Model-based structural health monitoring using an updating scheme can detect, localise and quantify damage. The model updating is achieved by means of numerical optimisation, which requires a large number of finite element simulations to achieve convergence. The computational cost of this process can be lowered by reducing complex finite element models to numerically more efficient representations. In wind energy, slender shell structures like the tower structure and the wind turbine rotor blades are reduced to beam models for structural mechanics computations. The usual way to obtain distributed beam properties is to extract axial stiffness parameters as well as moments of inertia for several cross-sections along the length of the considered structure. This can be achieved analytically or by two-dimensional finite element calculations. In case of widely varying cross-sectional properties, the discretisation along the longitudinal axis of the structure has to be very fine in order to accurately represent its global behaviour.