Civil Engineering Reference
In-Depth Information
h e statics of the dome, on the contrary, resolve into a horizontal analysis. h e
units (voussoirs) are held in place by compression operating around successive
horizontal “parallels”. h us the elemental form of a dome is not a vertical semi-
circle like the arch and the vault, but is a complete horizontal circle (a ring). A
moment's observation of many domes makes this clear. h e vertical slice (the
meridian semi-circular arch) is not complete. It is interrupted at the crown by a
space, the oculus, let free for lighting or other reasons. In such case no compres-
sive force at all is transmitted along the vertical semi-circle, yet the dome remains
structurally stable.
Arcuated
stone
rooi ng—
structure
& con-
struction
296
(i) h e Vault and the Arch
In form the vault is simpler than the dome—i.e. it is curvilinear in one plane
only (the vertical), whereas the dome is curvilinear in two (the vertical and the
horizontal). For this reason the vault will be discussed i rst. As distinct from the
detail of form (e.g. semi circular, parabolic, pointed, etc.) vaults are distinguished
by their construction. h ere are basically three methods of constructing a vault:
out of a slab; out of corbels; out of radially set units (voussoirs).
h e initial and limitrophic case is a vault, or a seeming vault, created by hol-
lowing out the soi te of a beam or slab. h is is ot en done for aesthetic reasons
(or to give greater head height). However it raises the question of terminology. Is
it correctly termed a vault? It would seem a vault in appearance only, i.e. a vault
in form but not in function. h e beam remains a beam; it is stressed in bending,
in spite of its arcuated soi te.
A common method of constructing a vault is by corbelling. Each successive
course of masonry is made to project somewhat beyond the face of the course below.
h e extent of the projection is governed empirically by whether the projecting
blocks remain in equilibrium. If construction is carried on in this way from both
walls, eventually the projecting blocks will abut or can be spanned by a closing
block. In this way the space between the wall is covered by blocks shorter than
those required to span across the space directly. h e extent the courses project
beyond the course below will govern the height of the vault and the proi le will
be presented as a series of steps. h e projecting parts of the blocks will be stressed
in bending not in compression. However it is possible to dress the succession of
steps into a continuous curve of one form or another, so that the construction is
vaulted in appearance.
h e stone vault appears to have taken form in Egypt—and in interesting cir-
cumstances, which may evidence a conl ict between structure and symbol. One
of the earliest expressions of Egyptian monumental building in stone was the
massif—the pyramid. h e design of these monuments called for extended passages
263, 266
