Reinforced concrete (RC) has been extensively used in the construction of long span box girder bridges. Box girder bridges are commonly designed to satisfy two criterions in terms of serviceability and safety. In order to ensure the serviceability requirement, it is necessary to predict accurately the cracking and deflections of box-girder bridges at working loads. To assess the safety of structures against failure, an accurate estimation of ultimate load is essential. Although experimental study provides good information about box-girder behaviour but they are time consuming and costly. Therefore, it is desirable to develop a more sophisticated and reliable numerical model to substitute the experiment testing of bridge. Because of the complex shape of box girder bridges, the prediction of elastic/ultimate load carrying capacity via analytical approaches is generally difficult. Therefore, the numerical means such as Finite Element Method (FEM) is often sought. Moreover, the analysis of reinforced concrete box girder is further complicated by several factors such as (ⅰ) Nonlinear stress strain response of concrete; (ⅱ) post-cracking softening behaviour of concrete; (ⅲ) softening due to excessive compression of concrete and (ⅳ) nonlinear bond behavior between concrete and steel. Reinforced concrete is considered as a type of heterogeneous, composite material. At the macroscopic level, it consists of two major components: concrete and steel reinforcement. In the modelling of its non-linear stress strain behaviour, a general dominant approach of representing reinforced concrete, consists of developing separate models for concrete and steel and combining these models at element level, through the addition of-constitutive matrices. In this paper an attempt has been made to carry out the non-linear analysis of box girder bridges by considering the post cracking behaviour. A degenerated shell element has been employed for the discretization purpose. Concrete in compression has been modeled using William-Warnke five-parameter model while the cracking phenomena has been numerically formulated using the fixed crack smeared approach.
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