Ancient masonry structures often rely on
the masonry arch as a load bearing element. The understanding
of its response under seismic actions is a
first fundamental step towards the comprehension of
the behaviour of more complex structures. It is well
known that the stability of masonry arches is primarily
related to the geometry. The safety assessment under
seismic actions is usually carried out by considering
known deterministic geometrical parameters, such as
thickness, rise and span, and the voussoirs are assumed
with equal dimensions. However, many factors, like
defects or irregularities in the shape of the voussoirs
and imprecise construction, produce variations of the
geometry with respect to the nominal one and, as a
consequence, may effect the ability of the arch to resist
seismic actions. In this paper, the effect of geometrical
irregularities on the dynamic response of circular
masonry arches is considered. Irregular geometries are
obtained through a random generation of the key geometrical
parameters, and the effect of these irregularities
is quantified by analysing the dynamic response
to ground motion. The masonry arch is modelled as
a four-link mechanism, i.e. a system made of three
rigid blocks hinged at their ends. The position of the
hinges at the instant of activation of the motion is determined
through limit analysis. Lagrange’s equations of
motion have been written for the generated irregular
geometries and solved through numerical integration.
The results are summarised by a fragility surface that
quantify the extent to which geometrical uncertainties
can alter the dynamic response of the masonry arch and
increase its seismic vulnerability