This paper deals with simple structures whose critical loads are infinite, but in the case of small changes in their geometry the critical loads become finite. Their equilibrium surfaces are analyzed, using large displacement theory. Since equilibrium surfaces have singular points, well-known methods of analysis; e.g., catastrophe theory, are not directly usable for the topological description of the neighborhood of a critical point. This is because the function that defines the equilibrium surface has no Taylor expansion at the singular point. It is shown that the Laurent expansion of the function is usable for investigation of the neighborhood of a critical point.
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