Combined Kitchen/Sanitary Venting in Chicago

Good day,

I am working on a 5-story residential complex in chicago, and I am trying to minimize the number of penetrations through the roof for the VTRs. I was hoping to combine sanitary and kitchen waste vent stacks, but a sr. engineer in the office has notifed me that it is against code. I have reviewed the Illinois Plumbing Code, the Chicago Building Code, and neither mention anything specific about combining kitchen and sanitary venting one way or the other.

Does anyone know whether, in the City of Chicago, one can combine sanitary & kitchen vents, and if so, is there a code section that references this? Thank you for you help!

I cannot imagine any reason why they cannot be, and it was NEVER a factor for the 20 years I was in the Chicago area. But, a 5 story building will probably require "relief vent" connections to the main drain riser every few floors, or feet.