Static and dynamic response of a structure subjected to ice forces – Evaluation of a lighthouse overloading event

Abstract

Exploitation of areas and natural resources in arctic and sub-arctic areas makes guidelines for designing structures exposed to ice-forces a necessity. Ice actions on a structure include both static and dynamic components, and methods to calculate the magnitude of the ice loads are given in several common design codes. The static load component is constant and dependent on structure geometry and ice thickness, while dynamic loading is given in the design codes as time varying forcing functions.

In the winter of 1985 Björnklacken lighthouse, located north in the Bothnian Bay, was overloaded by ice forces and displaced along the seabed. A numerical model has been created using the FEA software package ABAQUS to determine the static response and the structural properties of Björnklacken. The structural properties have further been used in the analysis of a single degree of freedom (SDOF)-system to determine dynamic response.

The static and dynamic ice load components given by common design codes have been applied to both the numerical model and the SDOF-system. Initial calculations revealed large differences between the predicted loads from the different codes. Dynamic analysis showed that the response caused by a harmonic forcing function was significantly higher than that which was caused by a sawtooth forcing function.

Results also showed that the amplitude of the dynamic forcing function is reduced if the structure’s velocity at loading point is scaled as a ratio of the ice velocity. The reduction is more severe with lower damping, resulting in higher reductions in systems with low damping fractions. Given the close relation between velocity at waterline and dynamic response, a recommendation is that guidelines for velocity scaling should be included in all of the design codes.