A few weeks ago Marie and I performed a structural assessment of the Guastavino vaults in a 1920s church in midtown Manhattan. Since there were no scaffolds to have a close look at the vaults, we had to find our way in into the attic to inspect them from above. After some crawling in dusty conditions, we got ourselves in the attic. To our surprise, above the Guastavino vaults of the main ceiling there was another level of vaults forming the gable roof of the church. Gable roofs of the main nave of churches are typically framed with steel or timber trusses, which can span long distances without intermediate supports. However, in this case the roof structure was a spectacular display of masonry catenary arches and catenary walls (yes, walls!!), the same type of vaults used in Gaudi’s masterpieces such as La Casa Milà in Barcelona.
The massive unreinforced masonry structure is a marvel of engineering and a mastership of graphical statics. The vertical arches are designed to not transfer a substantial amount of horizontal thrust to the small exterior buttresses. The whole structure works in compression, the tensions are absorbed mainly by the weight and shape of the structure. This helps to create a tall and slender main nave. To accomplish this, the arches have more than 20 layers of thin-brick and cement mortar. A particular feature is that the lower layer of brick was buttered with gypsum instead of cement mortar to act as formwork for the rest of the layers; this centenary technique is known as “Boveda tabicada” originally from Mediterranean Spain, where Guastavino and Gaudi (and myself) were from. There is no other material involved, no steel or reinforced concrete, just a lot of thinking and experience.
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