Load distribution factor for moment of composite bridges with multi-box girders

Abstract

Steel–concrete composite bridges with multi-box girders have been widely used in bridge structures owing to their advantages in terms of construction and mechanical behavior. The American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) provides a formula for calculating the load distribution factor (LDF), which is a key parameter in determining the internal force distribution in multi-girder bridges under live loads. To evaluate the performance of the AASHTO LRFD formula, the rigid-jointed girder (RJG) method is employed to obtain the transverse distribution influence line of each girder by considering the influence of the transverse connection beams, which is validated by field tests and finite element analysis. Based on the traversal algorithm composed of rough and refined calculations, LDF for moment can be obtained using the transverse distribution influence lines. In addition, information on 120 bridges, including 99 bridges in the literature and 21 actually built bridges designed or recommended by our research group, is collected for parametric analysis. On the basis of the parametric analysis, the relationship between LDF and the key parameters is determined, which helps derive an improved formula to calculate the LDF. Moreover, approximately 58% bridges’ errors exceed 10% based on the AASHTO formula, some of which even reach 30%. Compared with the AASHTO formula, only 5% bridges remain whose error exceeds 10% based on the improved formula for exterior girders. The improved formula for interior girders is the same as the former which is safe and precise because more than 80% bridge bridges’ errors are less than 10%. To sum up, the improved formula is found to be superior with higher accuracy and solid physical foundation.