Page 60 - Unistrut-Australia Catalog
P. 60

The following notes are presented in order to assist users to achieve maximum economy and convenience with the installation of cable support systems. As each application will have its own particular conditions and requirements, it is recommended that the services of Unistrut® sales personnel and engineering team be engaged, especially in the early stages of any major project, so that the best overall result can be achieved.
Standard ladder widths are 150mm, 300mm, 450mm and 600mm, being the inside dimension between side-rails and is the maximum width available for carrying cables. Straight lengths are 6m long. Standard rung spacing on all systems is 300mm nominal.
Each of the Unistrut® systems includes a full range of standard accessories, with a nominal radius of 300mm, 450mm and 600mm, depending on the system load class. Non-standard ladder widths and accessory radii can be manufactured by special order. Non-standard products are non-returnable and non-refundable.
a) CableLoad–Becausethecabledensityremainsfairlyconstantinatotalinstallation,thewidestladderscarrythemost load, and each smaller width carries proportionately less load. However, the load carrying capacity of any class of ladder is independent of the width.
For details on how safe working loads are determined, refer to the NEMA VE 1 Standard and to the published load graphs for allowable loads of each ladder type.
b) FixedLadderSpans–Itiscommonlyfoundthatthebuildingstructuresupportingthecableladderswilldictatethespan,butit is still possible to exercise some choice. Where the cabling is heaviest, and this is not usually extensive, it is possible to use two 300mm wide ladders side by side instead of one 600mm wide, in order to select a lighter category of ladder for the total project. It is often inconvenient to use more than one ladder category in the same installation.
c) Varying the Spans – Where the structure does not dictate the ladder span, the heaviest cable runs could be supported more frequently, again enabling a lighter category of ladder to be chosen.
Each of the Unistrut® Cable Ladder has a different cable laying depth. It is a general rule that the shallower the ladder, the lower
the cost per metre and the more frequently it needs support. It is sometimes found that the lightest, most economical ladders are excluded from consideration solely because a particular minimum cable laying depth is required and has been specified accordingly.
Cable ladders are essentially structural members designed to strength requirements only and are required to support pliable load elements. Therefore, the control of deflection is not necessary for durability or stability reasons and can probably only be justified on purely aesthetic grounds. If normally accepted deflection limits such as 1/360th of span are imposed, the resultant cable ladder will be grossly overdesigned and correspondingly expensive.
There may still be locations where the designer wishes to limit visual deflection. For example, prestige areas which may be open to public view or where the ladder is installed at eye level and deflection is accentuated. If these conditions exist, it is recommended that closer support spacings be used only in those important locations (to control visual deflection) and normal support spacings elsewhere (for economy). A maximum of 1/180th of span, when deflection is determined from the graphs published in this catalogue, should prove a satisfactory limit for visual deflection.
Otherwise, wherever overall economy is the principal consideration, no limits should be placed on deflection. This does not mean that deflection will be excessive but simply that a typically acceptable installation will result and optimum economy will be attained.

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