Continuous-span, cast-in-place box girders have been popular in modern bridge construction. Secondary moments due to prestressing in continuous-span, post-tensioned girders, however, have significantly complicated the structural analysis and design of the girders. The equivalent load method is a commonly used method in the analysis of continuous-span, post-tensioned concrete girders since the method reduces the analysis of a prestressed structure to that of a nonprestressed structure in which the consideration of secondary moments is not required. The basic concept of the equivalent load method is that the effects of prestressing are replaced by equivalent loads produced by the prestressed tendon along the span of the structure. The approximate equivalent load method significantly simplifies the procedure for the computation of equivalent loads for post-tensioned concrete girders with parabolic tendons and therefore has commonly been used by structural engineers. In this paper, three examples of simply-supported, posttensioned concrete girders with various combinations of locations of the centroid of tendons (c.g.s.) and the centroid of concrete (c.g.c.) are demonstrated to verify the accuracy of the approximate equivalent load method. Finally, an example of the analysis of a bridge composed of a continuous-span, post-tensioned concrete box girder superstructure and a concrete pier is also demonstrated using the approximate equivalent load method. Inconstant cross sections (inconstant c.g.c, lines) near the pier of the bridge are considered in this example.
Hsiao, Jen-kan K. and Jiang, Alexander Y. "POST-TENSIONED BOX GIRDER BRIDGE An Analysis Approach using Equivalent Loads." International Journal of Bridge Engineering 4, No. 1 (Jan 2016): 27-55.