Structural engineer here, Texas PE. With something like 6 minutes left in the LSU game, fans began bouncing in unison to music. I began to feel vertical movement in the 7th row of the south end second deck in synch with the bouncing of the fans. I could see substantial movement of the lower edge (1st row, which is the end of a cantilever truss structure) of the second deck, where it extends out over the upper part of the first deck. The dynamic movement was vertical, I estimate the deflection was at least several inches. Video analysis should be able to determine the actual movement. The differential movement was visibly evident between my location closer to the fixed end of the truss and the cantilever end of the structure. The seating deck is precast concrete which provides good horizontal diaphragm stiffness, but the vertical support structure is steel, specifically a horizontal steel truss structure back to the vertical steel structure at the concourse. The concrete is just mass as far as vertical movement is concerned, and, if near the natural frequency of the structure, movement can amplify increasingly if the forcing function continues, similar to the dynamic response of bridge structures. Fans noticed and stopped bouncing. Had the bouncing continued at the frequency of the music, deflection could have continued to increase, as it had been. The analysis and design of the south endzone structure for dynamic loading should be reviewed.