Rigging dynamic factors in t.d.s. ?

Why is it important to distinguish live loads and dynamic factors for chain-hoists in temporary demountable structures?

In entertainment rigging, it is common sense to add a dynamic factor (usually 1.2 to 1.6 ) to the live loads when hoists are used. Therefore, indoors it is best to use the gross values of the support loads, especially because the installation is always independent of weather conditions.

Open-air stages, on the other hand, differ from ordinary buildings in three main points:

  1. Assembly and operation takes place in normal weather conditions.
  2. The permissible live load is often a limiting factor.
  3. The wind load is the most frequent and determining load effect besides the live load.

If I neglect these differences, I probably underestimate the load-bearing capacity. Although this is not dangerous, it can quickly become uneconomical under certain circumstances.

What does this mean for the use of motor-driven live loads in open-air stages?

First of all, it must be distinguished whether the motor movement takes place only for set-up and dismantling or also during the show.

If the movement is also during the show, the gross load should be combined together with the maximum operating winds.

However, if the movement is only required for setup and dismantling, a distinction can be useful, especially for high live loads. On the one hand, because often not all live loads are moved at the same time, and on the other hand, because it can be ensured that movement only takes place in light winds for reasons of health and safety.

An simplified example to clarify:

Let’s assume the following worst case material utilization in a stage:
20% from dead weight
40% from wind
40% from payload (10t)
Total: 100% -> The stage is utilized to the maximum.

Production would now like to bring in 10t with a dynamic factor of 1.5, which would result in the following charge based on the above assumption:
20% from dead weight
40% from wind
60% from live load (10t x 1,5 = 15t -> 60%)
Total: 120% -> The stage is apparently overloaded. However, we also consider a situation that cannot occur.

However, if we now take into account that the movement will only take place in reasonably good weather for setup, the following results:
20% from dead weight
20% from wind
60% from live load (10t x 1,5 = 15t -> 60%)
Total: 100% -> The stage is not overloaded. The structure has sufficient load reserves to handle dynamic factors during setup and dismantling.

Of course, this simple calculation cannot be transferred to every stage without further consideration. However, it should be made clear that it is often uneconomical and also unnecessary if dynamic factors are calculated in a general way without thinking about the details.

For this reason, it is always important for the designer of temporary structures to know net weights and dynamic factors independently.

With existing calculations, users are naturally faced with the difficult question of whether or not load reserves for dynamic factors are available. Here we are happy to help in individual cases. Simply get in touch with us.