Box girders have a clean, uninterrupted design line and require less maintenance because more than half of their surface area is protected from the weather. The box shape is very strong torsionally and is consequently stable during erection and in service; unlike the plate girder which generally requires additional bracing to achieve adequate stability.
The disadvantage is
that box girders are more expensive to fabricate than plate girders of
the same weight and they require more time and effort to design.
Box girders were very popular in the late 1960's, but, following the collapse of four bridges, the Merrison Committee published design rules in 1972 which imposed complicated design rules and onerous fabrication tolerances. The design rules have now been simplified with the publication of BS5400 and more realistic imperfection limits have been set.
The load analysis and stress checks include a number of effects which are generally of second order importance in conventional plate girder design such as shear lag, distortion and warping stresses, and stiffened compression flanges. Special consideration is also required for the internal intermediate cross-frames and diaphragms at supports.