Stability of the structure of the basilican body of Santa Maria del Fiore in Florence during construction of the naves

The builders of the basilican body of Santa Maria del Fiore had to deal with complex problems, caused by the size of the four naves covered by vaults and the temporary state of incompletion of the structure during construction. As work progressed some areas became unstable due to stress which would disappear only on completion of the structure. As for the transversal structures of the basilican body, three principal and different static conditions occurred before reaching the final configuration. In phase one the lateral aisles covered by vaults were isolated structures, except for the communal attachment to the façade. During the successive construction of the vaulted structures of the central nave these three separate naves were connected to each other. Only after the execution of the large trusses supporting the roof did they achieve effective distribution of the central vertical loads. Particular structural weakness occurred after construction of the first two bays due to the stress of longitudinal forces caused by the asymmetry of the buttressing system. The massive masonry of the western façade was effectively opposed to longitudinal forces, while on the opposite side the structures of the naves temporarily ended in octagonal pillars too weak to withstand horizontal force. Many of these problems were foreseen and allowance made with the use of hidden, permanent or temporary ties. In other cases unexpected difficulties were overcome by the inventiveness of the masons. The naves of Santa Maria del Fiore still conserve many remnants of these special solutions, mostly omitted from documents but doubtlessly of great relevance in understanding the thoroughly unconventional medieval building techniques.