A semi-closed section is formed by attaching bottom plates of composite slabs to U-shaped steel girders under erection of a composite box girder bridge. In order to predict deformation in this structure system, it is necessary to evaluate the additional stiffness of the plates given to the girders. However, the present situation is that detailed examination is not performed since there is no commercial based software for analysis. The purposes of this study are to evaluate the stiffening effect of the bottom plates of composite slabs precisely and to propose the method for predicting deformation under erection of U-shared steel orders with sufficient accuracy in a short time.
