Group 21
The Cracking of St
Peter’s Dome
October 27th, 2006
The Cracking of St. Peter’s Dome
History of Dome
St. Peter’s Basilica was built to be the main church in the Vatican
City, but by 1506 the church had become rundown and shoddy. It no
longer conveyed the image which the Catholic Church wished to portray
to the rest of the world. With this in mind, Pope Julius II commissioned
the Italian architect Donato Bramante to design a dome which would
transform the Basilica into an international symbol of Christianity.
However, both the Pope and architect died at an early stage of the
project and a young artist from Florence
called Michelangelo was given total control
over the construction of the dome in 1546.
Michelangelo designed a dome which would
be much larger than the nearby Pantheon.
His design was for a hemispherical dome,
but this design changed before construction
had proceeded past the stage when the drum,
the base on which the dome rests, had been completed.
Michelangelo died before the completion of the works and the
project was completed by the architect Giacomo della Porta and engineer
Dominico Fontana. Della Porta redesigned the dome to be more egg
shaped, which would stand taller with a vertical thrust. While it would
have a smaller diameter than the Pantheon dome, it would stand a good
deal higher.
Design of Dome
The dome is in fact a double dome
made of made mostly of heavy masonry and
has an interior diameter of 42.3 meters and is
a height of 120 meters above the floor of the
Basilica.
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 form a mechanism that can
transfer not only the domes self weight but
any other additional weight by working interdependently to distribute the
loads through the series of arches.
The parallels develop tension at the bottom part of the dome which
causes them to act as hoops. This would not be enough on its own to
support a dome of this size.
Failure of Dome
The
redesigned dome was
more egg shaped than
hemi-spherical, which led
to greater forces being
exerted outwards at the
base of the dome. This
force had been looked at
by Michelangelo and by
the following architect
Giacomo della Porta and engineer Dominico Fontana, who decided that
the parallels would be enough to support the vast forces acting on the
dome, but the forces were underestimated. This outward force led to
cracking of the masonry at the base of the dome.
Solution
The solution to the cracking came in the 18th century when Vatican
engineers installed 4 iron chains between the two shells of the dome
which act like the rings on a barrel, and restrain the outward forces in the
same was the barrel rings do. These are known as tension rings. Visitors
to the Vatican may see these chains if they climb the steps between the
two shells. This solution has been in place since the mid 18 th century and
since then no more cracks have appeared, and so the repair can be
considered a success.