220.127.116.11 HVAC Equipment With Vibration Isolation
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This includes HVAC equipment, typically of sheet metal construction, that is floor- mounted with vibration isolators to prevent the transmission of mechanical vibrations into the building.
- Typical Causes of Damage
- Seismic Mitigation Considerations
Building Code Provisions
Seismic loads for HVAC equipment with vibration isolation are determined using using Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10) Chapter 13. The principal objective is to prevent the component from sliding or overturning.
- ASCE/SEI 7-10 requires anchorage for all equipment in Seismic Design Categories D, E, and F for all equipment weighing over 400 pounds, and items weighing over 20 pounds that are mounted over 4 feet above the floor. Lighter items may be exempt if the component Importance Factor Ip = 1.0.
- Items that are exempt from the anchorage requirements noted above are still required to be positively anchored to the structure. The anchorage need not be designed or detailed on the construction documents. Exempt components must be provided with flexible connections between the equipment and associated pipes, ducts, or conduits.
- The seismic design force for vibration isolated equipment is doubled if the “air gap” (the distance between the equipment support frame and the restraint) is greater than 0.25 inches.
Retrofit Standard Provisions
In Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41-06), HVAC equipment with vibration isolation is classified as force controlled. ASCE/SEI 41-06 requires compliance with the anchorage provisions of the standard when:
- The performance level is Immediate Occupancy
- The performance level is Life Safety in high seismicity areas, if
- The item is gas-fired.
- The item is part of an emergency power system.
- The items weighs more than 400 pounds and is 6 feet or more in height.
- The item is unanchored, weighs over 100 pounds, is 6 feet or more in height, and is subject to overturning. These items may be exempt if they have a factor of safety greater than 1.5 against overturning when design loads are applied.
- The item weighs over 20 pounds and is mounted over 4 feet above the floor.
Typical Causes of Damage
- Vibration isolated equipment is particularly vulnerable to earthquake damage unless some type of snubbers, bumpers, or vendor-supplied restraints are used. Open and housed springs do not have adequate capacity to resist shear and uplift.
- Vibration isolators tend to amplify seismic response of the equipment This requires the supporting frames or slabs to have adequate capacity to resist higher forces (Figure 22.214.171.124-3).
- Items can slide, tilt, overturn, or fall.
- Internal components may be damaged by shaking.
- Connections of fuel lines, electrical lines, or ductwork may be damaged; machinery may cease to function due to misalignment, failure of the isolators, or internal damage.
- Although the code exempts small components on vibration isolators from design requirements, they should be carefully evaluated, since vibration isolators tend to amplify the seismic response.
- Two methods are used for anchoring floor-mounted equipment on vibration isolators:
- Open springs used in conjunction with snubbers or bumpers.
- Restrained springs with rated capacity to resist the anticipated seismic shear and uplift.
- Rated housed springs with vertical travel limits should be used for seismic restraint applications.
- The “air gap”, or distance between the snubber and the component should be limited to less than 0.25 inches, to avoid significant dynamic amplification when the component contacts the snubber.
- Metal-to-metal contact between snubbers and the supporting frame or between the isolator housing and the mounting bolt to the unit should be avoided. The snubbers should have an elastomeric or resilient surface to lessen the impact effects during strong shaking. For housed springs, an elastomeric grommet will prevent hard surface impact.
- Flexible connections must be provided for fuel lines and piping.
- Refer to Installing Seismic Restraints for Mechanical Equipment (FEMA 412) and Incremental Seismic Restraints for Duct and Pipe (FEMA 414) for additional information and details.
- HVAC equipment or other items required for use in a hospital or essential facility would be classified as designated seismic systems and may require engineering calculations, equipment certification and special inspections. Check with the jurisdiction for specific requirements.