The ultimate moment of resistance of unbonded prestressed concrete beams

Abstract

Synopsis The method set out in CP 115: 1959 for the determination of the ultimate moment of resistance of unbonded prestressed concrete beams is simple but inaccurate. Alternative methods have been proposed which are more tedious to use, but because (in common with CP 115) they fail to consider.a factor which proves to be of importance, namely the span/depth ratio of the beam, the accuracy of prediction is little better. This is demonstrated by the results of tests on 38 beams, which are then used to investigate the validity of a revised theory. A substantial increase in accuracy of prediction is obtained, and this is confirmed by the results of 19 independent tests. Proposals are put forward for a safe design approach. The equations proposed predict the ultimate moment of resistance of the 57 test beams as having a mean value of 0·995 Mtest, and a coefficient of variation of 4·7%.