BEHAVIOR AND OPTIMUM DESIGN OF STEEL CONCRETE COMPOSITE BRIDGE GIRDERS

Abstract

In the construction industry, concrete and steel are being extensively used for decades. The exhaustive usage of these materials has posed both design-related and environmental challenges. Therefore, the efficient use of natural resources requires a better understanding of structural behavior and structural optimization. The steel concrete composite (SCC) girders are found to be a better structural system when compared to RC girders and steel girders due to attributes of the components (i.e., the concrete component being good in compression and the steel component being good in tension) forming SCC girders. In the last few decades, SCC girders are being extensively used in building and bridge construction because of their advantages, such as higher strength to weight ratio, higher stiffness, higher ductility, full usage of materials, and speed of construction. However, these advantages are maintained, provided that the composite action is achieved between the two connecting components. There are two bounds of composite action viz; full composite action or full interaction and no interaction. In practice, the interaction lies between these two bounds and is called partial interaction. If the partial interaction is not taken into account during design, it will lead to serviceability issues, and therefore, the proper assessment of deformations of SCC girders with partial shear interaction becomes imperative from the design point of view. Continuous composite girders offer higher load carrying capacity, higher span to depth ratio, and reduced deformations when compared to simply supported girders. However, concrete over the internal supports is subjected to negative bending moment, which causes cracking in the concrete deck. The general practice of design of continuous SCC girders is to ignore the contribution of the concrete component over the continuous supports and assume the steel girder only to be effective in resisting the negative bending moment, which may lead to conservative designs. Therefore, the evaluation of the assumption of ignoring concrete contribution in the negative bending region becomes important from the design point of view. Due to the efficient utilization of materials, SCC girders provide more economical solutions, and these savings can be further advanced by employing optimization techniques in the design of composite members. Further, structural optimization has become an important tool for structural designers because it allows better material exploitation, decreasing the self-weight of structure and saving material costs. In the case of bridge girders, the cost of the composite girder increases due to an increase in self-weight. Further, the design of SCC girder involves a large number of variables compared to RC girder and steel girder alone, and merely satisfying the design criteria as prescribed by the national standards may not provide optimum girder proportions for a given set of loading. Therefore, it becomes essential to develop an optimum design procedure for the SCC bridge girders. In the present work, a simplified numerical model for the analysis of simply supported SCC girders has been proposed, which considers the partial shear interaction. The main objective of the study is to bring out the relative significance of the partial shear interaction with respect to full interaction through a parametric study. It is observed that the partial shear interaction has a considerable influence on the deformations of SCC girders. Therefore, the effect of partial shear interaction should be taken into account during the design. A simplified numerical procedure has been proposed to model continuous SCC girders, considering the concrete cracking effect in the negative bending region. Here the objective of the work is to study the influence of the uncracked portion of the cracked concrete on the behavior of continuous SCC girders. It is observed that the deformations of continuous SCC girder and negative moment carried by the steel girder can be reduced by using the proposed numerical procedure. An optimization procedure using a genetic algorithm has been developed at the element level (i.e., girder level) for the optimum design of SCC girders. A parametric study considering different design parameters has been carried out to study the influence of these parameters on the design trends of SCC girders. The design trends provide vital information such as variation in the cost of SCC girder, percentage cost shared by the components forming SCC girders, variation in the geometry of the cross-section of SCC girder for varying parameters. This information is helpful to the designers while designing SCC girders. Further, the proposed optimization procedure has been extended to develop at the system level (i.e., whole bridge deck), considering the design variables that govern the system level design.