Cable tray transverse and longitudinal seismic bracing

Cable trays require both transverse and longitudinal seismic bracing to remain stable during earthquakes, with transverse braces perpendicular to the tray and longitudinal braces parallel, installed according to code-based spacing and load requirements.

Overview of Seismic Bracing for Cable Trays

Seismic bracing is essential to prevent cable trays from swaying, colliding, or collapsing during seismic events, protecting both the tray and the cables it carries . Bracing ensures that dynamic forces from earthquakes are safely transferred to the building structure, maintaining system integrity.

Transverse Bracing

• Orientation: Installed perpendicular to the cable tray to resist lateral forces across the tray width .
• Spacing: Typically, transverse braces are installed every 40 feet for piping and similar systems, and cable trays follow similar spacing guidelines depending on tray width and fill .
• Purpose: Absorbs lateral seismic forces and prevents side-to-side movement.
• Installation: Can use rigid struts, all-thread rods, or aircraft-quality tension cables. Rigid braces resist both tension and compression, while cable braces work in tension and require opposing brace assemblies at each location .
• Special Considerations: Each elbow, branch, or change in direction may require a brace pair within 24 inches of the turn to account for reorientation of forces .

Longitudinal Bracing

• Orientation: Installed parallel to the cable tray to resist forces along the tray length .
• Spacing: Typically every 80 feet, but spacing may be reduced based on tray fill, weight, or seismic zone requirements .
• Purpose: Prevents tray from moving along its length and maintains alignment during seismic events.
• Installation: Uses similar materials as transverse bracing, often with pipe clamps or certified riser designs for attachment to structural members .

Standards and Guidelines

• ASCE 7-22 §13.6.5–13.6.7: Defines thresholds for when bracing is required and the type of bracing based on system importance (I_p = 1.0 or 1.5) and tray size .
• SMACNA Seismic Restraint Manual: Provides pre-engineered details for duct and cable tray bracing .
• MSS SP-58/SP-127: Guidance for pipe hangers and supports, applicable to cable tray supports in some cases .
• Manufacturer Pre-Engineered Systems: Products like Eaton's B-Line series or nVent CADDY provide tested seismic bracing kits approved for specific tray types and loads .

Practical Considerations

• Tray Type and Load: Select bracing based on tray width, cable fill, and weight. Wider trays or heavy fills may require closer brace spacing .
• Attachment Methods: Use adhesive anchors, powder-actuated fasteners, or structural bolts rated for seismic loads. Connections are often the critical failure point .
• Testing and Performance: Modular bracing systems have been tested to withstand floor accelerations up to 1.8 g, simulating earthquakes of magnitude 7–9 .
• Flexibility: Modular components allow for prefabrication, easy modification, and disassembly without welding or drilling .

Summary

For effective seismic protection of cable trays:

1. Install transverse braces perpendicular to the tray at code-specified intervals to resist lateral forces.
2. Install longitudinal braces parallel to the tray to resist forces along the tray length.
3. Use certified materials and attachment methods compliant with ASCE 7, SMACNA, and manufacturer guidelines.
4. Consider tray width, fill, and seismic zone to determine brace spacing and type.
5. Ensure each change in direction or branch is properly braced to maintain system stability. Following these practices ensures that cable trays remain secure during seismic events, protecting both the electrical system and personnel safety.