Although the preponderance of studies of iceloadings on structures in sub-Arctic and Arctic regions have been directed towards determination of global forces and moments acting on the structure, recent investigations have identified local concentrated forces as imposing unprecedently high shear and flexural loads on the slabs or shells that form the structure's walls. Hard points of very strong ice embedded in a ridge or ice island fragment reportedly may exert pressures from 4 to 8 N/mm2 (580 to 1160 psi) over areas of limited extent.
The resultant shear and flexural forces would appear to require excessive wall thicknesses, up to 4 meters for typical configurations, if the resistance is calculated by conventional code formulae.
Recent research in The Netherlands and Norway on concentrated impact forces exerted by accidental contact between supply boats or barges and shafts of offshore platforms, and in this country on the end slabs of proposed nuclear reactor vessels shed light on the complex response of a thick shell or slab. Of particular importance is the formation within the wall after initial flexural cracking has occurred, of an "internal arch."
Afinite element method has been developed to permit a step-by-step evaluation of the change in response as flexural cracks develop, and is correlated with the data available from published tests.
In addition to the provisions of concentrated mild and prestressed reinforcement to resist the flexural stresses imposed, four methods are consideredfor developing the required resistance:
Steel fiber reinforcement.
Through-wall stirrups.
Composite steel-concrete sections.
Through-wall prestressing, similar in concept to rock bolt
The analyses and evaluation indicate that the use of through-wall prestressing is an extremely effective and practicable means for providing the required resistance to concentrated loads imposed by Arctic ice.
Structures constructed in Arctic and subArctic seas are designed to resist the force of seaice in its several forms. Sea ice features include sheet ice, annual ridges, rubble fields, multi-year ridges, ice island fragments, and ice bergs. When these features press against the structure, they exert not only a global force tending to slide oroverturn the structure, but may also exert very high local concentrated loads, (Figure 1). These loads may occur with impact, during the initial encounter of the ice feature with the structure, and subsequently may have a dynamic component as the ice crushes.
Local ice strengths (ref. 1) have reportedly been measured up to 10 N/mm2 (1450 psi). However, for such strengths to be effectively exertedagainst the wall of a structure, the hard point must be supported by the matrix ice behind.
