Prolyte Campus blog: Circles and Arches

Circles and arches can be found in a variety of applications, all of these bringing particular challenges on how to safely calculate constructions using circular or arched truss.

Although lots of examples of the use of circular truss can be found, the opposite is true for finding loading data. The information on these types of truss modules is sparse. Looking into this, it obviously comes as no surprise. When calculating a circular or arched truss module the number of parameters involved to generate the data is stunning.

This is why I think, when discussing circular and arched trussing, even without going into structural engineering, we need some classification to enhance our understanding of the subject.
We can identify the following:
•    Orientation of use
•    Bracing set-up in the truss module(s)
•    Number and position of supports

Dealing with all of these can’t be handled in one blog, so let's look at one of the most common situation: the horizontal circle (ranging from a flat plane till a 15-degree inclination will generate more or less the same results). This will at least give an idea of the loading effects that come into play when using circular trusses.

Circles in a Horizontal plane
A circle is a special structure, head and tail are connected, so there is no real span to refer to. In general, they can absorb less load that straight trusses. In all sections of the circle there’s always a superimposed cantilever effect.

In a circle segment the braces in the vertical plane for the inner and outer side are fundamentally different and the effective length of the braces is always greater on the outside than on the inside. This means that an asymmetrical distribution of force is built into the truss. The result is not only a different load on the chord tube, diagonals and connection elements through bending and shear forces, but also a rotation and torsion force which affects the bearing capacity of the truss. The torsion force influences the danger of buckling in the arched spans.

The minimum number of supports for circular trusses with diagonals on all sides used in a horizontal position without causing problems of stability or balance is 3.
The larger your number of support points, the lesser the cantilever effect (see picture, where Sh in relation to the length of the segment chord, defines the cantilever).

Read also Prolyte BlackBook – page 26

In my opinion this simple example already shows that rigging circular or arched trusses is a relatively complex thing to do. We must be happy with those manufacturers that provide circle loading data. Without any data, please turn to a structural engineer for proper advice and calculations before you start rigging such a structure!

 

 

I started working with an ST roof system back in 2000, it was one of the first ones Prolyte made. I've been using Prolyte ever since

Jon Reeves JDR Rigging, Finland