Prolyte Campus blog: Cantilever Conversations - Part 2

Cantilevers can be found in everyday life structures, such as bridges, stadium roofs, cranes, etc. We also find cantilevers in the truss structures in our industry, so it’s good to know how to deal with them.

TotemsProlyte Campus blog
My previous blog about cantilevers addressed the technical (strength & stability) issues in straight trusses. When talking cantilevers there are obviously more situations possible. For example, a Totem with a T-corner having two cantilever arms (see fig.1). As a rule of thumb the T-corner itself can become the limiting component, rather than the straight sections of truss connected to it. It is wise to always verify the capacity of corners with the manufacturer, as a lot of truss corners in the market prove to be either ‘substandard’ or just a plain copy. Saying this, it should be clear that in the case of Totems the strength is much less an issue than the stability. As soon as you load this tower you either have an unbalanced load or a stability issue, or both. In all these cases, you carefully need to calculate what’s happening in the total structure and to calculate your amount of ballast or counterweight needed.

Rules of thumb:
A classic rule of thumb is to make sure that the base is at least one quarter of the height. A 3m. totem must have a base plate of ca. 80cm wide. A similar rule of thumb applies for the loading: the ballast should be in a 4:1 factor. A 25kg fixture at the top needs 100kg ballast at the base.  Please note: This is only valid provided the load is in the dead centre of the tower or totem axis!

Sleeve Blocks Prolyte Campus blog
Talking about corners in relationship to cantilevers there is a special type of corner that does need some more detailed attention: Sleeve blocks in Ground Support systems. These are basically a ‘hollow’ structural element that guides the horizontal trusses along a vertical tower. (see fig.2).
In principle, such a structural part would predominantly be loaded in shear force, however if the Sleeve Block has a truss connected in opposite directions a bending moment acts across it as well. Clearly the ‘open’ structure in the sleeve block’s centre – allowing it to move along the vertical tower - can become limiting in capacity. It is the duty of the manufacturer to inform the user about any limitations in this respect, as well as provide data on the cantilever loading for their structural systems.

Giraffes or V-towers
A final remark on Cantilever related subjects is the fact that the ‘Giraffes’ (I love that name) or V-towers also are a type of cantilever, and even the head-section often is a cantilever in itself. These structures require careful engineering, and user must require all the details on the allowable loading, definitely when used in out-door conditions.

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Laurynas Paškevicius Stage Technical Service, Lithuania