INVESTIGATION OF THE BEHAVIOUR OF CURVED BOX GIRDER BRIDGES UNDER 10K STATIC AND DYNAMIC LOADUJGver

Abstract

Reinforced concrete single or multicell box girder bridges appear at present to be the most popular construction for curved highway bridges. The analysis of a curved box girder having single or multiple cells gets complicated due to deformability of the cross section and the arbitrari ness of the applied loads. The adoption of a box section for curved bridges is still an exception in India. This is as much due to the unfamiliarity with this form of construction as due to the absence of readily available design data. Since the behaviour of box girder bridges is complex to investigate and since the construction engineer in India has yet no access to even moderately sized computers, there is a general tendency to disregard the claim of this type of super-structure in favour of ordinary open-ribbed type construction, in spite of the acknowledged superiority of the former especially for curved bridges. The present project has been undertaken with the dual purpose of understanding the behaviour of box girder bridges curved in plan under static and dynamic loading and of present ing enough data in a suitable form that facilitates analysis and design of such bridges. Two computer programs, STAANA and DYNANA, have been developed, one for static analysis and the other for dynamic analysis of curved box girder bridges. Altogether eighteen structures obtained by various combinations of three span -Vllllengths (ifOm, 30m, 20m),three radii of curvature (l50m, 90m, if5m) , and two span/depth ratios (20,15) have been analysed. The Program STAANA computes bending moment and shear force in individual girders, transverse bending moment, and vertical deflection under webs at/near midspan due to a 'unit' load at midspan having nine different eccentricity ratios. The ratio of these design quantities to the corresponding quantities of a simplified structure, have been presented as design coefficients. These coeff icients, four in number, are named as coefficient CI for bending moment in girders, coefficient C2 for shear force in girders, coefficient C3 for transverse bending moment, and coefficient Clf for vertical deflection under webs. They provide a simplified method of analysis and. design of curved box girder bridges, either simply-supported or continuous, subjected to Indian Roads Congress loading. The coefficients are presented in a tabular form and also graphically for easy interpolation for a structure with given span length, radius of curvature and span/depth ratio, within the range considered. The effect of different parameters on percent sharing of tending moment by individual girders has been studied. The program DYNANA computes natural frequencies,mode shapes and dynamic response (vertical deflection) of a curved box girder bridge. The effect of different parameters on the fundamental natural frequency of a structure has been studied. -IXThe dynamic response is calculated at midspan when a 'unit' load, having nine different eccentricity ratios and moving with constant speed, is also at midspan. The ratio of this dynamic response to the corresponding static response, has been presented as dynamic factors, which provide an opportunity to incorporate the dynamic effect of a moving vehicle in the design of a smooth-surfaced curved box girder bridge. These factors are tabulated and presented graphically for easy interpolation for a structure with given span length, radius of curvature, and span/depth ratio. The effect of span, radius of curvature, and span/depth ratio on dynamic factors for individual girders has been studied. The effect of speed of the moving load on maximum dynamic factors for individual girders has also been investigated. Finally, from the large amount of data necessary for the preparation of this report, pertinent observations have been made and relevant conclusions drawn regarding behaviour of curved box girder bridge structures under static and dynamic loading.