Pole barn house
Ok Jennifer, I understand where what you are doing now and you are right there is not a lot of information out there on the web to help you understand how to build and this is what keeps folks like you and I up late at night trying to figure it out. But you are in luck, I can help you with this one. One of the things that are great with pole built structures you can put your floor at any height, so putting in your floor 4ft off grade can be done easily. You are on the right track of having a beam down the center of your building and having your floor joist rest on that and is the way I would go. Here is how I would do it using 2×10 lumber for your floor joist . First put two rim joist 2 x 12’s on the two out side walls of your building at the height of where you want the floor to go minus the thickness of 2×10 joist because this is what your joist will set on for the outside walls. Lag bolt them both through your buildings post, one rim joist on the inside and one on the outside of your building sandwiching your buildings post. This is basically the same way your contractor will do your roof trusses (if you use more trusses then just at every post).
For the center support post I would put in concrete piers that are on a footing below your frost line. The footing is nothing more then a concrete at the bottom of your pier holes about 8 to 10 inches thick or same as a lot of pole barn builders put in the bottom of your building post holes . To make the piers use those cardboard tubes that you pour concrete in that you can buy at any store like Home Depot .The piers need to be no less then10 feet on center and would be above grade at least six inches. You could just use treated poles to set on top of your footing if you want to fore go the concrete. You would do this the same as you would when building a deck except you are going to the inside of your home instead of outside. I would have a bracket on top of the concrete pier that will hold 3 2×6’s that are nailed together that sit vertical on your pier. On top of the 2×6’s is where 3 2×12’s that are nailed together and this runs down the center of your living quarters for your 2×10 joist to set on. You use 3 2 x 12’s in the center support because most of the weight of your floor system is on the center support. If your building is 30 feet wide I would buy my 2×10 joist 16′ long so they over lap some on the center support. This is how I will be doing my basement support for my floor system above my basement. Hope this helps, any more questions ask me here, and good luck on your build. One more thing, I wouldn’t have earth up against your steel, it will rust out your steel in short order.