The increasing demand of higher and higher speeds of ferries has been developed in recent years leading to a target ship mission profile: transport of cars and passengers along routes of about 150 / 200 nautical miles. In this context the University of Genova, Napoli and Trieste, in a joint research project, started studying some trimaran concept ships, competitors of the existing monohull high speed passenger ferries. Many investigations have been carried out about hydrodynamic and sea keeping performances aiming to improve speed and comfort of passengers. However, some of the structural design problems are still pending. As a matter of facts, in last years Classification Societies succeeded in developing quite complete codes suggesting simplified but reliable formulations for the scantling of fast units structures; High Speed Craft codes (HSC) are at present widely used for new monohull and catamaran passenger vessels. Nevertheless trimaran ships are not included and the application of HSC rules to this kind of vessels appears unfair without considering substantial differences in terms of ship’s motions and structural resistance definition. Direct calculations should then be considered for this kind of vessels particularly for the definition of the resistance of the whole structure which should take into account longitudinal, transversal and horizontal bending moments together with longitudinal and transversal torque acting on the three hulls. The definition of these loads can be performed with accuracy by using three-dimensional seakeeping analysis based on panel methods. In a preliminary stage simplified methods based on quasi-static wave approach can be sufficient for the definition of complete, three-dimensional load sets and the scantling of the main structural elements of the main and side hulls. The structural resistance can be assessed by finite element models of the structure studied under the action of the previously calculated load sets. A refinement of the model can be carried out and studied in the worst load condition. The aim of this paper is to describe the application of the above procedure to a large trimaran fast ferry. In the research framework the complete structural design of the vessel has been obtained.