Aim of this study is the development of an integrated methodology able to support the structural design of an UAV demonstrator, having a joined-wing configuration and a high structural flexibility. HAPD (High Altitude Performance Demonstrator) is an UAV multi-mission, very light vehicle under development at CIRA, the Italian Aerospace Research Centre. HAPD structure is redundant as regards constraints, so the internal forces depend upon the stiffness distribution. In addition the evaluation of flight loads has to be performed without neglecting the flexibility of the structure. Because of the extreme unconventionality of the configuration the design of the primary structures has been widely affected by aeroelastic analyses. The final output of such a methodology (fully developed in-house) consists of all primary structures stiffness distributions (fuselage, wings and vertical tail unit) compatibly with the absence of any aeroelastic instability and structural failure under operative loads. The results show how a very light joined-wing A/C can be designed just performing an integrated methodology that combines the aeroelastic and the structural analyses.