Hyperbolic paraboloids may be formed by using portions of the basic surface that was illustrated previously. In the above shell, the edges are parabolic arches and all the forming is made with straight lines running from equa-distant points on parabolas. The arch rib at the ends must be of sufficient strength to carry the principle loads.
On account of its double curvature, this shell may be made much less thick than the equivalent short shell. The stresses are almost entirely tension or compression.
Another variant is the trumpet intersection shell where crossed vaults are used rather than a single vault. This structure is shown on the next page.