Seismic Restraints for HVAC Equipment: Detailed Guide

Seismic Restraints for HVAC Equipment

Proper seismic restraint design protects HVAC equipment from earthquake-induced forces and prevents catastrophic failures. This guide covers force calculations, anchorage design, and installation requirements per IBC, ASCE 7, and SMACNA standards.

Seismic Force Calculations

Horizontal Seismic Force (Fp)

The horizontal seismic design force for nonstructural components is calculated using ASCE 7 equation 13.3-1:

$$F_p = \frac{0.4 a_p S_{DS} W_p}{(R_p/I_p)} \left(1 + 2\frac{z}{h}\right)$$

Subject to:

$$F_{p,\text{max}} = 1.6 S_{DS} I_p W_p$$

$$F_{p,\text{min}} = 0.3 S_{DS} I_p W_p$$

Where:

• $F_p$ = seismic design force applied to component (lbs or kN)
• $a_p$ = component amplification factor (typically 2.5 for mechanical equipment)
• $S_{DS}$ = design spectral response acceleration at short periods
• $W_p$ = component operating weight including contents
• $R_p$ = component response modification factor (varies by equipment type)
• $I_p$ = component importance factor (1.0 or 1.5)
• $z$ = height of attachment point above grade
• $h$ = average roof height of structure

Equipment Response Modification Factors

Anchorage Design Requirements

Minimum Anchorage Strength

Anchors must resist both horizontal and vertical seismic forces:

Horizontal Force: $$F_h = F_p$$

Vertical Force (concurrent): $$F_v = 0.2 S_{DS} W_p$$

Anchorage Safety Factors

Anchor design strength must exceed applied loads with appropriate safety factors:

$$\phi R_n \geq \text{Required Strength}$$

Where:

• $\phi$ = strength reduction factor (0.65 for cast-in anchors, 0.55 for post-installed)
• $R_n$ = nominal anchor strength in tension or shear

Equipment Anchorage Configurations

Curb-Mounted Rooftop Units

graph TB
    subgraph "RTU Seismic Anchorage"
        A[Rooftop Unit] --> B[Roof Curb]
        B --> C[Anchor Bolts Through Curb]
        C --> D[Structural Roof Deck]
        E[Seismic Restraints] --> A
        E --> F[Lateral Bracing]
        F --> D
    end

    style A fill:#e1f5ff
    style B fill:#fff4e1
    style D fill:#f0f0f0

Design Criteria:

• Minimum 4 anchor bolts per curb
• Bolt diameter ≥ 1/2 inch for units < 1000 lbs
• Bolt diameter ≥ 5/8 inch for units > 1000 lbs
• Embed depth per manufacturer and structural requirements
• Lateral restraints at 4 corners if Fp exceeds curb weight

Floor-Mounted Equipment on Isolators

graph TB
    subgraph "Equipment with Seismic Restraints"
        A[Equipment Base] --> B[Vibration Isolators]
        B --> C[Inertia Base/Housekeeping Pad]
        D[Seismic Snubbers] -.->|Gap δ| A
        D --> C
        C --> E[Anchor Bolts]
        E --> F[Structural Floor]
    end

    subgraph "Snubber Gap Calculation"
        G[Normal Operation: Gap Active]
        H[Seismic Event: Snubbers Engage]
    end

    style A fill:#e1f5ff
    style C fill:#fff4e1
    style D fill:#ffe1e1
    style F fill:#f0f0f0

Seismic Snubber Requirements:

• Maximum clearance gap: 1/4 inch
• Snubber capacity must resist full Fp
• All-directional restraint (both horizontal axes)
• Vertical restraint if required by ASCE 7
• Minimum 4 snubbers per equipment base

Suspended Equipment Bracing

For ceiling-hung equipment (fan coil units, air handlers), restraint consists of:

graph LR
    subgraph "Suspended Unit Restraint System"
        A[Suspended Equipment] --> B[Support Rods]
        A --> C[Lateral Bracing Cables/Rods]
        B --> D[Structure Above]
        C --> D
        E[4-Point Lateral Bracing] --> A
    end

    style A fill:#e1f5ff
    style C fill:#ffe1e1
    style D fill:#f0f0f0

Bracing Configuration:

• Minimum 4-point lateral restraint
• Brace angle 45° from horizontal (30° to 60° acceptable)
• Brace capacity ≥ Fp/number of braces in direction
• No reliance on ceiling grid for seismic resistance
• Direct attachment to structural elements

Code Compliance Requirements

IBC Seismic Design Categories

ASHRAE Guideline 13 Provisions

Equipment Weight Thresholds:

• Equipment < 400 lbs: Simplified anchorage acceptable (SDC A-C)
• Equipment ≥ 400 lbs: Full seismic analysis required
• Equipment > 10,000 lbs: Enhanced design and special inspection

Installation Height Considerations:

• Ground-mounted: Standard analysis
• Roof-mounted: Amplification factor z/h applies
• Equipment > 30 ft above grade: Increased force coefficients

SMACNA Seismic Restraint Manual

Key provisions from SMACNA guidelines:

Duct Bracing:

• Ducts ≥ 6 sq ft cross-section require lateral bracing
• Maximum brace spacing: 30 ft for longitudinal, 12 ft for transverse
• Brace capacity: 100 lbs minimum, or calculated seismic force

Pipe Bracing:

• Pipes ≥ 2.5 inch diameter require seismic bracing (SDC C-F)
• Maximum brace spacing: 40 ft for longitudinal, 12 ft for transverse
• Four-way bracing at changes in direction

Equipment Installation:

• Concrete housekeeping pads minimum 4 inches thick
• Grout or shim between equipment and mounting surface
• Anchor bolts installed per ACI 318 provisions
• Field verification of substrate strength

Vibration Isolation with Seismic Restraint

Restoring Force Check

For equipment on vibration isolators, verify:

$$F_p \geq 1.6 \times k \times \delta_{static}$$

Where:

• $k$ = isolator spring stiffness (lbs/inch)
• $\delta_{static}$ = static deflection under equipment weight (inches)

If this criterion is not satisfied, use Rp = 1.0 instead of 2.5.

Snubber Gap Calculation

Maximum allowable gap between equipment and snubber:

$$\delta_{gap} = \min\left(\frac{1}{4}\text{ inch}, \frac{\delta_{static}}{2}\right)$$

This prevents equipment from gaining excessive momentum before engaging seismic restraints.

Installation and Inspection

Critical Installation Steps

1. Substrate verification: Confirm concrete strength (minimum 2500 psi) and thickness
2. Anchor installation: Follow manufacturer torque specifications
3. Gap verification: Measure and document snubber clearances
4. Alignment: Ensure equipment level and properly supported
5. Documentation: Record anchor types, embedment depths, torque values

Special Inspection Requirements

SDC D, E, F require special inspection for:

• Equipment with Ip = 1.5 (essential facilities)
• Supports for equipment weighing > 400 lbs
• Post-installed anchors in concrete
• Field welding of seismic braces

Common Installation Errors

• Insufficient embedment depth for anchors
• Oversized holes reducing anchor capacity
• Missing or improperly adjusted seismic snubbers
• Inadequate edge distance for concrete anchors
• Reliance on non-structural elements (architectural features, ceiling grid)
• Incorrect torque application to anchor bolts

Design Calculation Example

Given:

• Rooftop air handler: Wp = 2,500 lbs
• Location: SDC D, SDS = 1.0g
• Height: z = 40 ft, h = 45 ft
• Importance factor: Ip = 1.0

Calculation:

$$F_p = \frac{0.4 \times 2.5 \times 1.0 \times 2500}{2.5/1.0} \left(1 + 2\frac{40}{45}\right) = 2,778 \text{ lbs}$$

Check maximum: $$F_{p,max} = 1.6 \times 1.0 \times 1.0 \times 2500 = 4,000 \text{ lbs}$$ ✓

Check minimum: $$F_{p,min} = 0.3 \times 1.0 \times 1.0 \times 2500 = 750 \text{ lbs}$$ ✓

Design force: Fp = 2,778 lbs

Required anchor shear strength (4 bolts): $$V_{required} = \frac{2778}{4} = 695 \text{ lbs per bolt}$$

Select 5/8" diameter expansion anchor with φVn ≥ 695 lbs.

Conclusion

Seismic restraint design requires careful attention to code-mandated force calculations, proper anchorage selection, and installation quality. Following ASCE 7 methodology, IBC requirements, and SMACNA guidelines ensures equipment remains functional and safe during seismic events. Proper documentation and special inspection provide verification that installed systems meet design intent and regulatory requirements.

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