(AGENPARL) – Wed 23 July 2025 THE MONITORING SYSTEM FOR BRUNELLESCHI’S DOME AND
THE MONUMENTAL COMPLEX OF SANTA MARIA DEL FIORE
Text by Samuele Caciagli, architect of the Opera di Santa Maria del Fiore
One of the most important activities carried out by the Opera is the structural monitoring of the
dome of Santa Maria del Fiore. Even today, this activity is passed down from generation to
generation.
With the exception of the earliest cracks involving the side aisles of the nave near the dome, now
unequivocally attributed to the structural settling of the foundations and recorded shortly after the
completion of the work, until 1639, there was no news of the structural problems of the largest
masonry dome in the world.
That was the year in which the architect of the Opera del Duomo, Gherardo Silvani, drafted a report
aimed at bringing attention to a series of cracks that went right through the layer. He also described
the solutions that would need to be applied in order to ‘bind’ the structure.
To date, this is the first document preserved in the historical archive on Via della Canonica that
refers to the cracks in the dome. Traces of a report on the first systematic survey of the ‘crack
patterns’ did not appear for about another century. This survey was designed to scientifically
analyse the damage to the dome using precise investigations and an accurate survey of the
measurements. In 1756, Leonardo Ximenes, who was interested in testing the stability of the dome’s
architectural structure to ensure the accuracy of the astronomical device made by affixing a bronze
bearing to the lantern structure, and therefore determined to test the accuracy of the cathedral’s
gnomon, began what could be considered the first ‘unintentional’ campaign of data collection and
scientific monitoring of the structures of the dome built by Filippo Brunelleschi.
However, the survey done in the 18th century on the structural damage was entrusted to
mechanical measuring devices and the expertise of the operators responsible for collecting the data.
It was only in the 20th century, with the appointment of the First Ministerial Commission for the
study of the complex of cracks and fissures (1934-1937), that more rigorous measurement systems
based on deformometry were adopted. These systematically compared the movements of the
cracks with the external and internal ambient temperatures. During those years, Father Guido
Alfani, director of the Ximeniano Observatory in Florence and inventor of numerous seismic
detection devices, developed a complex ‘continuous’ monitoring system designed to identify all
variations in the sizes of the cracks. This project was based on the installation of specially modified
seismographs, which were positioned at the edges of the cracks themselves in order to graphically
record all of the structure’s movements. At the same time, ambient temperatures were also
measured. The daily recording of data made it possible to define the relationship between the
dimensional variations of the cracks and those of the temperature; temperature variations and
related thermal movements were identified as ‘the main cause that produced and continues to
aggravate the initial cracks’.
The advent of the Second World War meant that the continuous monitoring and observation
activities recommended by Father Alfani in his final report had to be suspended. Not until the end
of the war, with the appointment of the Second Ministerial Commission in 1950, did operations
resume. These were based on a specific project that involved the periodic and continuous recording
of dimensional data concerning the cracks and the air temperature taken at predetermined points,
both inside and outside Brunelleschi’s dome.
According to the guidelines provided by this Second Ministerial Commission in 1955, two years after
the works had been completed, a network of deformometric bases was installed. This new network
consisted of 22 measuring stations where data was collected manually using an instrument
developed by Officine Galileo. With the appointment of the Third Ministerial Commission, which
operated between 1975 and 1976, a new monitoring system was set up, with detection stations: in
addition to the 22 existing ones, another 28 were installed, the control area was expanded, and a
new measuring instrument developed by Huggenberger was used.
The data collection, which was initially entrusted to Professor Salvatore di Pasquale of the University
of Florence and then to the technicians from the Superintendency of Monuments, has been the
responsibility of the technical department of the Opera di Santa Maria del Fiore since 1999. Today,
it performs surveys and collects data on a quarterly basis, at the solstices and equinoxes, at the 22
measuring stations installed by the Second Commission and the 28 stations installed by the Third
Commission.
This system of manual data collection and measurement was supplemented in 1986/1987 by a new
automated monitoring system, required as a direct result of the restoration work begun 10 years
earlier by the Superintendency for the restoration of the murals in the dome. Designed and built by
ISMES (the engineering and environment division of Cesi) and currently managed and maintained
by Cesi and Opera di Santa Maria del Fiore, the system uses deformometers, thermometers,
telecoordinometers, levelling instruments and piezometric probes to analyse the structures of the
dome and its base. Since its installation, automated data collection has made it possible to
constantly monitor the parameters necessary for complete control and static monitoring of the
structures.
Once the effectiveness of the system had been verified and the numerical data had been compared
with the data collected by the Military Geographical Institute through campaigns to level out the
piazza and its monuments, about twenty years later, in 2006, two additional peripheral measuring
units were put in place to collect data from the transducers (inclinometers, crackmeters and
thermometers) installed on Giotto’s bell tower, and the detection network was expanded to cover
the entire Cathedral.
In recent years, the Opera decided to expand the monitoring system by combining the collection of
data relating to the static control of the Dome, Cathedral and Bell Tower with a new data acquisition
network dedicated to the Baptistery of San Giovanni. What will be completed in the coming months
is a new static monitoring system for the entire monumental complex. A new automatic
seismic/dynamic monitoring system for Brunelleschi’s dome will also be completed, designed to
interact with the original monitoring system. This new system will make it possible to record seismic
events, including minor ones, and other phenomena that have generated accelerometric variations
of interest. This data will be useful for understanding the dynamic behaviour of the structure and
its possible dependence on environmental conditions. In addition to the monitoring system
permanently installed on the structures, work has continued over the years to check the detection
points, measured with a theodolite both from the external network and from inside the Cathedral.
The surveys are carried out every six months by the geodetic service of the Military Geographical
Institute of Florence.
Today, the various monitoring systems have made a great deal of data available regarding the
structures of the monuments, especially the structure of the dome. With this data, the technical