STRUCTURAL DESIGN FOR ARCHITECTURE

Fig. 5.2 Failure of a vault. Vaults exert horizontal force
on their supports. If the supports are high walls, these
must be sufficiently thick not to overturn and have suffi-
cient bending strength not to be overstressed by the
bending moment caused by the horizontal force at their
top. If significant horizontal movement occurs at the tops
of the walls the vault is likely to fail in the manner shown
here.

structure,^2 such as a vault or a dome, is neces-

sary to achieve the horizontal span without

incurring high bending and therefore tensile

stress. The action of a vault produces horizon-

tal thrusts at the points of support, however,

which, in the case of a vaulted hall, are the

tops of the supporting walls (Fig. 5.2). The

walls are therefore subjected to a combination

of axial compression, due to the weight of the

vault, and bending, due both to the lateral

thrust of the vault and to any eccentricity

which is present in the transfer of its weight to

the wall. The configuration of the structure

must be such that the axial compressive stress

in the walls is always greater than the tensile

component of the bending stress. This can

only be achieved if the wall is of adequate

thickness. The very thick walls required to

Fig. 5.3 Evolution of the form of the vaulted hall.

(a) The earliest vaulted halls relied on very thick walls for

stability. The volume of masonry involved was very large.

(b) The use of the concept of the buttress allowed a degree

of stability to be achieved which was equivalent to that

provided by the solid wall but with a greatly reduced

volume of masonry.

(c) The cross-vault concentrates the load at the buttresses

and creates flat areas of wall above the springing point of

2 See Appendix 1. the vault which may be used as fenestration. 149

Masonry structures

(a)

(b)

(c)