reputable testing agency (i.e., The Evaluation Service Report issued by the International Code Council). Anchor brands and styles used for wind loading are essentially the same as those for seismic loading.
Anchors must be installed to a minimum embedment depth of eight times the anchor diameter.
Anchors must be installed in minimum 4,000 psi compressive-strength normal-weight concrete. Concrete aggregate must comply with ASTM (American Society for Testing and Materials) C33. Installation in structural lightweight concrete is not permitted unless otherwise approved by the structural engineer of record.
Anchors must be installed to the required torque specified by the anchor manufacturer to obtain maximum loading.
Anchors must be installed with spacing and edge distance required to obtain maximum load unless otherwise approved by the structural engineer of record.
Wide washers must be installed at each anchor location between the anchor head and equipment for tension load distribution. See the applicable installation or dimension drawing for specific anchor information and washer dimensions.
Equipment installed on a housekeeping pad requires that the housekeeping pad be at least 1.5 times as thick as the anchor embedment depth.
All housekeeping pads must be securely anchored and dowelled or cast for security and approved by the structural engineer of record. Rebar reinforcing in the housekeeping pad is required for all installations.
Rebar reinforcement in concrete must be designed in accordance with ACI 318-05.
Wall-mounted equipment must be installed to a rebar-reinforced structural concrete wall that is wind load designed and approved by the engineer of record to resist the added wind loads from components being anchored to the wall. When installing, rebar interference must be considered.
- Structural walls, structural floors, and pads must also be designed and approved by the structural engineer of record. The installing contractor is responsible for proper installation of all electrical wiring, piping, ducts and other connections to the equipment.
When specifications call for IBC-wind rated products, the product must be tested or analyzed for IBC compliance. Since analysis is detailed, it can be very expensive for a ‘one-of’ job specific basis. In addition to the expense, the lead time on a ‘one-of’ project can be prohibitive. Therefore using products that have been prequalified to IBC requirements can save time and expense on projects.
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