Conventionally constructed steel bridge I-girders, with a concrete deck placed after girder erection, need bracing to
optimize the strength of their compression flanges. Bridge designers provide bracing in their designs which works to
prevent lateral-torsional buckling of the girders, with the critical construction stage for buckling usually occurring
during placement of the deck.
Girder bracing comes in two forms—lateral bracing and torsional bracing—with the effectiveness of each form being
determined by its ability to control girder twist and lateral-torsional buckling. A composite concrete deck, once it has
achieved its design strength, is an example of continuous lateral bracing for top flanges. Another common form of
steel girder lateral bracing is a horizontal truss placed between two girder flanges, which is frequently provided to
resist lateral loads such as wind.
Provision for I-girder torsional bracing has been the focus of AASHTO bridge design specifications since their
inception in 1931. This torsional bracing is in the form of cross-frames and diaphragms. Figure 1-1 and Figure 1-2
depict diaphragms and cross-frames, respectively. Prior to use of composite deck construction, cross-frames and
diaphragms were generally the sole source of girder compression flange bracing for all loads. With modern composite
girder design, cross-frames and diaphragms are primarily needed to brace for non-composite dead loads, construction
loads, and wind loads. Once the composite deck has achieved its design strength, it becomes a continuous lateral brace
for the top compression flange; bottom compression flanges remain braced by the cross-frames or diaphragms.
However, a composite deck will provide some level of torsional bracing for the bottom flanges. Other roles for crossframes
and diaphragms include:
