Cracking of Saint Peter’s Dome
History of the Saint Peter’s Dome
Saint Peter’s Basilica and its Dome are known
throughout the world as a symbol of the Roman Catholic
Church. The size and scale of the dome demonstrated the
power of the church in previous centuries and it stood as a
monument to god.
Famously designed by Michelangelo in 1546, the dome
has now stood for 412 years. Unfortunately Michelangelo died
(1564) before the dome completion and at the time of his death
it was only completed as far as the drum. Giacomo della Porta
with the assistance of Domenico Fontana, who was probably
the best engineer of the day completed the dome in 1593.
The double dome is brick, 42.3 m in interior diameter
(almost as large as the Pantheon), and rising to 120 m above the
floor.
Dome Design
A dome can be thought of as a series of vertical arches (its meridians) rotated
about a vertical axis and sharing a common keystone. These arches do not work
independently of each other but instead they work together to form a mechanism that
can transfer not only its self weight but any other additional weight.
A clear advantage of the dome is
there is no need for buttressing because the
parallels (the ideal horizontal circles of the
dome) act like hoops of a barrel and keep
the dome from collapsing. The parallels
must be able to develop a small amount of
tension at the bottom part of the dome to
allow them act as hoops. To relieve the
development of tension in the bottom of
the dome, 3 iron rings were installed. This
would later prove to be insufficient.
A diagram showing how forces are transferred and what happens when
there isn’t enough tensional reinforcement at the base of a dome.
Due to the combination of effects of the meridians and parallels have, domes
are exceptionally strong and rigid.
Problems Arise
In the early 18th century cracks began to appear and gradually propagate up the
side of the dome. These meridional cracks were appearing below 25 degrees on the
dome. An engineer named Polemi was asked to analyse the problem.
A Solution
Polemi analysed the dome as a series of 50 arches, then using Hooke
principles he established that the dome was still safe. Polemi hung a chain down from
the dome to form an inverted arch. This would determine whether the hypothetical
arches lay within the lines of thrust. He began by calculating the weight of sliced arch,
then dividing each half arch into 16 sections.
Polemi then loaded a flexible string with 32 unequal weights, each weight in
proportion to the corresponding section of the arch. As can be seen in the diagram
below when the chain is inverted the outline of the chain does lie in between the outer
and inner surfaces of the arc. This proved that the lines of thrust, while off centre,
still lie in the dome wall.
Polemi calculated this outward force and how much reinforcement was
required to prevent further cracking. In the end an additional 4 rings were added to
the dome.
Conclusion
There a number of reasons as to why the dome failed. At the time engineers
and architects relied on previous experience and trial and error. This is generally
sufficient for most buildings but Saint Peter’s Basilica wasn’t any ordinary building.
No one had ever attempted to construct a dome on this scale before. The designers
had no sophisticated method of analyse to understand the forces involved.