Cable Tray Loads
Cable tray loads are categorized into static loads, dynamic loads, and additional loads.
Static load refers to the load imposed by the cables installed within the tray-specifically their type, quantity, and weight per unit length (based on outer diameter)-tabulated according to the different cable routing paths.
Dynamic load refers to the weight of construction or maintenance personnel during the installation and upkeep of the cable tray. For lightweight cable trays, dynamic loads are generally not considered; that is, personnel are not permitted to stand or walk on the trays. If standing on the tray is required, the span between supports should be appropriately reduced. Additional loads apply only to outdoor installations and consist of loads caused by ice, snow, wind, and electromagnetic forces; these depend on local meteorological conditions and the nature of the energized conductors, and must be calculated based on these specific conditions during the design process.
Steps for Selecting Cable Trays
(1) Determine the tray width, number of tiers, support types and spacing, and the distribution of cables across the various tiers.
(2) Calculate the uniformly distributed load (kN/m²) for the cables on each tier and preliminarily select the tray model and specifications.
(3) Verify the structural strength of the tray based on the maximum total uniformly distributed cable load. The verification formula is as follows:
Q_service = q1 + q2
Where: q1 = Uniformly distributed cable load (taking the maximum value among all tiers) (kN/m²); this represents the load imposed on the tray, ladder, or trough;
q2 = Equivalent uniformly distributed load representing the weight of personnel during cable installation or maintenance (kN/m²); for the calculation of q2, the weight of a person is generally assumed to be p = 90 kg. Calculated based on the condition of equal maximum bending moments:
Where: P = load of one person (kg)
l = spacing between support points (use the maximum value if spacings are unequal) (m)
q2 = equivalent uniformly distributed load for one person (kg/m)
Based on the cable tray model, specifications, and support spacing preliminarily determined above, consult the manufacturer's catalog and repeatedly verify the spacing and model until load requirements are met.
(4) Deflection
There are no explicit regulations regarding the determination of deflection values; however, in areas subject to heavy loads, it is clearly necessary to minimize deflection-which implies a corresponding increase in steel usage. Therefore, provided that the steel's maximum allowable stress is fully utilized and an adequate safety factor is maintained, it is generally appropriate to set the ratio of maximum deflection to span (support spacing) between 1/250 and 1/150.
