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Pneumatic Cushion

Pneumatic cushions have always existed, but since the appearance of ETFE material (transparent or translucent) have become very popular.
It is a space enclosed by two membranes, one upper and one lower. They are kept separated by the internal pressure of the air which is higher than the external pressure. This pressure difference is usually greater than in pneumatic air-supported tires.

In the case of air-supported roofs, the difference between interior and exterior pressures is typically 200-400 Pa (20-30 kg/m2), while in cushions it is usually not less than 800 Pa (80 kg/m2)

The method to create a pneumatic cushion is the same as the one we used to create an irregular mesh. A double window appears on the screen. In the one of the left we fix the perimeter of the cushion. It can be a figure delimited by vertices, it can be a regular polygon or it can be a closed polyline imported from a DXF. We should observe in the window on the right that in the lower line the Fixed perimeter check box is marked, since this is typical in pneumatic cushions. If it were not so, we should uncheck it.
We follow the same steps as in the case of mesh generation: choose the type of mesh (triangular, square …), its size and position. Then we trim the mesh and save the file that has been generated.

We get this window:

In this window we can observe a node (269) and a distance (L). The node is the so called Reference Node. By default is more or less in the middle of the mesh, but the user can modify it by entering the number or by clicking a node on the mesh. The new value will be updated on the following window:

Well, with this data we can already fill in the Hs and Hi values which define the allowed movement of the node “n”. Hs refers to the upper layer and Hi to the lower layer. Instead of the absolute values Hs or Hi, we can use the approximate percent value H/2L. We can see that if we modify some of these values the others are automatically modified.

Once the values Hs and Hi are set, press the accept button and WinTess will search the shape of the cushion according to the data that we have entered.

Once the creation of the cushion is completed,  two groups of elements: Top layer and Lower layer have been created. With these groups, actions (wind, snow…) can be set separately for to the two layers that close the cushion.