I finally settled down on a structural design for the cement plant model. My key objectives were strong structure, accessibility to add details over time and as much as possible, a decent level of prototypicalness.
I certainly could build the actual structure as it was with beams and columns. However, since it would be made out of several different parts, I fear alignment problems. A base and a top board seems to me a better way to ensure a certain level of squareness. It also gives some room in case of slight variation on column height. And finally, both board can be drilled at the same time, clamped together for maximum accuracy.
It must be noted I'll probably need to brace the building in some way. Under each group of silos existed a central core that housed equipment. I'll probably use them to make sure the structure is braced sufficently. The weight of the ABS silos and the upper storey shouldn't be underestimated.