Rooftop units, primary sources of vibration problems, are often culprits of non-compliance with background sound criteria. To avert a vibration or structure-borne noise problem, engineers should specify Vibro-Acoustics' noise control curb, which provides external vibration isolation, not just isolation of the usual suspects inside the unit (fan, compressor, etc.). Rooftop units with curbs require additional calculations and bracing for extreme events to secure the two attachment planes: the unit to the curb, and the curb to structure. The Vibro-Acoustics VCR curb for extreme event applications comes with the necessary calculations, stamped by a PE/P.Eng.

VCR Curbs - Vibration Isolation Curbs series come with integral and adjustable spring vibration isolators. Galvanized steel construction with a wood nailing strip for attaching field supplied and applied flashing is standard.

VCR curbs can be constructed with a sloped bottom to match the roof pitch. As airborne noise and vibration are usually a simultaneous concern, complete systems are available incorporating vibration isolation, duct-borne noise control, radiated noise control and flexible connections.

Noise control elements are aerodynamically matched to the connections between the rooftop equipment and the ducting system to afford the lowest possible installed pressure drop and energy usage.

The Vibro-Acoustics VCR curb for extreme event applications comes with the necessary calculations, stamped by a PE/P.Eng.

Design Considerations

Structure-Borne Vibration

Problem: Fans inside rooftop units can create vibration. If this vibration is transferred to the structure, it can create serious noise problems in many different areas.

Solution: Isolate Equipment. Fans are often isolated internally. This does not eliminate other sources of vibration. The best way to insure there is not further vibration/noise issue is to support entire unit on spring isolated roof curb.

Uneven Weight Distribution

Problem: Internal components can cause major variances in point loads around the unit.

Solution: Isolate Equipment. It is important to obtain section weights to select the appropriate isolators for each location to avoid over or under-loading isolators.

Seismic/Wind Requirements

Problem: Forces are determined by a number of factors including: project location, site class, risk category, component importance factor and equipment location equipment relative to the overall height of the building.

Solution: Certified anchorage calculations must be submitted to verify loading conditions are met for each piece of equipment.

Problem: Horizontal and vertical seismic forces are applied at the center of gravity of the unit. The resulting overturning moment creates tension and shear forces that can be very large.

Solution: All connections from the unit to the restrained springs or restrained spring isolated curb and from there to the structure the must be designed for these loads. For concrete attachments, this requires ICC-ES certified anchors.

Problem: Poor installation of concrete anchors.

Solution: IBC 2015 requires ICC-ES approved anchors for cracked concrete and Omega Naught Factor (Horizontal forces multiplied by 2.5).

Problem: Differential motion due to equipment on isolators and piping attached to structure.

Solution: Flexible connectors are required to reduce equipment nozzle loads.

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