  It is well known that for higher order elliptic equations the positivity
preserving property (PPP) may fail. In striking contrast to what happens under
Dirichlet boundary conditions, we prove that the PPP holds for the biharmonic
operator on rectangular domains under partially hinged boundary conditions,
i.e. nonnegative loads yield positive solutions. The result follows by fine
estimates of the Fourier expansion of the corresponding Green function.
