I realize that you're looking to span 45' and want a clear span, but I don't understand why you've decided that you have to have posts inside your building. Have you talked to truss manufacturers? They have engineers and can do a clear span with wooden trusses over some very long spans. It's also not unusual to combine steel beams with wooden construction. I had a 2 story all steel building with a floor load rating of 125psf (house bedrooms are 30 psf) and they had a 40' clear span. I also had wooden floor trusses rated to 100 psf although that was only 24' clear span. It depends on how stiff you want the floor and how much height for the truss is available, but they should be able to span the 45', or at least only have posts on one side and clear span of 35' on the other.
P.S. I found that the truss manufacturers can build the trusses and deliver them on-site for less than it would have cost me for just the lumber.
A big cost savings in this project: We are milling the lumber ourselves, from trees on our property, with the exception of the posts (6x6x20 UC4B) and skirts (2x6x12 UC4A).
Looked at a bunch of truss designs and talked with a couple of companies that make them. Construction wood product costs are through the roof. Finished construction lumber is selling for four times what it was five years ago. And the backlog is also an issue. Six months or more for small orders in our area. Had one building supplies company telling me the posts were going to be three to four months. Fortunately, found another supplier stating two weeks for the posts.
The boat I need to store will occupy most of one of the three 12' aisles, probably next to a staircase for the upper level, from November to April each year. Because that will only go in or out twice a year, I'll probably store some winter/early spring use tools in front of it, like my log splitter, pressure washer and walk-behind tiller. All stuff on wheels, easily moved.
As long as each aisle has a door, it won't be a big hassle. My goal isn't to have a brightly lit workspace, beautified concrete floor, and spend time in there doing projects. I have other spaces for that type of work/leisure. It's effectly going to be a very large, three car garage. I may build additional side sheds off it for impments at some point, or I made leave one side partly open for easy implement access. Will ponder as I go.
