Seismic Snubbers for HVAC Equipment

Snubber Function and Principles

Seismic snubbers serve as critical restraint devices that permit normal operational movement while limiting displacement during seismic events. These components bridge the competing requirements of vibration isolation for operational performance and rigid restraint for seismic safety.

Primary Functions

• Allow controlled movement during normal operation (typically 0.25-1.0 inch clearance)
• Engage rapidly to restrict displacement during seismic excitation
• Transmit seismic forces to building structure without damaging equipment
• Coordinate with vibration isolation systems to maintain operational performance

The fundamental challenge in snubber design lies in providing sufficient clearance for vibration isolation while ensuring positive engagement before equipment displacement exceeds acceptable limits.

Snubber Types and Characteristics

All-Directional Snubbers

All-directional snubbers provide omnidirectional restraint, making them suitable for equipment requiring protection in all horizontal axes. These devices typically employ a housing-and-restraint element configuration that activates regardless of displacement direction.

Applications:

• Rooftop units on spring isolators
• Chillers with significant thermal expansion
• Equipment with unpredictable vibration patterns
• Installations where directional seismic forces cannot be predetermined

Design Considerations:

• Clearance gap must accommodate maximum operational displacement in any direction
• Load capacity must account for vectorial force combinations
• Housing alignment critical to prevent binding during normal operation

Directional Snubbers

Directional snubbers restrain movement along specific axes, typically installed in orthogonal pairs to provide complete horizontal restraint. These devices allow greater displacement in unrestrained directions while providing positive stops in designated axes.

Applications:

• Equipment with thermal growth in known directions
• Piping systems requiring longitudinal movement freedom
• Installations with space constraints limiting all-directional units
• Systems where directional isolation characteristics must be preserved

Configuration Requirements:

• Minimum of two perpendicular units for horizontal restraint
• Alignment precision essential for proper engagement
• Individual axis tuning possible for specific isolation requirements

Snubber Capacity Calculations

The required snubber capacity depends on the equipment mass, seismic coefficients, and distribution of restraint devices.

Horizontal Capacity

For each snubber in a multi-restraint installation:

$$F_s = \frac{W \cdot S_{DS} \cdot I_p}{n \cdot R_p} \cdot D_f$$

Where:

• $F_s$ = required snubber capacity (lbf)
• $W$ = equipment operating weight (lbf)
• $S_{DS}$ = design spectral response acceleration (dimensionless)
• $I_p$ = component importance factor (typically 1.0 or 1.5)
• $n$ = number of snubbers resisting force in the direction analyzed
• $R_p$ = component response modification factor (typically 2.5 for nonstructural components)
• $D_f$ = distribution factor accounting for non-uniform loading (typically 1.25-1.5)

Vertical Capacity

For equipment on vibration isolators, vertical snubbers must resist uplift forces:

$$F_v = \frac{0.2 \cdot S_{DS} \cdot W \cdot I_p}{n_v}$$

Where $n_v$ is the number of vertical restraints.

Clearance Gap Determination

The snubber clearance gap must accommodate normal operational displacement while ensuring seismic engagement before equipment damage occurs.

Clearance Calculation

$$C_{gap} = \Delta_{op} + \Delta_{thermal} + \Delta_{tolerance}$$

Where:

• $C_{gap}$ = total clearance gap (inches)
• $\Delta_{op}$ = maximum operational vibration displacement (inches)
• $\Delta_{thermal}$ = thermal expansion/contraction displacement (inches)
• $\Delta_{tolerance}$ = installation and manufacturing tolerances (typically 0.0625-0.125 inches)

Typical Clearance Values:

Maximum clearance should not exceed the lesser of 0.25 times the isolator static deflection or manufacturer’s maximum recommendation.

Coordination with Vibration Isolation

Effective seismic protection requires careful integration of snubbers with vibration isolation systems. The combination must satisfy both operational (vibration control) and safety (seismic restraint) objectives.

Design Integration Protocol

1. Establish Vibration Isolation Requirements

• Determine required static deflection for isolation efficiency
• Calculate maximum dynamic displacement under operational conditions
• Identify directional characteristics of vibration sources

2. Select Compatible Snubber System

• Choose clearance gap based on isolation displacement
• Verify snubber stiffness does not compromise isolation at operating frequencies
• Ensure snubber engagement does not induce shock loads damaging equipment

3. Verify Seismic Performance

• Confirm clearance allows seismic engagement before equipment damage
• Calculate combined stiffness of isolator-snubber system during seismic event
• Verify building structure can accommodate transmitted forces

Restrained Spring Isolators

Modern restrained spring isolators incorporate integral snubbers within the isolator housing. These units provide factory-set clearances and guaranteed compatibility between isolation and restraint functions.

Advantages:

• Pre-engineered clearance settings
• Reduced installation complexity
• Manufacturer-certified seismic performance
• Single-source responsibility for integrated performance

Limitations:

• Fixed clearance adjustments
• Limited directional customization
• Higher initial cost than separate components

Installation and Testing Requirements

Installation Procedures

Alignment Verification:

• Confirm snubber centerline alignment with restraint axis (tolerance ±0.125 inches)
• Verify clearance gap uniformity around entire perimeter (all-directional units)
• Ensure no binding or interference during full operational displacement range

Attachment Integrity:

• Verify anchor bolt embedment meets structural design requirements
• Torque fasteners to manufacturer specifications
• Confirm positive engagement of restraint elements (no loose fits)

Field Testing

Operational Clearance Test:

• Manually displace equipment through expected operational range
• Verify snubber does not engage during normal displacement
• Confirm no contact sounds or resistance at operational limits

Engagement Test:

• Gently force equipment to snubber engagement point
• Verify positive contact and resistance
• Confirm symmetrical engagement (all-directional units)
• Document actual clearance gaps for record

Manufacturer Specifications and Standards

Selection and installation must conform to manufacturer requirements and industry standards.

Key Specification Parameters:

• Maximum load capacity (horizontal and vertical)
• Clearance gap range and adjustment method
• Mounting configuration and attachment details
• Operating temperature range
• Material specifications and corrosion resistance

Reference Standards:

• ASCE 7: Seismic design coefficients and component factors
• ASHRAE Applications Handbook: Seismic restraint guidelines
• ICC-ES AC156: Seismic certification for nonstructural components
• Manufacturer’s ICC-ES evaluation reports for load-rated performance

Testing Certification:

Specify snubbers with independent testing verification demonstrating capacity under cyclic loading representative of seismic conditions. Certified products should reference ICC-ES evaluation reports or equivalent third-party validation.

Maintenance and Inspection

Periodic inspection ensures continued seismic protection capability.

Annual Inspection Items:

• Visual examination for corrosion, damage, or deformation
• Clearance gap verification (may increase due to isolator settling)
• Attachment hardware tightness
• No interference from adjacent equipment or modifications

Post-Seismic Event Inspection:

Following any seismic event exceeding 0.1g peak ground acceleration, inspect all snubbers for engagement evidence, damage, or permanent deformation requiring replacement.