Sectional load effects derived from strain measurements using the modal approach

Abstract It is not possible to measure cross sectional load effects (i.e. bending moments and shear forces) of ships directly. In complex structures, the sectional load effects must be derived from strain measurements. Knowing these load effects is important because they do determine the necessary overall structural strength of the ship, both in ultimate load but also in fatigue loads. Traditionally, load effects in specific prismatic shaped sections are derived using strain gauges placed in the section itself. An alternative approach is to derive the load effects in a cross section using measurements of the global strains across the length of the vessel. When this derivation of load effects is based on the first few global flexural vibration modes of the ship, this method is referred to as the modal method. This paper explains both traditional and modal methods and applies them to a simple barge and a more complex ship structure. Experimental data from full scale trials and model tests are used along with numerical simulations to compare the methods. Although the paper focuses on the sectional load effects, it also shows that the local strains can be calculated through the modal approach. The modal method gives better results and has more advantages compared to the more traditional sectional method in ships with complex configurations and effective house structures.