Lumber Mill Structure question

[ratled]

The area that I model, PNW, is heavily occupied by lumber mills. As I try to learn more about the industries I model I keep seeing one type of structure I can't find any info on. I've seen this A frame side supported structure in several Mill such as in this photo I took at the Roseburg mill in Weed CA




Or like in this picture of the Pacific Lumber Company near Eureka, CA


Why this type of construction?
These are all very long structures, what do they do inside? I am assuming a milling facility?
Anything else that you may know about these and care to share?

Thanks
Steve