Variation of Secondary Moment and Moment Redistribution in Prestressed Continuous Beams with Unbounded Tendons

Abstract:

An experimental study was conducted on four prestressed concrete continuous beams with unbonded tendons over entire loading. Support reactions and degree of moment redistribution at critical sections were analyzed. By secondary moment definition in post elastic range, total moment of a continuous beam in nonlinear stage could be decomposed into secondary moment and moment due to external forces, the variation of which over entire loading was studied. Based on revising the initial secondary moment and elastic external load moment separately, a redistributed moment formula for prestressed concrete continuous beams with unbonded tendons at the ultimate limit state were proposed. This formula was verified using a group of test specimens from literature. Results show that, the reason of moment redistribution in prestressed concrete continuous beams with unbonded tendons can be attributed to the stress increase of unbonded tendons and secant stiffness comparison charge between each part of the continuous beam. At the ultimate limit state, the higher global reinforcement indices at the section over inner support, the higher secondary moment reduction factor and the lower external load moment modulation factor. In the prestressed concrete continuous beams with unbonded tendons, the calculated moment of the section over inner support by the proposed formula is closest to the test results than the existing formulas. The proposed formula can provide a reference for the development of design code in relevant provisions.