I am designing a steel staircase and the customer wants a single center stringer design.  The stair treads are to be the "floating" type supported by a small box frame underneath.  My question is this?

• How thick should the steel plates be to support say 500lbs and not flex excessively assuming the load is at the far extreme end of the unsupported tread?  The unsupported portion of the tread overhangs the "box" support by approximately 14".  I am hoping to be able to remove unnecessary weight by not needing crazy steel thicknesses in the steps.  It's already going to be beast to install as a one piece unit.

I can't find anything for reference materials that would give me any clear idea of where to start.

Here are the basics of what I have.

• 1/4" steel "box" frame approx. 7.5" x 20.5" on the flat top surface.
• 1/2" steel plate 12" x 48"
• 1/4"machined aluminum tread
• center stringer is 8" x 8" x .188" square steel pipe

There will be no supports underneath the stringer, and most likely a cable or rod baluster system anchoring up through a grid ceiling out of sight.