|Dr. N. Subramanian
Joined: 21 Feb 2008
Location: Gaithersburg, MD, U.S.A.
|Posted: Wed Jul 01, 2009 5:35 pm Post subject: Felix Candela: Prototype for sustainability in structural e
|Felix Candela: Prototype for sustainability in structural engineering
Jennifer Anna Pazdon
Materials, structural systems, and construction methods developed with sustainable principles as their basis will be necessary to truly revolutionize the design and construction sector. Advancing sustainability will require more than just making current practice less unsustainable. The call for development of innovative structural ideas will allow structural engineers to take the role of leaders in defining the manifestations of sustainable design. Similarly, successful structural engineers throughout history often represented the vanguard in the use of new materials, and subsequently invented new forms to optimize the characteristics of those new materials. Maillart and Menn's development of efficient bridge forms in reinforced concrete and Kahn's development of integrated gravity and lateral systems for tall steel buildings offer a few examples. Because their forms defied convention, these notable structural engineers were often compelled to devise new and efficient construction techniques. Instances include the reusable formwork of Heinz Isler and Jack Christiansen, and Christian Menn's innovative use of the cantilever construction method for prestressed bridges.
Felix 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. 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.
Figure 1: Candela's most abundant structure was the umbrella, formed from four intersecting hypars with straight edges supported by a central column.
Figure 2: Interior of High Life Textile Factory (now Cavalier Industries Factory) Coyoacán, Mexico City, during construction.
Figure 3: Chapel Lomas de Cuernavaca, State of Morelos.
Figure 4: Construction of falsework shows the straight boards that were used to generate the hypar surface for the Chapel Lomas de Cuernavaca. 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 (Figure 1). 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. Highlife Textile Factory pierced the concrete shell with glass bricks in addition to tilting the umbrella (Figure 2). The hypar shells could also be formed with curved edges—a saddle-shape. Chapel Lomas de Cuernavaca (Figure 3) is an example of a curved-edge hypar. The construction photograph (Figure 4) shows how straight lines are used to generate the curved surface.
Photo of Santiago Calatrava (left) and Felix Candela (right)
Material minimization and synergies
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.
Thin-shell structures enclose maximum space with minimum materials by carrying loads most efficiently within the plane of the shell. Such efficiency makes long spans easy to achieve, and the resulting, sometimes column-free interiors may be attractive for industrial buildings, museums, and exposition centers, among other building types. These structures can provide attractive and clean finished surfaces inside and out that preclude the need for additional architectural finishes. For the structural engineer, the thin-shelled structure represents a refreshing expression of the structure of the building, which is so often cloaked and obfuscated by architectural cladding in contemporary stick-built construction. Examples of Candela's best work include the Bacardi Rum Factory and Restaurant Los Manantiales. Their striking thinness is an expression of material minimization revealed by Candela by avoiding beams or stiffeners on the shell edge. The inside is so well lit by natural lighting that during the day no additional light source is needed.
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. We consider Candela's concrete thin-shell structures to be a form of art, structural art to be precise. Just like works of painted art such as that of Rembrandt, or musical art such as that of Mozart, Candela's structures are timeless and immune from a definition of beautiful that is molded by current fashion. Candela was a structural designer who seized an opportunity to create a structure that not only satisfied the functional requirements of safety and serviceability but also is a work of beauty to be enjoyed for generations to follow, and thus his works represent a synthesis of structural function with sustainable design. Candela's buildings and his methods of simplified design and construction are an inspiration to those structural designers committed to conserving and protecting limited resources through their innovative structural solutions.
A sustainable education for all
It is important for structural engineers not only to receive an education in sustainable design, but also to learn from the best examples of structural engineering works and designers. As we have illustrated, these best examples were always "sustainable" by today's definition, but to the best engineers of the past it was common sense design driven by efficiency and economy. Such knowledge of the best engineering works does not need to be limited to engineers alone. Structures (buildings, bridges, vaults) are in and for society, therefore the education should expand to the general public as well. To this end, Princeton University has held several exhibitions related to structural art, the latest being "Felix Candela: Engineer, Builder, Structural Artist." The exhibition was on view from October 2008 until February 2009 in the Princeton University Art Museum and it is currently exhibited at the MIT Museum (April 2 through Sept. 27, 2009). Maria E. Moreyra Garlock and David P. Billington, the exhibition curators, wrote a book with the same name as the exhibit to accompany the show. Both the exhibition and the book are produced for a general audience to illustrate some of the best designs of the past as a stimulus for what is possible in the future.
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