SIGNIFICANCE OF DISTORTIONAL ANALYSIS IN DESIGN OF BOX GIRDER BRIDGE

Abstract

The use of concrete Box Girder in highway bridge construction has proven to be a very efficient structural solution. The advantage of the hollow box section is that the material is efficiently used in bending, torsion, and shear. Earlier, more conservative box girder designs employed large web thicknesses, for which Simple Beam Theory and St. Venant's Torsion Theory were adequate analytical tools. However, as bridge spans have increased, to reduce dead load and prestressing forces a thin walled box section is desirable. With reduced flange and web thicknesses, it becomes necessary to take in to account distortional and warping effects. The magnitude of the forces and stresses due to distortion introduced by eccentric loading is significant, and cannot be neglected. In the present work the effect of various parameters like span of bridge, thickness of deck slab, bottom slab and web, span to depth ratio on distortional and warping stresses are studied for single cell concrete box girder bridge. Distortional analysis is carried out by using Beam on Elastic Foundation analogy. BEF models area developed for different simply supported spans from 25 to 50m. STAAD Pro is used for all analysis purpose. Numerical study shows that warping and distortional stresses are significant for smaller thicknesses of flanges and web. It is also observed that distortional and warping stresses little decrease with girder span for constant span to depth ratio however the variation in stresses with girder span is marginal. The distortional and warping stresses increase linearly with increase in span to depth ratio hence girder depth should be kept more from distortion considerations. The thickness of deck slab and bottom slab helps in reducing stresses due to distortion but the reductions is less significant for greater thicknesses. Web thickness reduces warping stress negligibly and it does reduce distortional stresses here also the reduction in stress is marginal for greater thickness. Further it is observed that increasing thickness of various elements result in decrease in stresses up to certain limit only and after that there will be increase in overall stress value as the dead load stress becomes very high. Based on this study proposal on thicknesses of deck slab, bottom slab and web is made.