Surface untrimmed when used Isotrim

Doing a case study on the Peace bridge of Tbilisi, after trimming the surface to my needs, I needed to use it as an untrimmed surface, so I divided it using the "Isotrim" component method but, this un-trims it back to what it was before and I'm having a hard time trimming the subsurfaces, could you tell me how to do it? Or maybe there's a better method to do this? I could upload more pictures. Thanks a lot in advance! :)