  This paper addresses the robust stability of a boundary controlled system
coupling two partial differential equations (PDEs), namely beam and string
equations, in the presence of boundary and in-domain disturbances under the
framework of input-to-state stability (ISS) theory. Well-posedness assessment
is first carried out to determine the regularity of the disturbances required
for guaranteeing the unique existence of the solution to the considered
problem. Then, the method of Lyapunov functionals is applied in stability
analysis, which results in the establishment of some ISS properties {}{with
respect to} disturbances. As the analysis is based on the \emph{a priori}
estimates of the solution to the PDEs, it allows avoiding the invocation of
unbounded operators while obtaining the ISS gains in their original expression
without involving the derivatives of boundary disturbances.
