Civil Engineering Reference
In-Depth Information
Only a signpost to this ramii ed subject can be given here. Brick rib arches were
incorporated into all forms of arcuated concrete rooi ng: barrel vaults, cross vaults
and domes. h ey were not an original component, but their use developed during
the later stages of concrete rooi ng (3rd and 4th centuries AD). In this fashion it
must be assumed that their introduction and use was prompted by experiment and
that Roman builders recognised brick ribbing to possess functional virtues.
h e obvious theoretical question is whether brick rib arches were reinforcing
to a load bearing concrete structure or whether they constituted in themselves a
framed structure, so that they transmit the loads while the concrete is simply ini ll
panelling. To this question there is an immediate response that sometimes the rib-
bing was designed so that it could have functioned as a framed structure, cf h e
Temple of Minerva Medica (L.C. Lancaster, pp. 201-02; pl XI); and sometimes it
was designed so that it could not have functioned as a framed structure, cf “the
Tor de' Schiavi” (L.C. Lancaster, p. 99). h e evidence spoken of here is the disposi-
tion of the arches (or the major arches) so that they discharge onto the solid piers
of the supporting structure and are not directed above the weaker parts, e.g. the
niches. For a survey of this question v L.C. Lancaster,
Concrete Vaulted Construc-
tion
, Chap. 5; cf R. Mainstone, p. 119).
h is evidence, of course, does not necessarily establish that the brick ribbing in
practice did function as a framed structure, Adam (pp. 194-198) believes that it
did, at least to a signii cant degree; and that it gave an extra stif ening and rigidity
to the structure. h e only way to determine the question would be to subject the
structure to some form of stress scanning. Certainly 18th & 19th cent drawings of
the Temple of Minerva Medica (when the rib arches survived intact but much of
the concrete fabric had fallen away) gives the very image of a framed structure.
When the inset ribbing does not constitute a framed structure, but is reinforcing
to the load bearing concrete, an interesting question arises in view of the proper-
ties of Roman Concrete. Does the reinforced construction constitute a compound
structure or a mixed structure? A compound structure signii es that the concrete
and the brick elements have become unii ed and react as one substance to stress;
a mixed structure indicates that the concrete and the brick elements retain their
individual properties and behave separately when reacting to stress. Again this
question demands close investigation of the individual circumstances.
h e above outline indicates that the brickwork inset into concrete rooi ng serves
to canalise and/or better resist compressive stresses set up in the concrete. How-
ever it is not in the nature of domes to fail in compression. Concrete like stone is
strong in compression but weak in tension, and concrete domes generally fail in
tension so that the haunches spread and thrust outwards. To reinforce the tensile
strength of a dome it is necessary to inset within the tension zone a circumferential
hoop of material strong in tension. Brick is weaker in tension than concrete and is
Inset
brickwork
412
413
412
