MODEL STUDIES OF DOUBLE-CELL BOX GIRDER BRIDGE WITH INTERMEDIATE DIAPHRAGMS

An experimental study on the influence of intermediate cross-bracing diaphragms on the behavior of a simply supported double-cell box girder bridge has been carried out. A perspex model was tested under various loading conditions and the test results, namely the displacements, cross-sectional distortion, longitudinal and transverse normal forces were compared with theoretical values suggested by Nimityongskul, Pama and Lee. In this analysis, the elements in the box section are treated as rectangular plates subjected to lateral and in-plane boundary forces. The end diaphragms are assumed to be infinitely rigid in and flexible normal to their planes. The intermediate diaphragm is assumed to act in such a way that it exerts only concentrated vertical and horizontal reactions on the joints of the box section without introducing resisting moments against joint rotations. Test results indicated that the distortion of the cross-section of a box girder without intermediate diaphragm is more prominent when loaded along the side-joints. With one intermediate diaphragm the distortion at the loaded section remains practically the same when the diaphragm is sufficiently away from the applied loads, but is considerably reduced when the diaphragm is near the load. The use of intermediate diaphragm decreases effectively the cross-sectional distortion, increases the overall stiffness of the bridge and redistributes the longitudinal normal forces. In general, the experimental values confirm the theoretical predictions on the influence of intermediate diaphragms on the load distribution in a double-cell box girder bridge. Careful considerations must be taken in designing these intermediate cross-bracing diaphragms to satisfy the assumptions made in the theory. /Author/