Curbs and Bases - Tech Notes
VMC Group designs and manufactures a wide variety of concrete inertia bases, fan bases, roof curbs, and rails for mounting large equipment and machinery to floors or roofs. All of our products are custom built according to customer specifications to ensure proper fit and performance in any given application. Before any fabrication takes place, customers are provided a detailed dimensional drawing for approval. Once approved, the drawings are released to our shop for fabrication.
Concrete inertia bases are used primarily when a solid base is required for maximum stability under the heaviest equipment. Curbs and rails are generally used to reduce machinery vibration and noise and can be isolated or non-isolated. Isolated mountings add springs and/or elastomeric elements to reduce vibration and noise, and may also be specified to withstand high winds in rooftop applications and seismic events. Non-isolated curbs and rails do not have vibration-isolating springs and are used solely as stable mountings for the equipment.
VMC Group concrete inertia bases are typically used under pumps, fans having over a 40” wheel diameter, centrifugal fans driven by 75 hp motors or larger and certain types of compressors. These types of equipment usually possess low rotational speed, high static pressure, or a large unbalanced force that cannot be attenuated by the use of springs alone. These bases are designed as modular or welded frames with concrete poured on-site.
Integral steel bases, curbs or rails should be specified for all machinery that does not require concrete inertia bases. This includes a variety of air handling, HVAC, and motorized machinery. For example, rooftop air handlers are often mounted to factory-supplied curbs, usually manufactured from a thin gauge sheet metal. The curb is usually nailed to the roofing material and has an integral flange on the bottom that is sometimes shimmed for leveling. While these curbs are fine to support the equipment’s weight, they are not capable of isolating vibration or noise, or sustaining seismic forces or wind loads, and are not attached to the building structure itself.
Many factors are considered when designing for seismic or wind restraint with building location, building type, and mechanical system type comprising the top three primary factors. To resist seismic and wind forces, equipment must be securely attached to the building structure. To ensure that the equipment remains intact and functioning, it must move with the building during an earthquake or wind event and not break away from attachments.
VMC Group roof curbs and rails are fabricated from structural steel specifically designed to withstand seismic and high-wind forces. These rooftop support systems provide seismic spring housings within a structural steel frame. Our high-strength frames are capable of direct, positive attachment to both the building structure and the equipment. Each spring housing provides vibration isolation and both seismic and wind restraint and is certified to Cap S, D & C seismic design category ‘F’. All mountings offer ample stability and dramatically increase equipment life.
DESIGN AND APPLICATIONS
Special considerations must be followed when a project requires installation of rooftop units. First, the job specification must be reviewed to determine isolation, seismic and wind requirements. While all projects governed by IBC Code will have wind load requirements, many projects fall within a seismic region depending on their geographic location. All VMC Group P Series Curbs and R Series Rails are certified for use in seismic and wind applications.
VMC Group’s sales and engineering staff will offer any assistance necessary to properly select the correct curb for the application. Information required for proper selection includes approximate unit weight and dimensions (actual equipment model number is preferred), a copy of the job specification, the attachment method to the building, and project wind load data. The attachment methods may include welding to building steel, welding to individual leveling stanchions, lagging to wood beams, or anchorage to a structural concrete slab. Per building codes, seismic curbs must be rigidly and positively fastened to the building structure for the curbs to be effective. VMC Group will build curbs specifically designed for your installation.
The attachment of rooftop units to curbs is also a requirement for seismic certification. Our seismic curbs and rails provide contractors with a simple means of fastening to almost any rooftop unit. The attachment method is easily inspected by the local building inspector or code official and prevents costly and timely disassembly.
It’s important to note that seismic certification of curbs fabricated from sheet metal is dubious at best. Our rooftop curbs offer the added benefit of point support capability. Unlike curbs with bent sheet metal sides, our curbs do not require continuous structural support – saving the contractor time and labor.
FOR THE DESIGNER
Spring deflections up to 3” can be specified to compensate for large floor spans. Spring housings are positioned within curbs at the correct load points for proper support and seismic and wind restraint. All spring housings incorporate a minimum 1/4” elastomeric cup or pad in series with the springs to provide optimal isolation efficiency.
These fully flashable rooftop support systems also save contractors unnecessary labor costs. The curb and rooftop unit can be fully installed and adjusted before the roof is sealed, eliminating the possibility of damage to a finished roof.
Leveling the rooftop unit is easy since all spring housings are accessible from around the outside of the curbs. Springs are color-coded for ease of identification and any spring can be removed and replaced after the rooftop unit is installed. Once adjusted and leveled, the rooftop unit becomes free-floating – completely isolated from the building structure while completely restrained to it.
P SERIES CURBS COME WITH THESE OPTIONS:
HEIGHT – Any of our curb designs can be made taller by welding extension posts under each spring pocket. Taller curbs may require additional cross bracing to properly support the mechanical equipment.
ROOF PITCH – Unlike manufacturers who try to match the roof pitch shown on building plans, our design is simple and error-proof. Our standard, level curbs are shipped to the job site and steel stub support posts of varying heights are welded to the building steel. The P Series curb is then placed on the stub posts and welded into place. This method ensures a level installation, a proper seismic connection and cost savings over expensive “factory pitched” curbs.
PLENUM DIVIDERS – For supply and return plenums, VMC Group incorporates a flexible membrane within the curb that offers an airtight seal while maintaining its vibration isolation characteristics.
ACOUSTICAL PACKAGE – To suppress unwanted noise from fans and moving parts, an acoustical package is available consisting of angled steel welded to the inside perimeter of the curb and cross support members to support two layers of sealed 5/8” sheetrock (not included).
MPF/CPF SERIES CURBS AND BASES
MODULAR AND CONCRETE POURING FORMS
MPF (modular pouring forms) and CPF (concrete pouring forms) are designed for use with mechanical equipment that requires a concrete inertia base. Our MPF/CPF Series products are sturdy frames that, when filled with concrete, provide an effective means to isolate vibration and limit the motion of any demanding equipment including large pumps, fans, motors, and compressors.
Our MPF Series inertia bases should be used with pumps, fans having a 40” wheel diameter and larger, centrifugal fans driven by motors of 75 Hp or larger and certain types of compressors. These types of equipment usually possess low rotational speeds, high static pressures, or a large unbalanced force that cannot be attenuated by the use of springs alone. The benefits of using an inertia block include the addition of mass to the isolated system which reduces movement. For example, if a 1000 lb. machine develops 100 lb. of an unbalanced force, the machine would displace “X” inches. However, if you mount the machine to an inertia base weighing 1000 lb., the same 100 lb. unbalanced force must now move 2000 lb. The displacement has been reduced by 50%. VMC Group inertia bases also lower the center of gravity of the isolated system – offering greater stability and a rigid base to help the equipment maintain alignment.
The MPF series features a modular, bolt-together design that can be shipped assembled or knocked-down for field assembly. CPF type products feature a sturdy, welded design complete with reinforcing rods (rebar) welded in place.
MPF FEATURES AND BENEFITS
Our MPF Series Modular Pouring Form inertia base is available in 6” and 10” thicknesses. The bolt-together design comes complete with isolator brackets, anchor bolt templates, and 3/8” rebar cut to size.
When selecting an MPF inertia base, we recommend the thickness be a minimum of 1/12 of the longest dimension of the base, but not less than 6”. Should a base greater than 10” thick be required, use our CPF Series Concrete Pouring Form.
CPF FEATURES AND BENEFITS
Our CPF Series Concrete Pouring Form inertia base can be designed to accommodate virtually any application. The CPF design comes complete with isolator brackets, anchor bolt templates, and 3/8” rebar welded in place, ready for concrete.
When selecting a CPF inertia base, we recommend the thickness be a minimum of 1/12 of the longest dimension of the base, but not less than 6”. When an equipment submittal drawing is provided to VMC Group’s engineering department, a properly designed base drawing, indicating isolator model numbers, is produced for customer approval. Once approved, drawings are provided to our manufacturing department for fabrication and shipment.
WFB/WFR SERIES CURBS AND BASES
CUSTOM-BUILT WIDE FLANGE BASES
VMC Group manufactures a complete line of fan and motor bases specified for all belt-driven centrifugal fans that do not require a concrete inertia block. Available with an adjustable motor slide rail, our WFB Series bases are designed to maintain proper drive alignment and resist starting torque without the use of restraining snubbers.
Whenever mechanical equipment lacks the proper rigidity for point support, our WFR Series rails should be utilized. Like our bases, WFR Series rails are available in wide flange, angle or channel steel and can be designed for any isolator in our product line.
When an equipment submittal drawing is provided to VMC Group’s engineering department, a properly designed base or rail drawing, indicating isolator model numbers, is produced for customer approval. Once approved, drawings are provided to our manufacturing department for fabrication and shipment.
FEATURES AND BENEFITS
? Our complete line of fan and motor bases can be sized to meet a wide range of equipment requirements.
? Wide flange steel beams and rails with spring or elastomeric isolation mounts prolong equipment life.
? Height-saving brackets limit elevation and provide additional equipment stability.
? Our simplified design permits quick and easy installation.
? Structural steel rails are available in wide flange, angle or channel steel designs to meet a wide range of equipment requirements.
APPLICATIONS
? Fans
? Pumps
? Compressors
? Air handling units
? HVAC equipment
? Package boilers and steam generators
? Chillers
P & R SERIES CURBS AND BASES
CUSTOM-BUILT SPRING CURBS AND BASES
P Series curbs and R Series rails are high quality, fully adjustable support systems specifically designed for rooftop unit vibration isolation and seismic/wind restraint. Each custom-built unit incorporates both roof curb and spring isolation into one complete structure. VMC Group engineers design each curb and rail to custom fit any rooftop mechanical equipment.
Spring deflections up to 3” can be specified to compensate for large floor spans, and spring housings are positioned within the curb at the correct load points for proper support and seismic restraint. All spring housings incorporate a minimum 1/4” elastomeric cup or pad in series with the spring to provide optimum isolation efficiency.
These fully flashable rooftop support systems also save contractors unnecessary labor costs. The curb and rooftop unit can be fully installed and adjusted before the roof is sealed, eliminating the possibility of damage to a finished roof.
Leveling the rooftop unit is easy since all spring housings are accessible from around the outside of the curbs. Springs are color-coded for ease of identification and any spring can be removed and replaced after the rooftop unit is installed. Once adjusted and leveled, the rooftop unit becomes free-floating – completely isolated from the building structure while completely restrained to it.
P Series roof curbs and R Series rails are fabricated from force-resisting structural steel specifically designed to withstand seismic and high-wind forces. Unlike standard sheet metal curbs that typically come with a rooftop air handling unit, our structural curbs incorporate a seismic restraint, and our rooftop support systems combine our seismic spring design within a movement-resisting frame. Our high-strength frame is capable of direct, positive attachment to both the building structure and the roof-mounted equipment while each spring pocket provides vibration isolation and both seismic and wind restraint.
Standard sheet metal curbs that typically come with rooftop equipment are not capable of resisting these forces and do not allow for direct, positive attachment to the roof structure and mechanical equipment. Our high-strength frame can be bolted or welded to the roof structural steel and will not break away during an earthquake or high-wind forces. Sheet metal curbs cannot accept point-loading of these forces and will tear away from the building structure due to thin construction.
FEATURES AND BENEFITS
? Roof systems and spring isolators are combined in one complete structure and are custom built to fit any rooftop equipment.
? Spring housings offer up to 3” deflection and are seismically rated to ensure equipment captivity.
? Fully adjustable spring housings are accessible from around the outside of the curbs and rails, providing easy leveling of the rooftop units.
? Factory installed top flashing ensures simple, weathertight installation.
? Structural steel frames resist seismic and wind forces and allow for direct attachment to roof structural steel and to the equipment.
? For R Series Rails, CCA grade plywood covering permits contractors to completely roof the rail systems for a weather-tight finish.
? Our Test & Measurement division provides special seismic certification for the International Building Code (IBC) compliance for seismic and wind events.
? P Series Curbs, R Series Rails, and TeleCurb? are now approved per Miami-Dade County NOA (Notice of Acceptance) for Wind Loads – having been structurally designed, tested, and approved to comply with the High-Velocity Hurricane Zone of the Florida Building Code. We also offer wind tunnel testing specifically required in Miami-Dade County.
APPLICATIONS
? Rooftop equipment
? HVAC equipment
? Fans
? Compressors
? Chillers
? Air-cooled condensers
? Air handling units
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