Irregular structures have a more-complex behavior that it is not always considered during their analysis and design. In some design codes, irregular condition is assumed by means of factors that modified the response of simple systems to define the final transversal sections. Although some elastic analyses were developed, they do not always evaluate the effect of different irregular conditions in the dynamic response of bridges. In this work, irregular bridges were analyzed to define the influence of irregular conditions in the variation of dynamic properties, as frequencies and modal forms. For a regular bridge condition, a structure with three single piers by bent of the same size and four spans with equal dimensions were considered. To define irregular bridges, systems with superstructure and substructure irregular conditions were characterized: the first by means of structures with variations in the length of extreme spans, in comparison to the central length spans, in relations of 1:0.25, 1:0.50, 1:0.75, 1:1.25, 1:1.50, 1:1.75, 1:2; and for curved structures, with curved angles of 30° to 180°, every 30°. Substructure bridges were considered by changes in the length of central piers, in relation of extreme elements, in relations of ±0.25, 0.50 and 0.75. For regular and irregular structures, elastic dynamic properties and its variations were registered. Responses show that modal shapes are different when the central pier length is modified that when the extreme pier length is different. Results also indicate that as the length of the extreme spans or the angle of curvature of the bridges change, the modal forms and periods become more complex.
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