another option, if you don't want to clip/split the cylinders, you can always function detail them and then bury rectangular nodraw brushes inside them. this will allow you to block visibilty without murdering the silos into pieces. granted, theres a graceful way to slice em up (shmitz's way), but this third method (illustrated below) is effective and not as 'messy' as it first appears
purple are regular buildings, tan are the silos, and yellow is your nodraw work.
and shmitz, not to be a know it all, but func_brushes can easily block light... it all depends on if enable shadows is checked.
