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stress, before the development of large plastic strains and defor- mations shown in Figure 2. Shell plate thickness requirements are derived using ultimate strength criterion where the ultimate state is determined when plas- tic folding occurs due to perfectly plastic hinge formation. e local frames in side structures and bottom structures are to be dimensioned such that the combined eects of shear and bending do not cause the development of a plastic collapse mecha- nism. e plastic section modulus requirement is derived from an analytical energy method considering three limit-states shown in Figure 3, two of which involve shear and bending while the third one is purely shear. e Requirements Concerning Polar Class rig- orously treat bending and shear interaction by taking into account actual section shape in the calculation procedure. e application of an iterative procedure may be advantageous for the designer to optimize the frames for the shear requirement and section modu- lus requirement. e scantling requirements are provided for both transversely- and longitudinally-framed structures. Machinery requirements Part III of the Requirements Concerning Polar Class provides specic machinery requirements related to the strength of main propul- sion, steering gear, emergency, and other essential auxiliary support systems. Propeller ice interaction load formulas form the basis of the propulsion line component strength calculations. e load formulas were developed though extensive international research and development eorts which included numerical simulations of propeller-ice interaction validated with full-scale measurements and analyses of propeller damage service histories. e calculated loads are the expected, single occurrence, max- imum values for a ships entire service life in normal operation conditions. Design load formulas are provided for both open and ducted propellers and include the maximum backward and forward blade bending forces, blade spindle torque, propeller ice torque, and propeller ice thrust applied to the shaft. The propeller blades should be designed with respect to two overall limit states, namely extreme static and fatigue. e extreme criterion is based on the calculated maximum expected loads applied via nite element analysis with acceptance criteria for permissible stress levels. Propeller blade fatigue criterion is based on a load distri- bution for the ship service life and an S-N curve of the blade material. e propulsion line components should be designed according to the selective strength principle,? so that the rst damage does not cause signicant risk to the ships safety and other shaft line components. In most cases, the propeller is considered the sacricial component. Limitations Although the IACS Requirements Concerning Polar Class adopts many modern technologies, it should be considered as the mini- mum requirement for ice-capable ships for arctic operations. Some important issues which are normally addressed in other ice class rules are subject to the requirements of each of the classication societies. ese gaps include icebreaker notation, propulsion power requirements, scantling requirements for large structure members, inertial force for internal structures, ice loads for non-icebreaking bow forms, and ice loads for stern icebreaking, among others. e current ABS General Ice Class Rules adopted a modern ice collision model similar to those adopted in Requirements Concerning Polar Class since 1992 and they are widely recognized by industry as one of the most advanced ice class rule sets. To support the industry demand for the harmonization of the ice class requirements, ABS plans to withdraw the existing General Ice Class Rules and oer the ABS Polar Class Rules for all vessels operating in polar regions eective from 2012. To make the Polar Class Rules a complete set of ice class rules, the gaps in the IACS Requirements Concerning Polar Class and other necessary subjects will be supplemented by the applicable requirements in the current ABS General Ice Class Rules. e Finnish Swedish Ice Class Rules, also known as Baltic Ice Class Rules, will remain as part of ABS Ice Class Rules for ves- sels intended for sub-arctic operations. Novel ship designs e IACS Requirements Concerning Polar Class provides a basis for design but, depending on the novelty and scope of the design chal- lenge, may need to be supplemented with additional methods and data. For example, the industry has no experience operating LNG carriers in the Arctic, or the size of ships may be much greater than FIGURE . TYPICAL LOAD DEFLECTION CURVE FOR A FRAME SHOWING THE DESIGN POINT Development and Harmonization continued p? represents ice pressure p January 2011 www.sname.org/sname/mt (mt notes)