Not sure I have any underframing.
The table is (I think) 3 solid oak planks joined together. The only fixing at the 'big end' is a vertical pole under the middle So I can't see anything pulling against anything. I think it is just the effect where the upper side of the wood dries more than the underside and the whole thing bends. The same would happen if it wasn't attached to anything. At least that is what happens to our cork bath mat which is rather an extreme example :-)
If you leave a plank in the sun so the upper side is dried but the lower side is cooler and a little damp then the plank bows up at the edges. I think this is what is happening (but much more gently) here.
I was hoping I could reverse the shrinkage in the upper surface and slowly bring the wood back to true.
Are there accepted ways to add moisture (or oil) to wood to encourage it to expand?
Cheers Dave R