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A modified Morison model is presented for calculation of hydrodynamic forces on aquaculture net cages. The model is based on a simple method for conversion of ”screen model force coefficients” to approximate equivalent directional dependent Morison coefficients. The motivation for this is that experimentally obtained force coefficients for net panels are generally presented as screen model force coefficients, while commercial analysis software are often restricted to a Morison model. Based on the screen model force coefficients defined by Løland’s formulas, the method is implemented in the nonlinear finite element program USFOS. Analyses with the modified Morison model are performed, investigating the viscous forces on a net panel for different inflow angles. The numerical results are benchmarked against Løland’s original screen model, showing good agreement for all inflow angles. Comparison with the classical Morison model is made to illustrate the advantage of the proposed method. The model is also applied for calculation of hydrodynamic forces on the net of a real aquaculture structure exposed to a steady current. Again, the results are compared to Løland’s original screen model, showing an almost exact reproduction of the global drag force.

A Timoshenko beam finite element model is formulated based on the Lagrange and Hermitian interpolations, in which the transverse shear deformation is explicitly evaluated. To account for the local inelastic buckling, the finite element formulation is coupled with a Smith-type progressive collapse method where the nonlinear responses of local structural members can be assessed. The effect of shear lag is accounted by an effective breadth theory. This coupled model enables an efficient prediction of the flexural behaviour of a thin-walled box girder under lateral loading. To demonstrate its capability, a case study is completed on a single-skin box girder under four-point bending. Equivalent finite shell element analysis is carried out for validation. The proposed method shows a close correlation with the shell element model in both strength and stiffness predictions. It is found that the effect of shear lag is significant in this case study, which substantially reduces the stiffness of the box girder.