A PC beam was made and different prestressing force was inflicted to the beam to research the relationship between the prestressing force and the short-term stiffness of the full-PC beam and the partial-PC beam. Under all levels of prestressing force, a concentrated force is exerted at third point. At the same time, the deflection of this beam was collected under different kinds of concentrated load value. Through the calculation and analysis of the calculated deflection value, it is found that the short-term stiffness of the beam increases with the increasing of the prestress, which leads to a certain deviation from the calculation formula of the short-term stiffness of PC beams in which, no cracks, are stipulated in Code for Design of Concrete Structures and Specification for Design of Highway Reinforced Concrete and Prestressed Concrete. In order to calculate the deformation performance of the PC beams more accurately, the paper suggests that it needs to take the influence of prestressing force into account in the calculation of short -term stiffness.
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