  We introduce a new measure for the stability of structures, such as the
cross-section of the deck of a suspension bridge, subject to a 2D fluid force,
such as the lift exerted by a laminar wind. We consider a wide class of
possible flows, as well as a wide class of structural shapes. Within a suitable
topological framework, we prove the existence of an optimal shape maximizing
the stability. Applications to engineering problems are also discussed.
