The bending moments and the torsional stiffness of trapezoidal longitudinal stiffeners are calculated according to the Pelikan-Esslinger method, neglecting the flexibility of crossbeams. In fatigue constraints, recently published experimental research results are taken into account. A computer program of the Rosenbrock mathematical programming method is used to determine the following optimum dimensions: thickness of the deck plate, thickness and heights of stiffeners and crossbeams as well as the distance between crossbeams. The optimization procedure is illustrated by a numerical example.