Candela's stimulus for the form of Los Manantiales Restaurant in Xochimilco, Mexico City came from Colin Faber, who was working with Candela at Cubiertas Ala. Faber had made a rough sketch that somewhat resembled the final form of the restaurant; Candela liked the idea, so he took it and redesigned it into a more graceful shape.
Form
The form of the shell was a play of the hypar with free curved edges, that is, the edges of the shell are parabolic and free of any edge stiffeners that would conceal the thinness of the shell. The groined vault consists of four intersecting hypars, a structure that he had not yet attempted.
The Cosmic Ray Pavilion, Candela's first hypar shell, has curved edges just like those of the restaurant in Xochimilco. Where the pavilion has two saddles, one in front of the other, the restaurant belongs to a type of shell structure called groined vaults. The groins are the valleys in the shell formed at the convergence of the intersecting hypars. The restaurant was not Candela's first groined vault. He had designed a few others before Xochimilco, but none so striking.
Structure and realisation
As was his practice, at Xochimilco Candela used V-beams for groin stiffening, which was not visible, either from the inside or the outside, thus adding a bit of mystery to the educated observer of shell behavior. The V-beams are reinforced with steel, while the rest of the shell has only nominal reinforcing, not for added strength, but to address temperature effects and other properties of the concrete material that can cause cracking.
At the supports, Candela anchored the V-beams into inverted umbrella footings, which cup the earth to prevent the shell from sinking into the soft Mexican soil. To resist lateral thrusts, he linked adjacent footings with steel tie-bars, thus allowing the umbrella footings to carry only vertical loads. Candela was pleased with the unique visual design of his support detail, where instead of a sharp V-shape, he formed it into a curve to give it continuity. He commented that this is "what makes (the shell) so graceful, the regularity and the proportion . . . it looks good and I like it."
The form boards for construction followed the path of the straight-line generators that formed the hypar surface. With the form boards in place, and then the reinforcing steel laid on them the concrete could be cast by hand, one bucket at a time. When the concrete had hardened to sufficient strength, the scaffolding and form boards were removed. It is at this point that the shell comes alive and is seen in its purest, most refined form. Following the addition of architectural details (windows, doors, etc.), the structure remains the most prominent feature.
