Steel Box Girder Lateral Bracing -- LRFD C6.7.5.3-1

[TampaBridgeDesign]

I don't have the US Steel manuals so I'm hoping someone might be able to give me some insight into LRFD Equation C6.7.5.3-1. The bridge I'm working on is 50ksi. The engineer of record has requested that all lateral bracing and cross-frames be 36ksi steel. LRFD equation C6.7.5.3-1 states that the minimum cross-sectional area for the steel box girder top lateral bracing be 0.03 times the center to center distance between the top flanges. Does this equation imply that the area used will have the same yield strength as the the steel box girder? I've known of hybrid 50ksi/70ksi steel box girder designs so I wonder if they would switch the lateral bracing yield strength from 50ksi to 70 ksi in the middle of the bridge? I highly doubt it! Again, any background on this equation would be appreciated.