ULTIMATE STRENGTH OF PRECAST CONCRETE MEMBERS ASSEMBLED BY UNBONDED TENDONS

Abstract

　In arch model, stresses in longitudinal steels are supposed to be constant along the length of the steels, and hence to investigating the shear transfer mechanism of the arch model, concrete members having unbonded longitudinal steels have been tested. Consequently, the arch model has been applied to the prediction of ultimate shear strength of precast concrete members assembled by tendons.

　This paper analyzes test results in past investigations on mechanical behavior of members having unbonded

longitudinal steels, and it is found that:

1) The modes of failure are classified flexural compression failure and diagonal tension failure.

2) The distributions of concrete compressive strain are consistent with the assumption of plane sections remains plane, and those corresponding to the diagonal strut in the arch mechanism are not found, in the members that fail in the diagonal tension as well as those that fail in flexural compression.

3) Under the same amount of longitudinal reinforcement and concrete compressive strength, the ultimate bending moment of the test beams are the same whether their modes of failure are flexural compression or diagonal tension.

　These findings provide the evidence that the mechanical behaviors of the members having unbonded longitudinal

steels are governed by flexural theory, and their ultimate strength should be expressed by ultimate bending moment.

　Based on the evidence mentioned above, equations using a lower bound theory of plasticity are proposed for predicting ultimate bending moment of the precast members assembled by unbonded tendons. Comparisons are made between the measured, Meu and estimated, Mu ultimate bending moment using data from 64 test specimens with unbonded longitudinal steels. Forty two out of the 64 test specimens fail in the flexural compression and remaining specimens fail in diagonal tension. The average of Meu / Mu and the coefficient of variation are 1.016 and 7.56%, respectively and the probability that Meu / Mu > 1.2 or Meu / Mu < 0.8 is 0% with respect to the 64 specimens. The average and coefficient of variation with respect to the specimens that fail in flexural compression are 1.024 and 7.71%, and those for the specimens that fail in diagonal tension are 1.00 and 6.91%. These statistical results show that

1) The proposed equations have high accuracy.

2) The average and the coefficient of variation do not depend on the modes of failure.