Elasto-plastic out-of-plane buckling strength of through type and half-through type arch bridges are analyzed by a finite element method which is capable of considering finite displacements and yielding of materials. Examining the numerical results and using an analogy between an arch and a column, we propose a method to determine the slenderness parameter for arch bridges. A designer can use the slenderness parameter to obtain an ultimate stress for arches by substituting it into a column

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... gth formula. The predictions by the proposed procedure are shown to be fairly good correspondence with all of numerical results.