Seismic Support Requirements for Cable Trays

Seismic cable tray supports must be designed to resist lateral, longitudinal, and vertical forces, following project-specific seismic criteria and applicable codes such as IEEE 344, NEMA VE 1, and AISC-N690.

Key Design Considerations

Seismic Design Basis: Before specifying supports, confirm the project-specific seismic design criteria, including expected ground motion, building drift, and acceleration levels. This ensures that tray selection, brace layout, splice design, and anchor requirements are appropriate for the seismic environment . Tray Type Selection: Ladder trays are often preferred for primary distribution in high-seismicity areas due to their structural stiffness and efficient weight-to-strength ratio. Perforated or trough trays may be used but require careful evaluation of mass, support spacing, and cable retention. Wire mesh or basket trays are suitable in some cases but need detailed review of splice and support connections . Bracing and Attachment: The support and bracing system is critical. Standard gravity-only supports are insufficient. Seismic bracing must resist lateral, longitudinal, and uplift forces. Bracing can be rigid (resists tension and compression) or cable-based (tension only, requiring opposing braces). Rod stiffeners and transverse/longitudinal braces are used to maintain system integrity .

Loads and Load Combinations

Dead Load (D): Includes the weight of trays, cables, covers, supports, and permanently attached items. Temporary construction items are excluded . Seismic Load (E): Must account for lateral and vertical accelerations, vibration, and building drift. Load combinations typically follow code requirements, such as D+E or D+L+E, where L is live load. Support Spacing and Connections: Brace spacing, orientation, and attachment must be engineered based on tray type, cable weight, and seismic forces. Non-ductile or customized connections should be avoided, as they have been shown to fail in shake table tests .

Codes and Standards

• IEEE 344-1987: Seismic qualification of Class 1E equipment for nuclear facilities
• NEMA VE 1-1998: Metallic cable tray systems design
• AISC-N690-1994: Steel safety-related structures for nuclear facilities
• AISI Cold-Formed Steel Specification: For structural members
• IBC and ASCE 7: General building seismic design requirements

Practical Recommendations

• Use pre-approved seismic bracing kits (e.g., Eaton B-Line with TOLCO) for faster installation and code compliance .
• Include hold-down clamps and cable guides to prevent cable displacement during seismic events.
• Conduct walkdowns and limited analytical reviews to identify non-ductile connections or outliers that may compromise seismic performance .
• Ensure all bracing assemblies are designed and stamped by a licensed engineer for the specific project conditions. By following these specifications, cable tray systems can maintain functionality and protect critical power, control, and life-safety systems during seismic events.