Shake Table Testing for HVAC Equipment Certification

Overview

Shake table testing provides the most rigorous and realistic method for seismically qualifying HVAC equipment. This dynamic testing method subjects equipment to simulated earthquake motions on a programmable platform, demonstrating performance during and after seismic events.

Test Standards and Protocols

AC156 Requirements

ICC-ES Acceptance Criteria 156 establishes shake table testing protocols for nonstructural components. Key requirements include:

Biaxial or Triaxial Testing: Equipment must be tested with simultaneous motion in two or three orthogonal directions
Test Specimen Mounting: Representative of actual installation conditions, including support structure flexibility
Input Motion: Required Response Spectrum (RRS) enveloping the target spectrum across all frequencies
Amplification Factors: Component amplification factors (ap) from 1.0 to 2.5 applied based on mounting height

IEEE 693 Standard

IEEE 693 applies primarily to electrical substation equipment but provides relevant guidance for HVAC electrical components:

Performance levels: High, Moderate, and Low qualification
Time history generation matching target response spectra
Fragility testing to determine ultimate capacity
Post-test functional verification requirements

Required Response Spectrum

The target response spectrum for shake table testing must envelope the design requirements. The 5% damped spectral acceleration is calculated as:

$$S_a(T) = \frac{S_{DS}}{T} \cdot \frac{a_p}{R_p} \cdot \left(1 + 2\frac{z}{h}\right)$$

Where:

$S_a(T)$ = spectral acceleration at period T (g)
$S_{DS}$ = design spectral response acceleration at short periods
$T$ = natural period of component (seconds)
$a_p$ = component amplification factor (1.0 to 2.5)
$R_p$ = component response modification factor (typically 2.5 for mechanical equipment)
$z$ = height of component attachment point above grade
$h$ = average roof height of structure

The test response spectrum (TRS) must meet:

$$TRS(f) \geq 0.9 \cdot RRS(f) \text{ for all frequencies}$$

$$\frac{1}{n}\sum_{i=1}^{n}\frac{TRS(f_i)}{RRS(f_i)} \geq 1.0$$

Where the average ratio across all frequency points must equal or exceed unity.

Shake Table Test Setup

graph TB
    subgraph "Shake Table Configuration"
        A[Hydraulic Actuators X-Direction] --> B[Shake Table Platform]
        C[Hydraulic Actuators Y-Direction] --> B
        D[Vertical Actuators Z-Direction] --> B
        B --> E[Support Frame Structure]
        E --> F[Test Specimen Mounted]
        F --> G[Accelerometers on Equipment]
        F --> H[Displacement Transducers]
        F --> I[Strain Gauges Critical Points]
    end

    subgraph "Data Acquisition System"
        G --> J[Signal Conditioning]
        H --> J
        I --> J
        J --> K[High-Speed Data Logger]
        K --> L[Real-Time Analysis]
        K --> M[Response Spectrum Generation]
    end

    subgraph "Control System"
        N[Target Response Spectrum] --> O[Motion Controller]
        O --> P[Servo-Hydraulic System]
        P --> A
        P --> C
        P --> D
        L --> O
    end

    style F fill:#e1f5ff
    style K fill:#ffe1e1
    style O fill:#e1ffe1

Test Procedures

Pre-Test Requirements

Functional Testing: Verify equipment operates correctly before testing
Instrumentation: Install accelerometers at critical locations (base, center of mass, extremities)
Documentation: Photograph installation, record equipment condition
Resonant Frequency Search: Low-level sine sweep to identify natural frequencies

Test Sequence

Step 1: Exploratory Tests

Low-amplitude motion (0.25 × RRS) to verify:

Data acquisition system functionality
Absence of installation problems
Initial resonant frequency confirmation

Step 2: Simulated Design Basis Earthquake (DBE)

Full-amplitude test matching required response spectrum:

Duration: Minimum 20 seconds of strong motion
Three complete runs in principal directions
Monitor for functional issues during shaking

Step 3: Maximum Considered Earthquake (MCE)

Enhanced testing at 1.5 × DBE level (optional):

Demonstrates margin of safety
Identifies ultimate capacity
Documents failure modes if equipment fails

Step 4: Post-Test Verification

Visual inspection for damage (cracks, loose components, deformation)
Functional testing to verify operational capability
Resonant frequency comparison to detect structural changes

Acceptance Criteria

Equipment passes shake table testing if:

Structural Integrity: No structural failure, fracture, or detachment of major components
Functional Performance: Equipment operates within specifications after testing
Anchorage Performance: Mounting system remains intact and secure
Deformation Limits: Permanent deformation does not impair function or create safety hazard
Frequency Shift: Natural frequency change less than 10% indicates no significant damage

Minor cosmetic damage (paint cracking, small dents) is acceptable provided function is maintained.

Instrumentation Requirements

Accelerometer Placement

graph LR
    subgraph "Equipment Under Test"
        A[Base Mount Point] --> B[Accelerometer 1]
        C[Center of Mass] --> D[Accelerometer 2]
        E[Top of Unit] --> F[Accelerometer 3]
        G[Fan Housing] --> H[Accelerometer 4]
        I[Compressor Mount] --> J[Accelerometer 5]
    end

    subgraph "Measurements Required"
        B --> K[Base Input Motion]
        D --> L[Primary Response]
        F --> M[Amplification Factor]
        H --> N[Component Response]
        J --> N
    end

    style C fill:#ffebcc
    style L fill:#ccffcc

Minimum three triaxial accelerometers required:

One at mounting interface (input motion)
One at equipment center of mass (primary response)
One at highest point (amplification verification)

Documentation Requirements

Test Report Contents

Equipment Description: Model, capacity, dimensions, weight, mounting configuration
Test Setup: Photographs, drawings, support structure details
Input Motions: Time histories, response spectra, peak accelerations
Response Data: Accelerations, displacements, strains at all measurement points
Acceptance Verification: Comparison to criteria, functional test results
Certification Statement: Declaration of compliance with applicable standards

Quality Assurance

Independent third-party testing laboratory accredited per ISO/IEC 17025
Calibration certificates for all instrumentation within 1 year
Witnessing by engineer licensed in jurisdiction of equipment installation
Video documentation of all test runs

Cost and Schedule Considerations

Shake table testing represents significant investment:

Testing Costs: $25,000 to $150,000 depending on equipment size and complexity
Schedule: 2 to 6 weeks from equipment delivery to final report
Specimen Requirements: At least one production unit, sometimes two for repeatability

For equipment produced in high volume, shake table certification amortizes cost across many installations in seismic regions.

Alternatives to Full Shake Table Testing

When shake table testing is impractical:

Experience-Based Certification: Demonstrated seismic performance in past earthquakes
Analysis with Limited Testing: Computer simulation validated by component tests
Similarity Certification: Testing of representative unit extended to similar models

These alternatives require substantial engineering justification and may not be accepted in high-seismicity jurisdictions.

Conclusion

Shake table testing provides definitive proof of seismic capability but requires substantial investment in specialized facilities, instrumentation, and expertise. Compliance with AC156 and IEEE 693 protocols ensures equipment can withstand design-level earthquakes while maintaining critical function. Proper documentation and third-party certification enable broad acceptance of test results across jurisdictions, making this approach cost-effective for equipment deployed in seismic zones.