Civil Engineering Reference
In-Depth Information
and this sometimes occasions confusion in references. In any event the nature
and disposition of both centering and shuttering in Roman concrete rooi ng have
occasioned dispute. Some reference has been made to this question (in Vol. 2, pp.
188, 189), and only a summary statement is presented here.
1.
Centering
Although the function of centering is constant, the construction to support a dome
40 m in diameter is of a dif erent order from one supporting an arch spanning
4 m. h e following remarks pertain to centering for expansive structures.
h ere are two systems for installing centering: standing centering and l ying
centering. Standing centering is held in position by vertical supports rising up from
the ground. Flying centering has no contact with the ground but is made to spring
from some sort of lodgement in upstanding walls. If the vaulted construction is
high above the ground, standing centering requires a great quantity of timber for
the uprights, and it obstructs circulation at ground level by a forest of timbers.
Flying centering leaves the l oor unimpeded and is more economic in the timber
required. However its construction is more demanding.
h ere is no installation necessary for installing standing centering. It provides its
own installation. It is assembled
in situ
, timber baulk by timber baulk, thus requir-
ing no hoisting of heavy burdens. On the other hand l ying centering can only
be installed in the form of prefabricated units. In the nature of things where the
span is great, these are ribs similar to bow string trusses (segmental trusses). Such
members for the centering of a large concrete dome constitute weighty burdens
(ca 5-7 tons), and require installations and lit ing devices to install them h us the
economy calculus for standing centering and l ying centering is not clear. Saving
in timber with l ying centering may be outweighed by demanding construction
with its own necessary installations.
Centering for wide span concrete roofing has been considered mainly in
connection with large domed monuments (e.g. the Pantheon), where in a
number of instances, much of the dome structure survives. In general it may
be said that little detailed attention has been given to the provision of “stand-
ing centering” for such monuments. At the middle of the 19th century Viollet
le Duc in his Dictionnaire (
s v
Voute) illustrated a project for l ying centering
structured on trussed segmental ribs. And this approach has remained inl u-
ential (v Adam, p. 176). That centering of this basic type was known in
Antiquity is well demonstrated by the Pont du Gard (ca 5 AD). Here projecting
corbels in the haunches of the arches etc. are clearly lodgements for l ying center-
ing (for a reconstruction v Adam, p. 176, i g 421).
It is regrettable that all the reconstructed drawings of such centering include
the i nished masonry structure together with the reconstructed centering. h is
gives a false impression suggesting that the centering is suspended or attached in
Standing
& Flying
Centering
400
401
401
