As I discussed earlier, if masonry is considered from a
contemporary engineering perspective then three assumptions are made about it
(none of which are actually true).
First, it is infinitely strong in compression. Second, blocks (or voussoirs in an arch)
never slide against each other, they remain in fixed position. Third, masonry has no tensile strength. These three assumptions are useful to the
engineer in seeing how a masonry structure will respond to forces and how the
structure will bear its load.
This perspective of viewing masonry leads to some
interesting insight. Of particular
interest is the idea that for some arch forms there is no mechanism of
collapse. As I discussed here, masonry
arches collapse from hinges being formed at the intrados and/or extrados of an
arch; where the catenary thrust force line either touches or exits either the intrados
or the extrados.
As discussed by Jacques Heyman in his book The Stone Skeleton (Cambridge University Press, 1995) by applying the 3 assumptions of engineering for masonry with
the arch forms shown below, a hinge cannot be formed; buckling cannot happen
and a mechanism for collapse does not exist.
Of these arch forms, a unique application is that of the flying arch buttress. While Heyman notes "in passing" that this is the case and assigns little significance to the phenomenon (p.19), it seems quite significant and noteworthy to me.
A flying arch buttress cannot fail; something else has to
fail for the buttress to fail. For
example, in any known case of a collapsing cathedral flying buttress, it was either
due to settling of soft ground at the foundation, or repeated lightning strikes. No fault of the flying arch, which has no mechanism
for failure.
