IBC ASCE 7 Requirements

Seismic Design Categories (SDC)

SDC Classification

Buildings are assigned a Seismic Design Category (A through F) based on:

• Occupancy category (I, II, III, or IV)
• Mapped spectral response accelerations (Ss and S1)
• Site class (A through F)

SDC Levels:

Determination Process

1. Establish occupancy category from IBC Table 1604.5
2. Obtain mapped spectral accelerations Ss and S1 from ASCE 7 maps
3. Determine site class from geotechnical investigation
4. Calculate design spectral response accelerations SDS and SD1
5. Assign SDC from ASCE 7 Tables 11.6-1 and 11.6-2

Component Importance Factor (Ip)

Ip Values

The component importance factor reflects the hazard to human life and need for continued operation:

Ip = 1.5:

• Life safety components (fire suppression systems, emergency ventilation)
• Systems required for post-earthquake operation (hospital HVAC, emergency power cooling)
• Components containing hazardous materials
• High occupancy assembly spaces
• Essential facilities

Ip = 1.0:

• Standard commercial and residential HVAC equipment
• Non-essential components
• Standard occupancy buildings

Application

The importance factor directly multiplies the seismic force:

• Higher Ip = larger design force
• More robust anchorage required
• Enhanced quality assurance during installation

Seismic Design Force (Fp) Calculation

Primary Equation

The horizontal seismic design force for nonstructural components:

Fp = (0.4 × ap × SDS × Wp / Rp) × (1 + 2z/h) × Ip

Where:

• Fp = component seismic design force
• ap = component amplification factor
• SDS = design spectral response acceleration (short period)
• Wp = component operating weight
• Rp = component response modification factor
• z = height of attachment point above grade
• h = building height (roof above grade)
• Ip = component importance factor

Upper and Lower Limits

Maximum Fp: Fp ≤ 1.6 × SDS × Ip × Wp

Minimum Fp: Fp ≥ 0.3 × SDS × Ip × Wp

These limits prevent unreasonably high or low design forces regardless of calculated values.

Component Amplification Factor (ap)

Typical values for HVAC equipment per ASCE 7 Table 13.5-1:

Key Consideration: Equipment on flexible supports (vibration isolators) experiences amplified motion, requiring larger seismic forces.

Component Response Modification Factor (Rp)

Typical values for HVAC equipment per ASCE 7 Table 13.6-1:

Key Consideration: Lower Rp = larger design force. Rp accounts for ductility and energy dissipation capacity.

Height Factor (1 + 2z/h)

Calculation

• z = elevation of component attachment above grade
• h = roof elevation above grade
• Ratio z/h ranges from 0 (ground level) to 1.0 (roof level)

Examples:

Result: Rooftop equipment experiences 3× the base seismic force due to building amplification effects.

Anchorage Design Requirements

Strength Design

Anchorage must resist:

Horizontal Force: Fp (calculated above)

Vertical Force: Typically 0.2 × SDS × Wp, applied concurrently

Allowable Stress Design Conversion

If using allowable stress design (ASD) for anchors:

Fp(ASD) = Fp / 1.4

This converts load combinations from LRFD to ASD basis.

Anchor Bolt Design

Tensile Capacity:

• Calculated per ACI 318 Chapter 17 (concrete) or AISC 360 (steel)
• Must account for concrete breakout, pullout, and steel yielding
• Apply appropriate reduction factors (φ)

Shear Capacity:

• Concrete pryout
• Steel strength
• Concrete edge distance effects

Combined Loading:

• Interaction equation: (T/φTn)^2 + (V/φVn)^2 ≤ 1.0
• Where T = applied tension, V = applied shear

Concrete Anchor Requirements

Minimum Edge Distance:

• Typically 4× anchor diameter or per manufacturer
• Increased for anchors near slab edges

Embedment Depth:

• Minimum per ACI 318: typically 4× anchor diameter
• Cast-in-place anchors preferred over post-installed

Reinforcement:

• Supplementary reinforcement may be required for shallow embedments
• Anchor reinforcement detailed per ACI 318 Appendix D

Base Plate Design

Minimum Thickness: Calculate to prevent bending between anchor bolts:

t ≥ √(3 × M × L² / (Fy × W))

Where:

• M = moment between anchors
• L = span between anchors
• Fy = steel yield strength
• W = effective width

Weld Requirements:

• Full penetration welds for equipment legs to base plates
• Fillet welds sized for calculated forces
• AWS D1.1 requirements apply

Drift Accommodation

Relative Displacement

Dp = (0.5 × SDS / Rp) × (z / h) × h

This represents the differential displacement components must accommodate.

Seismic Separation

Clearance required for:

• Piping penetrations through walls and floors
• Ductwork through seismic joints
• Equipment mounted across building joints

Minimum Clearance: 2 × Dp (both sides of joint)

Flexible Connections

Required for:

• Utility lines crossing seismic joints
• Connections to equipment on vibration isolators
• Piping attached to equipment subject to differential movement

Vertical Seismic Force

Calculation

Fv = 0.2 × SDS × Wp

Applied concurrently with horizontal force for:

• Equipment on vibration isolators
• Suspended components
• Cantilever supports

Load Combinations

LRFD: 1.2D ± Eh ± Ev

• D = dead load
• Eh = horizontal seismic effect (Fp)
• Ev = vertical seismic effect (Fv)

The ± indicates both upward and downward vertical forces must be considered.

Component Period and Dynamic Analysis

Qualifying Dynamic Equipment

Equipment requires dynamic analysis when:

Simplified Period Calculation

For rigid equipment:

T = 0.32 × √(W/k)

Where:

• T = period (seconds)
• W = weight (lb)
• k = stiffness (lb/in)

Testing Alternative

In lieu of calculation, equipment can be shake table tested per ICC-ES AC156 protocol.

Certification and Documentation

Design Professional Requirements

SDC D, E, F: Seismic restraint design must be performed by licensed engineer.

SDC A, B, C: Engineering may be required by authority having jurisdiction.

Submittal Requirements

Design Calculations:

• Seismic force calculations (Fp)
• Anchorage design
• Base plate design
• Material specifications

Installation Drawings:

• Anchor layout
• Connection details
• Required torque values
• Special inspection requirements

Special Inspection

Required for SDC C and higher:

• Anchor bolt installation verification
• Torque verification
• Visual inspection of welds
• Vibration isolator restraint installation

Documentation:

• Installation inspection reports
• Material certifications
• Anchor test reports (if post-installed)
• Final approval from engineer of record

Acceptance Criteria

Load Path Verification

Complete load path must exist from:

1. Equipment center of gravity
2. Through supports and base plates
3. Through anchors
4. Into structural elements

Installation Tolerances

Anchor Location: ±0.5 inches from specified Bolt Torque: Per manufacturer (typically 75% proof load) Base Plate Contact: Minimum 80% full bearing Weld Quality: Per AWS D1.1 acceptance criteria

Exemptions

Equipment Not Requiring Restraints

SDC A: No restraint requirements

SDC B and C:

• Equipment weighing ≤ 400 lb
• Attachments to light-frame construction

All SDC:

• HVAC equipment rigidly attached to structure that moves uniformly with equipment (e.g., small roof units with integral curbs adequately anchored)