Subfloor on Steel beam

[prassi]

Hello Experts,
I plan to install marble tiles in the living room soon but need to take care of the sub floor issue first. I'm hoping someone here will help me out with this.
The sub floor is 3/4 plywood resting directly on steel beams (Not "I" but box) that are 48" apart ( yes no joists). Please see attached pictures.

If I added a 3/4 inch plywood to existing and provide 2x8 bridging 24" apart between the steel span, would that help me achieve the 720 deflection ?

I've always used hardibacker but plan on using Ditra this time. I welcome any suggestions regarding this as well.

Thanks,

 

[FloorMaven]

This might help-
http://www.johnbridge.com/vbulletin/deflecto.pl

 

[prassi]

Thanks.. I've seen that, but wasn't sure since it involves both lumber and steel here.

 

[FloorMaven]

If you input dimensional lumber into the formula and it passes I would imagine it'd be even stronger with steel.