Bed bending

[Wojciech]

I am now doing some full working area prints which takes up to 20 hours and I see bed is bending at different stages of a print.
When leveling a bed it's shape is like hill in the middle and going down at sides but when print's weight gets around 20-30g it's going down in the middle and sides are higher. I see that printed objects ABS/PLA are bending because of bed and not because of low heat in the chamber.
Looks like heating PCB is way too thin and it should have an extra support structure under bed to stop bending.
I think it's really bad especially it is only 12x12cm.
Community what You think? Anyone has found similar problem/solution?