A Practical Method for the Calculation of Secondary Prestress Moments

This paper presents a practical method for calculating the secondary moment due to prestress of a continuous, prestressed concrete beam or frame. The method involves the determination of influence lines due to a moving unit load and a moving unit moment, which can be easily obtained by any commercial software package. Conventional beam-type shape functions are then utilised to compute the bending moment distribution corresponding to the influence line due to unit load. Secondary prestress moment, which is given by the integral of the product of this distribution and the primary prestress moment distribution, is subsequently carried out using a closed-form expression. The method is conceptually simple and easy to implement, and it does not involve numerical integration, complicated mathematics or special computer software. To demonstrate the accuracy and efficiency of the method, a few numerical examples are used to compare with results obtained using equivalent load method.