Civil Engineering Reference
In-Depth Information
column must be available to operate it, since the haulage must be applied with a
reasonable horizontal component to draw down the vertical arm and thus raise
up the supine column into the vertial. In the plan of the Pantheon porch there
is not the free linear space to lay out the two central columns of the innermost
row and operate the tilter for 50' shat s (v R. Taylor, pp. 129-31). h ese circum-
stances were discovered too late to make any adjustment to the plan (the rotunda
was already built), hence the alternative was to reduce the length of the shat to
40' and thereby to lower the entire elevation of the porch and so bring it out of
adjustment to the already completed rotunda. However this explanation rests on
the categoric necessity of using the Adam tilter to raise up these monolithic shat s,
which is dii cult to maintain. Certainly the marble columns inside the rotunda
were not raised by the tilter. h ese shat s are roughly the same dimensions as the
porch columns, but they are not monoliths (they appear to be composed of two
frustra). Presumably they were set in place with a two legger crane.
h e Pan-
theon
(2) h e Procedure for building the Dome
h e wonder of the Pantheon is its concrete domed roof, one of the earliest to be
constructed and one of the widest span domes ever constructed in traditional
building materials. And it still stands intact at er nearly 2,000 years. h is wonderful
structure was built by virtue of a temporary wooden installation which both gave
it its designed form and also supported it while the concrete material was becom-
ing rigid and self supporting. How this wooden installation of great strength was
fashioned and set in place climbing to 40+ m above pavement level is a wonder
in itself, equal to that of the concrete dome it made possible. It represents a pin-
nacle of Roman building procedure and something must be said of it here. h e
installation was designed as a temporary structure, to be removed entirely when
it had performed its function, and to leave no trace of its presence in the i nished
monument. In this way there is no direct archaeological evidence of its nature and
discussion of this must proceed from comparitive, historical evidence.
As noted above this temporary installation was multi functional. It provided a
surface which delimited the form of the dome, and thus in a general way it may
be termed “formwork”. Since the form of the Pantheon dome was hemispheri-
cal the wooden installation was “centering”, evidencing that the form was struck
from a centre. h is is the term applied to form work used for all arches, vaults
and domes. However, in addition to dei ning the form of the dome, the centering
was required to support the load of the dome under construction until it became
competent—i.e. could support its self load. In these respects the centering for a
concrete dome was no dif erent functionally from the centering for a stone masonry
dome where the voussoirs required support from beneath until the structure was
completed when they were held in place by compression.
