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U-shaped steel girders (tub girders) composite with a hardened concrete deck form a closed- box,  torsionally strong bridge system.  A typical cross section of a twin tub girder system is shown in below.

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Prior to the development of composite action, the tub girder itself is a torsionally-weak, open steel section that must be braced to support the erection and construction loads.

Therefore, there are a number of bracing systems that are required to improve the performance of the girders The primary role of the internal intermediate diaphragms is to control distortion of the box girder.

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K-frame  is most often used since the opening in the middle facilitates access in walking down the middle of the box during construction or routine maintenance inspections.

Actual spacing of internal diaphragms is coordinate with the spacing of top flange lateral bracing system