Need for Diaphragms in Concrete Box Girders

The action of diaphragms in single-cell concrete box girders is covered with particular reference to torsional loading. A simple analysis of the distortion of boxes is presented in which the webs are treated as beams on elastic foundations. An empirical factor for the effective span of the top and bottom plates has been selected to agree with experimental results and this factor is consistent with values obtained from more elaborate analyses from other sources. Current designs are analyzed and the need for diaphragms is considered in terms of web moments and shears and transverse moments. The simplified analysis enables a designer to visualize readily the behavior of the box section. Diaphragms are generally required at bearings. In those few cases where diaphragms are required in the spans their spacing can be determined rationally.