Ultimate stress increase in unbonded tendons in prestressed concrete beams

Since the assumption of plane sections cannot be applied to the strain of unbonded tendons in prestressed concrete beams subjected to loadings, a moment-curvature nonlinear analysis method is used to develop analytical programs from stress increases in unbonded tendons at the ultimate limit state. Based on the results of model testing and simulation analysis, equations are proposed to predict the stress increase in tendons at the ultimate state in simple or continuous beams of partially prestressed concrete, considering the loading type, non-prestressed reinforcement index beta (p), prestressing reinforcement index beta (s), and span-depth ratio L/h as the basic parameters. Results of 380 beams studied here and test results for 35 simple beams obtained by the China Academy of Building Research were compared with those from prediction equations given in codes and other previous studies. The comparison reveals that the values predicted by the proposed equations agree well with experimental results.