The tension in wires will be different at different climatic conditions, thus, the sag will be different as well.
We need to define the maximum sag.
Let there me “m”- climatic conditions with the following characteristics:
Let us assume, that the climatic conditions have changed. Now we have some “n” climatic conditions: In this case:
. The length of the wire in the span could be defined for “n”-conditions, if we know We can write down: we insert and into this expression.
Now we have a complicated equation of third order. It is acceptable that the last member in brackets could be neglected, (=23·10-6; =6300) – as an example for aluminum.
The rest of the equation is re-arranged:
Let us right down this expression in a more convenient way. If we accept that «» differs from «» by 2…3%. «» could be replaced by «». Each member of the equation is divided by «». When transposing the members of the equation from the left to the right side, the signs are changed to opposite ones.
- the equation of the wire state (the basic reference equation).
This is a cubic equation - most often, it is solved by trial method (this what it used to be like, now, with the help of computers, it is solved by bipartitioning and some other methods).
The calculationof this equation could be performed for both mono metal and composite wires. For composite wires the values caracterizing the entire wire are inserted into the equation. (; and others).
Tus, when we have the equation of the wire state and we know the tension at “m”-conditions, we are able to sort out the tension in the wire material.
The question is what tension is the source tension. The starting point will the permissible tension. In any case, the tension in the wire material should not exceed the permissible value. We have three values for permissible tension, we should sort out the one we need.