Keywords: box girder, spatial analysis, concerned stresses, spatial grid model, reinforcement design, membrane element.

Durability and sustainable development of bridge structures is an important problem in the engineering field. Avoiding and handling structural cracking is one of the important components to ensure durability and security of bridge structures. This objective can be obtained through structural spatial analysis, understanding and bettering reinforcement design methods in current codes, and a unified approach connecting structural analysis and reinforcement design.

Due to the limitations in current codes, which are based on slender beams, the corresponding stresses only consist of normal stresses in the top/bottom flanges and the principle stress in the web. In fact, these three stresses are only suitable for slender beams (i.e. rectangular or T section beams) and cannot reflect the real structural behaviour of complex bridges; in other words, there are some deficiencies in the conventional structural analysis and reinforcement design in the current codes.

In fact, for box girder bridges, the top and bottom plates are also subjected to in-plane two-dimensional principal stresses. Two-dimensional stress in the top and bottom plate should also be checked as a one-dimensional stress, which means all the concerned stresses should be considered during the reinforcement design of concrete structures. In addition, there are no provisions for shear reinforcement design in the membrane element at the top and bottom flange, and reinforcement design for the web is imperfect in current codes.

For the concrete structures, the basis to connect structural analysis and reinforcement design can be shown as follows: obtaining all the concerned stresses and complete the reinforcement design methods for all the concerned stress.

For the models often adopted in structural analysis: plane analysis and quasi-spatial analysis can not completely solve the statically indeterminate effect in structures, so simplification methods and different coefficients are introduced; the block element is more suitable for local analysis and has little relationship to reinforcement design. Though comparing with other finite element analysis model, a unified model, a spatial grid model, is proposed, which can analyse all the spatial effects clearly, including shear lag, statically indeterminate shear flow inside box girders, and produce all the concerned stresses that meet the requirement of reinforcement design. The analysis results are reinforcement design oriented, and such an analysis method connects the structural analysis and reinforcement design.

The spatial grid model is recommended by the authors as a reasonable model that connects the structural analysis and reinforcement design in concrete structures. It is hoped to play a beneficial role in the durability and sustainable development of bridge structures.

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