Sunday, May 13, 2012

No mechanism for failure

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.