Cross frames are critical members for the stability of straight and curved steel bridges. Conventional cross frames are often fabricated from steel angles; however these members have relatively poor structural behavior. Because of the low buckling strength, cross frames with angle diagonals are often designed as tension-only systems, therefore increasing the necessary steel. The angles are also connected through one leg resulting in eccentric connections causing bending of the members and potentially reducing the fatigue performance. Improved behavior may result if concentric members are utilized for the cross frames. The increased buckling strength of tubes and double angles results in effective members in both tension and compression, and a single diagonal cross frame can provide effective bracing. Although there are structural advantages of utilizing concentric members, a suitable connection must be developed. Tubes are often connected by slitting the tube in the center and welding to a connection plate, which requires precise fabrication and relatively poor fatigue behavior. One proposed solution is to use a steel casting designed to connect easily to the beams and to seal the end of the tube (preventing exposure to atmospheric conditions). In addition, the report covers in detail the categorization of the single angle detail for both X and K frame configurations. To date, the determination of the single angle fatigue detail has been largely based on component tests only. The project incorporated both component and full-scale cross frame fatigues tests to fully examine the interaction of the cross frame members with the overall structure. Finally, the project also examined the stiffness behavior of cross frames, using a combination of full-scale laboratory tests and computational finite element model analysis.