The concept of "Cables in Free Air" for power distribution and control cables has been adopted primarily for economic reasons. In order to achieve this objective, the engineer must bear in mind that the general design rules established for aluminum and steel structures are not always compatible with design rules for a cable tray system. This is particularly applicable in the case of restrictions on deflection.
Since the most economical cable tray system utilizes heat treated aluminum alloys, or high strength steels with long spans, any limitation on deflection which will not permit the best utilization of material and design will increase the cost. By limiting the maximum fiber and shear stress used in the design the adequacy and safety of the structure is assured.
Why Limit Deflection?
The primary reason to limit deflection in cable tray systems is appearance of their installations. So rigid restrictions on deflection of cable trays installed at eye level or in prominent location are common. However, it is neither economical nor good engineering practice to restrict deflection of a cable tray system in less prominent areas.
Methods of Decreasing Deflection
There are various ways to limit deflection of a cable tray. If the objective is minimum installed cost, they should be considered in this order:
1. Decreasing the Stress by Decreasing the Bending Moment
This can be accomplished by introducing restraining moments at the end of a span in the form of a rigid support. The deflection in a continuous beam, with negative bending moments at the intermediate support points, is only a fraction of the deflection in simple beam.
2. Increasing the Depth of the Cable Tray
Deflection in any location can be reduced by increasing the depth of the load-carrying side members and / or by adding to their cross-sectional area. Adding to the depth generally utilizes the material most economically.
3. Increasing the Modulus of Elasticity
Since the modulus of elasticity of steel is 29 x 106 psi, and that of aluminum alloys is only 10 x 106 psi, greater deformation aluminum alloy trays is to be expected at any given stress level. Under its own weight, an aluminum beam will deflect the same amount as an identical steel beam, since not only the weight, but also the modulus of elasticity is only one-third that of steel. However, under the same applied load (disregarding he beam's own weight), aluminum will deflect almost three times as much as steel. Therefore consideration must be given to the choice of material for any one location. For an isolated run or for an entire installation.
Deflection Criteria Applied to Cable Tray
Design rules and specifications developed for steel should not be applied to aluminum alloys since this would not permit the most economical use of these materials. Deflection criteria which apply only to steel and should not be used when the most economical system is desired, include:
Example: deflection is limited to 1/300 of the span by the National Electrical Manufactures Association specifications for structures supporting air switches. While very important in that instance as even slight deflection could cause misalignment in the operating mechanism and result in binding and difficult switch operation, the application of this specification to a cable tray system is uneconomic and not recommended.
Depth to span ratio
Example: The American Institute of Steel Construction, in their specifications for buildings, specifies the depths of beams and girders in floors to be not less than 1/24 of the span, or not less than 1/20 of the span where shock or vibration may be encountered. This specification ensures a certain rigidity and levelness of the structure which is important in that instance, but cannot be justified for cable tray systems because of the higher cost involved.
Example: Deflection is limited to a certain amount by an engineering company for a tray system. While such a specification might make a system using 8-foot spans look better, it prohibits use of more economical designs with longer spans which can have much greater deflections and still look acceptable. Such a specification increases the cost of the tray system unnecessarily, especially if the trays are to be installed well above eye level.
As a guide, a span-deflection ratio of around 1/200 satisfies most owners. This ratio provides an allowable deflection of 0.6" in a 10-foot span, 0.72" in a 12-foot span and 1.20" in a 20-foot span under the actual loads encountered. Data for calculating deflections is presented above.