In 1981 Wartsila (Turku, Finland) was commissioned the construction of three large catamaran type crane vessels. In the design of the vessels the determination of the wave loads acting on, and the strength of the bridge structure connecting the hulls received considerable attention. They were subject of a thorough investigation in the course of which theoretical calculations and model tests were performed at MARIN (formerly NSMB) in Wageningen, The Netherlands. The paper describes the design procedure for the bridge structure, which is based on a probabilistic approach. It also highlights some typical results of the calculations and tests.