Prolyte Campus blog: Cantilever Conversations

Cantilevers can be found in everyday life structures, such as bridges, stadium roofs, cranes, but also the wings on an airplane fuselage. Maybe not so every day for the common person, but don’t we all have this work orientated look on life?

The thing is, we do find cantilevers in the truss structures in our industry, so it’s good to know how to deal with them.
To keep things simple in the loading tables, most truss manufacturers assume that the user places the supports at the very end of a span. Thus, the length of the truss is equal to the span and forces and stresses inside the truss are relatively simple (see fig.1).

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Daily practice, as we all know, is different. We don’t really mind if the truss span is a bit shorter, and part of the truss length sticks out at each end.
This practice results in two cantilevers, with lengths of 0,5 up to 2 meters being very common (see fig.2).
By doing so we can observe that the pattern of the forces does change in a more complex way. The centre of the span might have lower stresses in bending and shear, but stress increases over the support position. With this the number of parameters for various load types can sky rocket and the loading tables for just one truss type could run in many hundreds of variations.

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Extending the length also increases the risk of failure of the cantilever, and thus manufacturers must provide the data to help the user to stay in safe working conditions. It is the responsibility of the user to understand the type or combination of types, and the position of the loads, and verify them to be okay in capacity (=strength). For example, a line-array of 1m wide could be suspended from a 3m long truss cantilever. But if the attachments are applied at 1m and at 2m away from the support point, the length of the cantilever can be checked for the full array load at 1,5m.
Failure of a cantilever at the position of the support point could initiate the collapse of the complete truss (fig.4).

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Obviously with a lot of weight on just one of the two cantilevers, the whole system can become unstable and behave like a seesaw (see fig.5). The stability always is the responsibility of the user, and unstable trusses can also lead to unsafe conditions.


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Finally, to allow the user to check the loads per support we can fill in the data in the General Cantilever Formula (see below). This can help in choosing the adequate type or capacity of machinery or structure in the support position and even tell if there will be an uplifting effect, that must be adequately restrained or solved otherwise to make the structure safe.

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All in all, it’s just important that you are aware of the cantilever effect and that you check your structures beforehand. I hope this simple formula might offer a tool to build safer structures.


Have a look at the download section at the Prolyte truss pages for the applicable cantilever loading tables.

 

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Sammy Freh GSF Sound & Light Rental, Belgium