Evaluation of ultimate strength and history of collapse of structures in their intact and damaged conditions is one of the key issues in advanced design techniques based on explicit safety evaluation and collective optimization. This paper describes an efficient method, the idealized Structural Unit Method, to evaluate ultimate strength of structures. Its application to offshore frame structures ~s presented taking account (If the nonlinear behavior of members, joints and the structure as a whole. Methods to account for strain hardening, damage effects and local shell buckling arc summarized. Example are presented to demonstrate the accuracy and efficciency of this method.


Traditionally, structural design has been based on providing a higher structural capacity at first failure (such as buckling or yielding) than the anticipated load multiplied by a certain factor of safety. Loads considered are those of frequent occurrence, while extreme loads were not explicitly dealt with. Safety has been taken into account implicitly based on the value of the factor of safety and past experience with similar structures. With the trend towards building ever larger structures and devising different structural layouts to suit new types of structures in different environments, safety could not be evaluated based on solely past experience. At present, two main streams of safety assessment may be recognized. A Many designers also show a great interest in the history of collapse in order to design efficient layouts of structural members with ample redundancy. Effects of damage, in the form of fatigue cracks or deformation caused by accidental loads (such as dents on offshore structures caused by collision) are also gaining a lot of attention not only for the purpose of evaluating the safety and serviceability of a damaged structure, but Also for integrated design/inspection/repair optimization at the design stage.

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