Load-path modeling of pier diaphragms under vertical shear in concrete box-girder bridges

摘要

Pier diaphragms are usually adopted in concrete box girder bridges to transmit shear forces from the webs to the bearings. The pier diaphragm under vertical shear acts as a side-shearing loaded deep beam that is rarely studied in the literature. Through the load-path modeling, this paper provides an analytical method for the design of pier diaphragms under vertical shear in concrete box-girder bridges. Explicit equations are derived for the direct calculation of transverse tension forces in the pier diaphragms, which are critical for the determination of the amount of transverse reinforcement. Further comparative studies in this paper show that: (a) the side-shearing loaded deep beam differs greatly with the top-loaded beam in the aspect of elastic internal force distribution when the span-to-depth ratio of the diaphragm section is less than 1; (b) the proposed quantitative strut-and-tie models agree well with the elastic internal force distribution in the pier diaphragms with various configurations; and (c) the commonly used "direct-strut model" is acceptable for the design of pier diaphragms with span-to-depth ratios greater than 1.5, while it would greatly underestimate the top tension force when the span-to-depth ratio is less than 1 (referred to as the "ultra-deep member").