Effects of Antisymmetric Load Component on Collapse of Concrete Box-Girder Bridges

Space truss analogy and collapse mechanisms are the theoretical methods which are available at present to predict the collapse loads of single-cell concrete box-girder bridges. It is observed that of the two formulations, the one based on a collapse mechanism is found to be more versatile and better suited to box sections. The eccentric load acting on the top flange of a box girder may be resolved to symmetric and antisymmetric components. The antisymmetric component may again be resolved as torsion component and distortion component. However, no researcher has extended this concept to determine an equation for the estimation of collapse load. The present study proposes a theory for calculating the collapse load of single-cell concrete box-girder bridges considering pure torsion and distortion mechanisms. The box girder is treated as a folded plate structure subjected to the vertical and horizontal components of the load in the plane of each plate. It is assumed that plastic hinges are developed at the four corners of the box girder for distortion mechanism and the cross section causes pure twisting without distortion for torsion mechanism. The proposed pure torsion and distortion mechanisms are incorporated in the existing collapse mechanism method and the modified method is validated with the experimental results available in the literature. Estimation of the ultimate load carrying capacity of existing concrete box-girder bridges is discussed in this paper. Among the various simplified theoretical models available to predict the collapse load of single-cell concrete box-girder bridges, the collapse mechanisms theory yields the most accurate results according to the existing literature. To the existing, pure bending and distortion-bending mechanisms, two more mechanisms (torsion and distortion mechanisms) are proposed in the current paper to make the collapse mechanisms theory more conclusive. With the introduction of these two new mechanisms, the collapse load of existing single-cell concrete box-girder bridges subjected to unsymmetrical loading can be predicted by taking the lowest value obtained from: (i) pure bending mechanism; (ii) distortion-bending mechanism; (iii) torsion mechanism; and (iv) distortion mechanism. This will help the bridge engineers to ascertain the functioning of existing as well as new concrete box-girder bridges.