Civil Engineering Reference
In-Depth Information
When the dome was of concrete a further function was demanded of the wooden
installation. Since concrete was an aggregate of small stones together with semi-
liquid mortar, when applied it was cohesionless and the centering needed to be
continuous and impervious so as to coni ne the material from escaping. h is func-
tion can be required in any concrete construction (walls as well as rooi ng) and
the wooden installation providing it is called shuttering. h us the installation for
building the Pantheon dome was of necessity both centering and shuttering—the
centering provided the form and supported the construction in place for the
required interval of time, and in addition the shuttering coni ned the plastic mate-
rial while curing until it became fully cohesive and competent. It is now necessary
to consider the background to these various functions.
h e idea of Roman builders to roof a large open space with concrete (a cohesion-
less material when applied) was an astounding enterprise. Accumulated experience
of ages revealed that neither stone nor brick served well for purposes of traditional
rooi ng. Brick (terra-cotta) was useless, and for a stone beam or slab to span any
appreciable distance (e.g. 5 m) it had to be so massive as to be impractical for any
except the most monumental structures. h e only material generally useful for
rooi ng was wood. Poles or beams could be laid horizontally over a space to sup-
port l at terrace roofs or inclined upwards together to support ridged rooi ng. Also
the device of corbelling outwards successive courses of stone or brick until they
covered a restricted space was a traditional practice (v. H. Soeder, “
Urformen der
Abendländischen Baukunst
” Köln 1964,
pass
). But none of these measures suggested
the idea of concrete for rooi ng—and they were all restricted to limited spans.
In addition to these traditional methods of rooi ng, from sometime in the latter
part of the 4th century BC Greek builders had come to construct arches and bar-
rel vaults in ashlar masonry with radially set voussoirs. Such vaults again were of
limited span, but their construction demanded that the voussoirs be supported from
below until the structure was completed. h is support was arranged by forming
stout wooden arches to set as bearers to the stone construction in progress—i.e.
the ashlar vaulting was erected by use of wooden centering. h us when Roman
builders began to build spacious concrete vaults and domes during the Augustan
period they were familiar in principle with this device. However, what a dif erence
in practice existed between wooden centering spanning a few metres and set a
few metres above the ground for centering ashlar masonry vaults and arches and
centering arches or segmental arcs 20 m or more in diameter and set at similar
heights above pavement level. Furthermore in this connection it should be noted
that the i rst ashlar domes to be erected (again all of a few metres span) can not be
dated earlier than the Christian era; that is to say there is no historical priority in
the building of (small) ashlar domes over the building of (large) concrete domes.
In short in the construction of wooden centering for concrete domes Roman
h e Pan-
theon
Concrete
Dome
98
99
