Temporary Truss Systems - Load Tables

In my series of posts, answering some of the most common questions among my clients, this post will try to answer some of the most common questions regarding the load tables of temporary truss systems.

What legislation is applicable for truss?

It depends on the usage of the system. In the EU a truss, will either be part of a machine and applicable to the EU Machine Directive, be part of a temporary or permanent structure applicable to local construction legislation, or even in some cases applicable to both. This type of "diversity" is also seen with the popular lifts/telescopic/winching stands - when hoisting the load its a "machine", but when left standing its a temporary structure applicable to the same loads as demanded by construction legislation (More on this in an upcoming post).

In the EU, truss´ must be calculated to the Eurocode applying the national annexes of the country they are applied. As the partial coefficients are specific for each member state, the properties used to create the load tables are typically calculated to either the country of manufacture or by using the suggested min. values of the Eurocode. Adding the issue of some manufactures not providing the partial coefficients used in their calculations increases the difficulties of a simple comparison of different truss systems.

How are the load tables calculated?

Load tables are typically calculated on the basis of the mechanical properties of a straight piece of truss, using the "weakest" parts of the truss as the basis of the further calculation.

Due to the lack of a mandatory standard on how to calculate temporary truss´ in the EU, manufactures are essentially free to choose the type of supports used for the static calculation of the truss as a theoretic beam. This leads to some significant differences in the allowable loads - In my effort to keep this post as short as possible, illustrated above, one of many examples of the differences, the formulas for the calculation of the moment at the center of a beam with a uniform load depending on the support type.

The load tables are typically based upon ideal static calculations, providing the end-user with the lowest of the calculated loads (q) at a fixed length (L), depending on the allowable bending moment (M), shear resistance (Q) and axial force (N). Below, a short example of how the UDL could be calculated for a simply supported setup.

Pls. do note this post is a brief insight of some of the "magic" behind the commonly used load tables and not applicable to DIY static calculations of truss systems.

When can I use the load tables provided by the manufacturer?

You should only use the load tables provided by the manufacturer in your initial planning - during the process of making rough sketches and estimates. As described above, the tables are based upon the simplest and ideal static conditions, not considering all applicable loads.

Load tables neglect forces generated by wind, snow, ice, the combinations of them, nor the most vital for the install - the dynamic loads generated by hoisting the truss, which depending on the hoists used, etc. can decrease the applicable load by 20-60%. A static calculation considering the effect of some or all of the above, should always be performed for the specific application of any truss system.

I hope this post gives you some insight into the field of Entertainment Engineering.