Deflection of unbonded partially prestressed concrete continuous beams

Abstract

Continuous beams are preferred to simply supported beams because of economy, fewer expansion / contraction joints and possible benefits from moment redistribution. In the design of unbonded partially prestressed concrete (UPPC) continuous beams, it is necessary to estimate their deflections under service loads in order to satisfy the requirements of serviceability limit state. A method is developed to convert the cross sectional area of unbonded prestressed tendons to the equivalent cross sectional area of non-prestressed steel. Then the moment of inertia of cracked section as well as Branson’s effective moment of inertia in a UPPC continuous beam can be easily determined. The computed deflections are compared with some available experimental results, including beams with external unbonded steel tendons and those with external unbonded aramid fibre reinforced polymer tendons. The proposed method gives satisfactory predictions of deflection till the yielding of non-prestressed steel. Another equation for moment of inertia of cracked section, which was originally suggested by the PCI design handbook for bonded partially prestressed concrete beams, is also evaluated in the study. In most cases the PCI equation can also give satisfactory results but in some cases its discrepancy of deflections is larger than that of the proposed method. Compared with the method recommended in the current Chinese Code, the proposed method is applicable not only to members with the conventional high-strength steel prestressing tendons, but also to those with tendons made of other materials such as fibre-reinforced polymer.