Construction TechniquesThe construction of concrete shells requires scaffolding (sometimes referred to as falsework), a temporary structure of wood or metal that supports the form boards and wet concrete. These boards are placed on top of the scaffolding and allow the concrete when hardened to take its proper form. Designers and contractors have often shied away from curved concrete shapes believing that they would require expensive, custom, curved forms. Candela, however, understood that hypars, while doubly curved, can actually be formed by straight lines. Once the forms are placed, steel reinforcing is added and finally the workers cast the concrete, typically 4 centimeters (1.5 inches) thick. Candela indicated that on steep surfaces the placing of the concrete was done using a dry mix with the reinforcement bars laid out close together so that they could keep the wet concrete from sliding down.
Because Mexico City is on the site of what once was a lake, its soil, composed of lacustrine and volcanic sediments and organic matter, is soft and compressible, one of the worst on which to build. In addition, the city is in a region of high seismic activity as illustrated by the earthquake of 1985, which resulted in enormous settling and tilting of the substructures of numerous buildings, followed by total collapse or substantial damage to superstructures. Candela was aware of the poor soil conditions and the earthquakes that would challenge the integrity of his structures. But according to Candela, no earthquake has ever damaged his structures. Since his structures were light, due to the thinness of the concrete shell, the forces induced by the earthquakes would be relatively small (the earthquake force imposed on a structure is proportional to the building weight times the ground acceleration).
With regard to foundations, Candela developed a solution that was simple and clever: umbrellas as footings. This type of foundation provided significant economy since, due to the poor soil conditions, the traditional flat footing would have been quite large and expensive.
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