compound countertop support strength

[Dennis Putnam]

I have a real challenge for you experts which is not strictly wood working but I don't know where else to start. I have a stainless steel countertop (14ga) with 3/4" ultra-lite MDF as the substrate. The countertop is 11' but I have a 6' span and no idea how much weight it can support. Does anyone know what kind of load this might support or how to figure the load? I need to keep out as much weight as possible so I want to minimize supporting structure beyond the ends. TIA.

[Wayne Lomman]

This will not support itself over time, much less any load. It will need support. Steel or stainless steel RHS subframe required. On the other hand, if the splash back and front edge are folded, they impart structural elements that call for engineering calculations. My rule has always been to build to the substrate's limitations and the stainless skin is not considered in the calculations. Cheers

[Andy Giddings]

Agree with Wayne. There is a calculator you can use for deflection of shelving which will give you some idea of how much MDF would bend over that span under different loads. It won't be pretty and I suspect it will even deflect on its own over 6' without support. Here's a link http://www.woodbin.com/calcs/sagulator/

[Jamie Buxton]

Use sagulator. The sheet metal doesn't add much strength at all, so just use the values for MDF. Another challenge with MDF is that it just keeps sagging. That is, if you put it in place and measure the sag, then come back in a year and remeasure the sag, it will be more. So you need serious reinforcement for the MDF counter. Andy's suggestion a few weeks ago of a torsion box is a good one. Properly-built torsion boxes can have a high stiffness to weight ratio.

[Curt Harms]

Use sagulator. The sheet metal doesn't add much strength at all, so just use the values for MDF. Another challenge with MDF is that it just keeps sagging. That is, if you put it in place and measure the sag, then come back in a year and remeasure the sag, it will be more. So you need serious reinforcement for the MDF counter. Andy's suggestion a few weeks ago of a torsion box is a good one. Properly-built torsion boxes can have a high stiffness to weight ratio.

Maybe thinner MDF in torsion box configuration seems like a good choice. Another possibility to limit thickness might be a truss setup. I read some years ago reading about using thin ply underneath sheet goods. Fasten the ply securely at each end and put a spacer in the middle. The spacer didn't need to be very thick, I'm remembering something like 1/2". The principle was that when a load was applied to the top, that load was partially transferred to the thinner ply on the bottom. Similar to a torsion box but thinner though probably not as strong. The thinner ply was loaded in tension and it takes a fair bit of of force to cause even thin ply to fail in tension. I've never used this but always kept it in the back of my mind for if one day I need a fairly thin but stiff counter top or shelf.