Some updates on the build:
I've been getting a small chunk of time to work on the thing here and there. As usual, the amount that I learn after the project starts far exceeds the amount that I know beforehand.
The general plan has been to design a mezzanine loft using 4x4 3/16" wall square tube pillars at the four corners. This is to support a floor framed with 6" I beams and angle iron, which will then skinned over with wood 2-by material and OSB.
Problem number 1 came up as I had to decide whether to bolt or weld the thing together. I decided to bolt in order to allow for modification in the future, and also to keep the heat off the walls. The I beam rails will be right up against the drywall and I wouldn't be able to weld anything from the back.
Problem number 2 followed immediately. If I can't weld on the backsides of the I beams, then I surely can't apply a wrench there either. All the connectors that hold beams together up against the would have to be manageable from the facing side, only. Consequently, I had to design and build a set of connections that had welded parts on the backsides, with welded nuts for bolts. Only parts that came from the front would be boltable.
This had implications with regard to the order of assembly as well.
Here's what I came up with:
This is the top of one of the 4x4 pillars. It's 7.5' tall. All 4 are slightly different. There are two back-corner versions (a right and a left) and two front-corner versions as well. This is the front left.
Each pillar has a pair of supporting corbels made from 2x3 .250" wall angle iron that the the I beams rest on. The bottom flange of the I beam through-bolts with the corbel using half inch grade 8 bolts. The web of the I beam is then bolted between two cleats of .188" wall angle iron. One is welded to the pillar and the other bolts up. On top of the pillar is another triangular piece of plate which is just there to take a screw which will hold down the plywood. I didn't cover the pillar completely because I need to be able to get a nut and a wrench down the hole.
Here is the rear left pillar, right-side up:
The rim of the structure all the way around is made from wide flange I-beam (also called "H" beam). The specific designation is W6x12#. W="wide flange", 6=inches in height, and the 12# is the weight per foot. W6x12 is 4" wide and has a web thickness of 0.230".
This is opposed to standard 6" I-beam which is designated S6x12#. Standard I-beams are mostly phased out, but the smaller stuff is still readily available. I decided to use it for the internal cross members because I was only going to be anchoring to the web.
Here are the two different kinds of I beam side by side:
The one with tapered flanges is the standard type. It's an extruded and rolled product. The central webs are about the same, but you can see on the one kind, the flanges are tapered and shorter, and on the other, they're flatter and wider. The older "standard" type of I beam with tapered flanges is only 3.25" wide and still weighs 12 pounds per foot. The wide flange 12# version is 4" wide.
As you might imagine, the tapered flange is lousy to drill and bolt through, but either could be welded without a problem.
I used wide flange I beam around the edges, but the central cross members are standard type so that I can fit them thus:
That's the part I'm doing today. I cut those beams to let the flange stick out. It will be sandwiched with two bolts, between angle iron cleats welded onto the other beam.
I should have all the I beam work done here today, and then I'll start cutting angle iron. There will be angle iron parts every 2 feet bridging between the I beams. The angle irons will support wood 2x4's or 2x6's, which will then support the OSB floor.