Method for constructing girder using pre-flex load

The present invention invention to aim to be installed so as to be fixed but inverted central portion of the girder on the girders making process, followed by the grant Preflex load on the girder to offset the loads generated by the girder and then making girder weight increases the cross-sectional efficiency of the girder to be. Bridge girder and method for increasing the efficiency of cross-section by using a pre-flex effect according to the present invention includes the steps of mounting a mold steel girder (10a) for making a girder for 40 in the reverse direction (S10); The step of making the girder (10) on the basis of a steel mold (10a) for commercial girder (S20); And is characterized in that the flip-made girders (10) comprising the step (s30) for installation on a wall disposed in the forward direction.