Synclastic form

Double-curved surfaces in pure tension may also be achieved by inflation e.g. soap bubbles, balloons and car tyres. However, these are synclastic forms where the curvatures in two perpendicular directions are the same (i.e. either both concave or both convex). 

The foil layers of pneumatic ETFE-foil cushions are pre-stressed by internal pressure, which can be formed as either positive or negative pressure. Those multi-layered ETFE-foil cushions form predominantly synclastically curved surfaces, i.e. surfaces curved in the same direction in both main stress-axes. The Gaussian curvature is greater than zero for all surface points within the area of the synclastic curvature. 

The above descriptions of the various projects show that there are no limitations in principle to the great variety of shapes in ETFE-foil constructions. As pneumatically pre-stressed foil cushions or as single-layer mechanically pre-stressed foil membranes, anticlastically or synclastically curved surfaces can be formed across nearly any ground plan. The feasible spans of the foil constructions vary according to the ground plan geometry, boundary conditions, construction system, and load effect. 

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