In my experience, Gratings are designed only for UDL. Mostly industries buy Standard Gratings.
My advice- Take any standard grating and check for maximum permissible span. If grating can support an UDL of 502 PSF at Y Span max.
Design grating support runners at maximum at Y feet and design...
I think the grating should be designed for a UDL of 506 psf and allowable stress of 0.2 FY based on WSD method. But this loading is very high. If I understood correctly, you have to use heavy sections. Rods or flats will not work. ( FOR FOS 1.5 it is 0.66FY. For FOS 5 it will be 0.2FY)
