Transverse & Longitudinal Ductwork Supports

Fundamental Bracing Orientations

Seismic bracing for ductwork requires restraint in two perpendicular horizontal directions to resist lateral forces during seismic events. Transverse braces resist forces perpendicular to the duct longitudinal axis, while longitudinal braces resist forces parallel to the duct run. SMACNA and ASCE 7 require both orientations for complete seismic protection.

Transverse Bracing Requirements

Transverse braces prevent lateral movement perpendicular to the duct axis. These braces typically attach to the duct at 90° to the longitudinal centerline and transfer seismic loads to the building structure.

Key design parameters:

Maximum spacing: 40 feet for ducts up to 8 square feet cross-section
Maximum spacing: 30 feet for ducts 8-20 square feet
Maximum spacing: 25 feet for ducts over 20 square feet
Attachment within 6 feet of duct direction changes
Braces at 45° angle (maximum 60°) to horizontal

Longitudinal Bracing Requirements

Longitudinal braces resist axial movement along the duct run. These restraints prevent telescoping or accordion-like compression during seismic events.

Spacing criteria:

Required at every fourth transverse brace location
Maximum 80 feet spacing for ducts up to 8 square feet
Maximum 60 feet spacing for ducts over 8 square feet
Mandatory at changes in duct elevation
Required near heavy equipment connections

Seismic Force Calculations

The lateral seismic force for ductwork bracing design follows ASCE 7 provisions:

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

Where:

$F_p$ = seismic design force (lb)
$a_p$ = component amplification factor (2.5 for ductwork)
$S_{DS}$ = design spectral response acceleration
$W_p$ = component operating weight (lb)
$R_p$ = component response modification factor (6.0 for ductwork)
$I_p$ = component importance factor (1.0 to 1.5)
$z$ = height of attachment point above grade (ft)
$h$ = average roof height of structure (ft)

Force limits:

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

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

The distributed load per linear foot of duct:

$$w_d = \frac{F_p}{L}$$

Where $L$ = spacing between braces (ft).

Brace Load Distribution

Each transverse brace pair resists the tributary seismic load:

$$F_{brace} = \frac{w_d \cdot L}{2 \sin(\theta)}$$

Where $\theta$ = brace angle from horizontal (typically 45°).

For longitudinal braces carrying cumulative forces:

$$F_{long} = F_p \cdot n$$

Where $n$ = number of transverse brace intervals between longitudinal braces.

Bracing Configuration Diagrams

graph TD
    A[Duct Seismic Bracing System] --> B[Transverse Bracing]
    A --> C[Longitudinal Bracing]
    A --> D[4-Way Bracing]

    B --> B1[Perpendicular to Duct]
    B --> B2[Maximum 40' Spacing]
    B --> B3[45° Angle Preferred]

    C --> C1[Parallel to Duct]
    C --> C2[Every 4th Transverse]
    C --> C3[Maximum 80' Spacing]

    D --> D1[Complete Lateral Restraint]
    D --> D2[At Critical Locations]
    D --> D3[Equipment Connections]

    style A fill:#2c5f8d,stroke:#1a3a52,color:#fff
    style B fill:#5a9bd4,stroke:#2c5f8d,color:#fff
    style C fill:#5a9bd4,stroke:#2c5f8d,color:#fff
    style D fill:#5a9bd4,stroke:#2c5f8d,color:#fff

graph LR
    subgraph "Transverse Bracing Plan View"
    A[Structure] -->|45°| B[Brace]
    B --> C[Duct]
    D[Structure] -->|45°| E[Brace]
    E --> C
    end

    subgraph "Longitudinal Bracing"
    F[Duct Direction] --> G[Longitudinal Brace]
    G --> H[Structure]
    I[Structure] --> J[Longitudinal Brace]
    J --> F
    end

    style C fill:#ff9900,stroke:#cc7700,stroke-width:3px
    style F fill:#ff9900,stroke:#cc7700,stroke-width:3px

Combination Bracing Systems

Four-way bracing combines transverse and longitudinal restraint at single attachment points for optimal seismic performance.

4-Way Brace Locations

Install combination bracing at:

Duct rises and drops
Heavy equipment connections (fans, coils)
Branch takeoffs from main trunks
Locations where duct size changes
Within 6 feet of seismic joints
Both sides of building expansion joints

Design Advantages

Combination bracing provides:

Complete lateral restraint in all horizontal directions
Reduced installation complexity
Fewer ceiling penetrations
Better load distribution
Compliance with both transverse and longitudinal requirements

Attachment and Connection Details

Duct Attachment Methods

Sheet metal ducts:

Welded or bolted steel angles
Minimum 12-gauge steel straps
Full perimeter of rectangular duct
180° wrap for round ducts over 28 inches

Structural attachments:

Welded to steel structure
Through-bolted to concrete (embedments or post-installed anchors)
Minimum 3/8-inch diameter threaded rod
Locking nuts or tack welds required

Load Path Verification

The complete load path must transfer seismic forces:

Duct wall → duct attachment
Duct attachment → brace rod/cable
Brace rod → structural connection
Structural connection → building frame
Building frame → foundation

Each element must satisfy:

$$\text{Capacity} \geq 1.4 F_{brace}$$

The 1.4 factor accounts for material overstrength requirements per SMACNA.

SMACNA Standard Compliance

SMACNA Guidelines for Seismic Restraint of Mechanical Systems establish minimum requirements:

Brace sizing:

Minimum 1-1/4 inch diameter rod for braces under 30 feet
Minimum 1-1/2 inch diameter rod for braces 30-40 feet
Steel cables minimum 1/4 inch diameter (7×19 construction)
Cable turnbuckles required for tensioning

Quality requirements:

All-thread rod meeting ASTM A36 or better
Grade 5 or better fasteners
Factory-fabricated brace assemblies preferred
Field welding by certified welders

Documentation:

Shop drawings showing all brace locations
Calculations stamped by licensed engineer
Inspection and testing records
Material certifications

Installation Best Practices

Field Verification

Before installation:

Verify structural capacity at attachment points
Confirm duct gauge meets design requirements
Ensure adequate clearance for brace angles
Identify conflicts with other systems
Verify seismic joint locations

Common Installation Errors

Avoid these deficiencies:

Braces attached to non-structural elements
Excessive brace angles (over 60° from horizontal)
Missing or inadequate duct attachments
Improperly sized rods or cables
Loose connections without locking devices
Bracing attached to sprinkler pipes or conduit

Testing and Inspection

Post-installation verification:

Visual inspection of all connections
Torque verification on bolted connections
Cable tension measurement
Clearance verification at full seismic displacement
Documentation photography

Seismic bracing represents critical life-safety infrastructure. Proper design and installation of transverse and longitudinal supports ensures ductwork remains functional after seismic events, maintaining building pressurization, smoke control, and critical ventilation systems.