Thanks you Kraid for your tips
But in fact, those “bars” are in reality holes in the mesh (negative boolean). I use them to imitate a concrete floor with expansion joint. here an image in UE4 (without the texture). I don’t know if it will be possible to make only one plan for the top surface with this detail. But you are right, the unwrap map isn’t simple. There is probably a better way to do it manually. This is one of most complex mesh in my project. that is why I tested the uvw creation on this one first.
