Deflection of the cable tray affects the appearance of an installation, but it is not a structural issue. In the case of nonmetallic cable tray, deflection may be affected by elevated temperatures.
The NEMA/CSA load test is a simple beam, uniformly distributed load test (see Figure 1).
This type of test was initially selected because:
• It was easiest to test.
• It represents the worst case beam condition compared to continuous or fixed configurations.
When consulting the manufacturer’s catalogue for deflection information, the designer must verify whether the data shown represents simple or continuous beam deflection.
If continuous beam deflection is shown, the calculation factor should be given.
NEMA/CSA has one criterion for acceptance under their load test: the ability to support 150% of the rated load.
Location of couplings
Since different bending moments are created in each span, there is no simple factor to approximate deflection as the number of spans increases. It is possible to calculate these deflections at any given point by using second integration of the basic differential equation for beams. Testing shows that the center span of a three-tray continuous beam can deflect less than 10 % of its simple beam deflection.
The support span should not be greater than the straight section length, to ensure no more than one splice is located between supports.
Location of couplers
(See Figure below ) The location of the coupler dramatically affects the deflection of a cable
tray system under equal loading conditions.
Testing indicates that the maximum deflection of the center span of a three-span tray run can
decrease four times if the couplers are moved to the one-quarter point from the above supports.
This can be a major concern for designers considering modular systems for tray and pipe racks.