Félix Candela’s thin- shell concrete structures offer a particularly instructive and enduring example of the structural engineer’s role in sustainable design. Candela was a designer/builder who valued methods of construction as integral to the ultimate design of his structures. Through his design process, Candela realized works that are durable, employ minimum and locally sourced structural materials, and achieve various synergies.

Candela’s most abundant structure was the umbrella, formed from four intersecting hypars with straight edges supported by a central column.

Although Candela’s work precedes the lexicon of sustainability used today, his writings and lectures point to conservation of resources, as exemplified by his structures, as a fundamental goal of the engineer. “Any development that saves money and effort in construction contributes more to the general well being of mankind than all the messianic claims so common in the profession,” he wrote in World Architecture in 1991. Today, Candela’s beautiful, economical structures provide a road map toward sustainable design and construction for contemporary engineers.

The Hypar Form
To guarantee the economic competitiveness of his works compared with conventional construction, Candela used the hyperbolic paraboloid (hypar) form — a saddle shape described by straight lines that is inherently stiffened by its curvature. By using this ruled surface, or one that can be described by a series of straight lines, Candela greatly expedited the construction of formwork. The hypar shape exploits concrete’s strength in compression, minimizing material use by operating in a primarily in-plane stress state. Modern analyses confirm that demand stresses within Candela’s shells are well below the capacity of the concrete with a characteristic thickness of 4 centimeters and a span of 15 meters. Although the curved form may seem exotic, building experience and a concern for economy motivated Candela’s use of the hypar. Unlike today’s contemporary trend toward visually complex designs lacking structural logic, Candela utilized his thorough understanding of the hypar form to simplify analysis and construction.

Candela’s most abundant structure was the umbrella, formed from four intersecting hypars with straight edges supported by a central column. The free edges of each of the umbrella quadrants define the set of straight line generators that describe the surface. Candela used a simple cantilever analogy to design his shells, concentrating reinforcement at the edges and valleys where the principal stresses are highest. The umbrella units were repeated in rows to cover large spaces such as markets, warehouses, and subway stations. Candela created skylights to allow daylighting for interiors by either varying the umbrella height or tilting them.

Faced with the necessity of building economically, Candela reused his timber formwork. For example, after casting several umbrellas, he decentered the forms and translated them to the site of the next set of umbrellas to be cast. This clever reuse of formwork expedited construction and reduced materials costs. An additional benefit of the simplicity of straight-line formwork construction was that Candela’s design-build company, Cubiertas Ala, could employ local laborers and use locally sourced materials.

Candela’s structures, most built in the 1950s and 1960s in Mexico City, stand today in excellent condition despite more than 50 years of weather and earthquakes. Candela’s concrete thin-shell structures are a form of art, structural art to be precise. They are timeless and immune from a definition of beautiful that is molded by current fashion.

This article has been edited for length. The unabridged version is available online at www.gostructural.com

Jennifer Anna Pazdon is a member of the SEI Sustainability Committee. She is completing her Master’s of Science in Engineering Thesis at Princeton University where she contributed to the Candela exhibit and the accompanying book. The SEI Sustainability Committee website is www.seinstitute.org/committees/sustainable.cfm