In the field of the tensile structures, especially those of membrane but also in the cable nets, it is well known that we should give a pre-stress to membrane. It will provide the structure of a rigidity needed to better withstand the loads and, above all, have some deformations considerably less than those that would have been with the structure without prestress.
This pretension can occur, at the time of installation of the structure, in various ways. It depends on many things:
- through the supporting elements (cranes, scaffolding, … ) and tools we can use.
- if the membrane can grow in dimension or if it is framed in a perimeter that cannot grow (to see Offset (interior))
- if there are elements outside the membrane that can be used to remove slack: external wires, extensible masts, turnbuckles, …
WinTess3 provides some options to be able to simulate this prestressing. The most common is to preload the membrane directly.
We can give a prestress equally in all directions (isotropic, as in ETFE, e.g.) or differentiated according to the direction of the warp and weft of the fabric (as it would be the case of a polyester PVC or PTFE fiberglass fabric). This means that we are going to cut the smaller membrane (1%, for example) and then we will have to stretch it to put it in its proper place, causing the pretension. This process is called compensation of the membrane or the pattern.
This is one of the most common methods, albeit not the only one. There is another method that is also very common. It is stretch the membrane through tensioning free cables that balance the exterior structure of the tripods or other provisions of structural elements.
Let’s suppose that we have the hyperbolic paraboloid of the previous figure. It has 4 apexes with a tripod at each of them. It is easy to think that we might pretighten this structure by means of the free cables of the tripods. WinTess3 allows you to give a prestress to an external cable.
Now, it is known that prestressing a membrane by method of tightening a cable modifies tension of others. We might think that by means of the program we can cheat the structure so that all the cables have the prestressed same. Really, the program allows to pretighten 8 free cables simultaneously, but we will see that, on having carried the analysis, the structure becomes unstable and collapses down, or stays in a position without balance.
What the program is telling us that it is impossible tension this paraboloid so that all external cables have a tension of 2 T. Perhaps, the program allows us to tighten two cables of the same tripod at the same time. But if the tripod is not symmetrical, we may not be able to do so.
How to tackle with prestressed external cables? Just as we would in reality, prestressing cable one after another.
When we pretighten a cable, the structure looks for the balance, in such a way that the cable in question has the prestress that we have applied. If the applied prestress is absurd, obtained result will also be absurd. But if it is correct we will find a correct situation. If we notice well, we will see that WinTess3 not only has looked for the balance with this prestressed cable but it has also modified the initial length of the cable in question, so that in the balance position has the tension we wish. The cable is “locked” to the effect that its initial length changes to maintain the prestress. If we want to pretighten another cable, we must “unlock” the previous cable.
To do this we will return to cable edit window and enter a prestress value of 0.
Now, the cable continues to have the pretension that you have imposed, but you can modify the pretension of other cables. This will also change the prestressing of this cable. It is the same thing as if we are now under loads of wind, snow, etc.
Just like as it happens in real life!
What if you want to remove the prestressing? We have already seen that entering a 0 in the box of pretension in the bar edit window doesn’t cancel the prestressing but unlocks the cable. To remove the prestressing on a cable, we use the menu Bars | Initial length = Final length. Using this, we give the cable a length which does not impose any pretension.