Concept of prestressing secondary moment in nonlinear stage and its application in moment redistribution.

The variation of secondary moments in post-elastic range, including whether the secondary moment shall be considered in ultimate limit state calculations, has been a subject of much controversy for many years. Towards a better solution to this problem, the concept of secondary moment due to prestressing is extended into nonlinear stage. By using the method of stiffness variation, fifteen prestressed continuous beams with bonded tendons were numerically simulated in which the ratio of initial secondary moment (M2) to ultimate elastic loading moment (Me) was 0. 15-0. 36. Based on the extended secondary moment concept, a formula for moment redistribution at the ultimate limit state was put forward with relative depth of compression zone (ξ) and M2/Me as the influencing parameters. A test specimen (ξ = 0 .3 5) from literature was analyzed to compare the proposed formula and existing formulas for moment redistribution. A two-span prestressed continuous beam (ξ = 0. 18) with large degree of moment redistribution was designed and tested under monotonic loading whose support reactions were measured and used in checking the prediction by the proposed formula. It is shown that the proposed formula is conservative and gives predictions that are closer to the test results than the existing formulas. [ABSTRACT FROM AUTHOR]