Marisela Mendoza and Juan Ignacio del Cueto Ruiz-Funes Preservation and conservation of Felix Candela’s historic concrete shell: Los Manantiales restaurant. Marisela Mendoza1 and Juan Ignacio del Cueto Ruiz-Funes2 1. Course Leader MArch, School of Architecture Design and Built Environment, Nottingham Trent University; 2. Professor in Architecture, Faculty of Architecture, Universidad Nacional Autónoma de México Introduction Félix Candela Outeriño was born in 1910 in Madrid. Candela studied architecture at Escuela Superior de Arquitectura, Madrid and completed his studies in 1935. It was during his time at school that he became acquainted with thin-shell concrete structures. During the Spanish Civil War (1936-1939), he enrolled in the Republican Army as a volunteer [1] and in 1939 he fled to Mexico as a political refugee. Almost ten years after his arrival in Mexico, Candela co-founded the construction company Cubiertas Ala. During the tenure of Cubiertas Ala Candela mastered the design and construction of thin-shell concrete structures, in particular the hyperbolic paraboloid concrete shells or commonly known as ‘hypars’. The use of the hyperbolic paraboloid geometry to design concrete shells was not new in the realm of engineering and architecture and Candela had researched the work of other experts in the field to inform the design and structural analysis of his shells [2]. However, what can be attributed to Candela is the ingeniousness and inventiveness in the construction process of ‘hypar’ concrete shells . Once the geometry of the shell was determined and the structural analysis verified, its construction techniques and construction management required a skillful group of workers to place the formwork, lay out the steel work, pour the concrete by hand and the careful formwork removal. In 1958, and commissioned by the Joaquín Álvarez Ordóñez, Candela built Los Manantiales restaurant, one of his most famous thin-shell concrete structures on the banks of the Xochimilco canals. To this date Xochimilco is primarily a recreational area in the Southern part of Mexico City and in 1987 it was listed as UNESCO World Heritage Site. Formed by an eight-sided groined vault, Los Manantiales concrete shell covers an area of 900 square meters, and which represents the best of many examples of ‘free edge’ concrete shells built by Cubiertas Ala. Design and construction of Los Manantiales Lake Xochimilco was one of the five pre-Hispanic lakes in the ancient valley of Mexici City was the core of the chinampa agriculture site (man-made floating agricultural gardens built during the Aztec ruling). However, destruction of dams and gates after the Spanish conquest of Mexico led to the decline of agriculture and Lake Xochimilco gradually dried. Although on a smaller scale, agriculture activities continued on the banks of the remaining canals and by the mid-twentieth Century the canals became an important recreational site for trajineras 577 Preservation and conservation of Felix Candela’s historic concrete shell: Los Manantiales restaurant (gondola-like) weekend rides [3]. To service the recreational site a wood-built restaurant on the banks of Lake Xochimilco was constructed in 1938 (Fig.1). Figure 1. Los Manantiales wood-built restaurant, 1938. Courtesy of Archivo de Arquitectos Mexicanos (AAM-FA UNAM) In 1957, the wood-built restaurant was destroyed in a fire and the owners asked the architect Joaquín Álvarez Ordóñez to rebuild it using non-flammable materials. The young architect Álvarez Ordóñez (25 years old at the time) proposed a concrete roof and contacted Candela to collaborate with him. By that time Cubiertas Ala had already built experience and reputation as experts in the construction of thin-shell concrete structures in Mexico. Furthermore, Candela was very excited about this project as it was a great opportunity to put in practice one of his most recent achievements, the free-edge concrete shell. In 1953 and in collaboration with the architects Enrique de la Mora and Fernando López Carmona, Candela built his first groined vault for the Stock Exchange Market (Fig.2). Figure 2. Stock Exchange Market, Mexico City, 1953. Courtesy of Archivo de Arquitectos Mexicanos (AAM-FA UNAM). 578 Marisela Mendoza and Juan Ignacio del Cueto Ruiz-Funes In this groined vault Candela achieved the most-desired smooth surface that the architects were looking for by concealing the border-beams. However, Candela was convinced that border beams could be omitted, and he began working on different structural design solutions and construction strategies to achieve a free-edge-beam hypar groined vault. Three years later, in 1956, Candela built his first free-edge ‘hypar’ concrete shell for the church of San Antonio de las Huertas in 1956 also in collaboration with De la Mora and López Carmona (Fig.3). Figure 3. San Antonio de las Huertas Church, Mexico City, 1956. Courtesy of Archivo de Arquitectos Mexicanos (AAM-FA UNAM). Although Candela was not able to provide a full structural analysis for this free-edge hypar concrete shell, the built structure demonstrated that Candela’s success was a combination of his vast knowledge and intuition [4]. Another example of free-edge hypar shells built by Candela in 1957 was at Hotel Presidente in Acapulco and in collaboration with the architect Juan Sordo Madaleno [5]. 579 Preservation and conservation of Felix Candela’s historic concrete shell: Los Manantiales restaurant The shape for Los Manantiales restaurant followed the trend of the recently developed groin vaults built by Candela and was conceived as an octagonal groined vault composed of four intersecting hypars (Fig.4). The groined vault covered a span of 32.4 meters and the groins were stiffened using V-beams. These V-beams were specially designed to integrate smoothly and to follow the slenderness and curves of the concrete shell. The free edge form was achieved by ‘cutting in outward-tilting planes and the resulting perimetral arches have the shape of hyperbolas’ (Fig.4), [6]. Figure 4. Felix Candela’s architectural drawing for Los Manantiales restaurant. Source: Colin Faber, The Shell Builder. The site where the thin-shell concrete for ‘Los Manantailes’ restaurant was built sits on the Lake Zone of Mexico’s City Seismic Hazard Maps. Mexico City is well known for its high seismic activity, those originated in the subduction zone of the Pacific Ocean, where the Cocos plate subducts the North America plate [7]. Although the epicentres of some of these earthquakes have originated at a distance of more than 250 km far away they have been disastrous to the city’s built environment. The apparent reason for this seems to be ‘a combination of frequency content, regional geological and structural features which can produce huge amplifications and very long-time durations in Mexico City’ [8]. The seismic hazard maps of Mexico City divide the basin in three zones: The Lake Zone (I), the Transition Zone (II) and the Hill Zone (III) of which the Lake Zone is at greatest risk due to the presence of lacustrine clays. The mapping of Candela’s structures developed by Mitchels [9] against the three basin zones of Mexico City demonstrates that Los Manantiales thin-shell concrete was built in Zone I (Fig.5). 580 Marisela Mendoza and Juan Ignacio del Cueto Ruiz-Funes Figure 5: Map of Félix Candela’s structures in Mexico City against the three seismic hazard zones of Mexico City. Image courtesy of T. Michiels. The shell formwork was made with 1.27cm wide tongue-and-groove timber boards and placed according to one generating system. At the time Candela built Los Manantiales, he had already established a standard thickness of 4cm for shells: ‘The Standard thickness of 1½ inches has been found the most convenient for these shells, since its concreting does not require special care, and ordinary gravel, up to 1 inch, can be employed in the mix.’ [10]. Surveys conducted of most groined shells that candela built at that time showed that they were built with a thickness of at least 5cm. 581 Preservation and conservation of Felix Candela’s historic concrete shell: Los Manantiales restaurant Steel reinforcement consisted of a 5/16-inch-diameter mesh of steel bars spaced at 10 centimeters (Fig 6). The five-centimeter-thick edge of each intersecting hypar was reinforced with two 16 mm (5/8 in) bars following the sinuous perimeter of the shell. Figure 6. Los Manantiales during construction, steel reinforcement. Image courtesy of Archivo de Arquitectos Mexicanos (AAM-FA UNAM). Concrete pouring of Los Manantiales shell was made by hand using buckets (Fig.7). The shell’s foundation consisted of umbrella footings that were linked with 1-inch-diameter steel tie-bars to resists lateral thrusts [11]. Umbrella footings had been pioneered by Candela, first used in 1953 at Las Aduanas project (Fig.7). Furthermore, and to secure the continuous curvature, the original sharp v-shape of the supports (where the groins met) was modified to integrate into the curve of the shell [12]. Figure 7. Umbrella footings, Las Aduanas project, Mexico City, 1953. Image Courtesy of Department of Rare Books and Special Collections, Princeton University Library 582 Marisela Mendoza and Juan Ignacio del Cueto Ruiz-Funes Figure 8. Los Manantiales during construction, concrete pouring. Image courtesy of Archivo de Arquitectos Mexicanos (AAM-FA UNAM). Current state of the shell, conservation and preservation. The 7.1 magnitude earthquake that shook Mexico on September 19th, 2017 had devastating effects in the central and south areas of Mexico City including Xochimilco borough where Los Manantiales restaurant was built. As illustrated in Fig. 9 Los Manantiales concrete shell was significantly affected, buckling occurred on one the eight hypars composing the groined shell and altering its form, thus compromising the structural behaviour of the shell (structural integrity of hypar shells is by and large dependent on the integrity of their form).