  We investigate, at a laboratory scale, the collapse of cylindrical shells of
radius $R$ and thickness $t$ induced by a granular discharge. We measure the
critical filling height for which the structure fails upon discharge. We
observe that the silos sustain filling heights significantly above an
estimation obtained by coupling standard shell-buckling and granular stress
distribution theories. Two effects contribute to stabilize the structure: (i)
below the critical filling height, a dynamical stabilization due to granular
wall friction prevents the localized shell-buckling modes to grow irreversibly;
(ii) above the critical filling height, collapse occurs before the downward
sliding motion of the whole granular column sets in, such that only a partial
friction mobilization is at play. However, we notice also that the critical
filling height is reduced as the grain size, $d$, increases. The importance of
grain size contribution is controlled by the ratio $d/\sqrt{R t}$. We
rationalize these antagonist effects with a novel fluid/structure theory both
accounting for the actual status of granular friction at the wall and the
inherent shell imperfections mediated by the grains. This theory yields new
scaling predictions which are compared with the experimental results.
