What do you mean do they fit 105 5800 chainring? Most (not all, e.g. some models don't and some chainring sizes are separate from cranks) Shimano 105/Ultegra chainrings come together with crank arms. You have to replace both cranks and the larger chainring - not because it's not incompatible, but because they are a single unit.

I think your best bet is converting to something like Rotor cranks, but not sure if that's doable without a powermeter spider. Then you can cheaply replace just the cranks. However, you will also have to play with spacers and maybe even replace your bottom bracket (I had to, even though all manufacturers were saying it will be compatible (however, not all BB386EVO 24mm BBs are the same...)).