I have an attic above my attached garage that I want to finish…

I have an attic above my attached garage that I want to finish to make a family room. Access to this space is from a large stairway landing in between floors. It is used for storage now but the floor above the garage is spongy and I don't want to load it down with sheet rock, carpet, etc etc. I would like to use a steel beam to shore the floor system up. Garage is 26ft wide and 23ft long, the 18ft overhead door is centered on the long wall and the floor joists are 2x12, 12" o.c. running from the back wall of the garage 23' to sit on top of a triple Lam-beam header (#3, 1.5"x11.5") over the overhead door. The roof has a gable end and is a 12/12 pitch with rafters extending 24ft from the peak down to the eave with a 2' overhang included. The roof forms a cathedral ceiling inside the attic space and has two 5ft knee walls on either side of the attic supporting the rafters and sitting on the floor of the attic 5ft out from the outside walls. I would like to install a 26ft long steel beam about 5ft to 6ft back from the overhead door to shorten the span of the 23ft long floor joists to about and position the beam under the knee wall in the attic to take some of the weight of the roof and the additional weight of the interior finishes in the attic space and some of the weight off of the Lam-beam above the overhead door. I was thinking of placing steel posts or columns in the walls at either end of the beam if that is possible. The foundation walls 8" block that could be slugged with concrete unless I need to cut the garage floor and pour larger footings. So the question is what size of steel beam, steel post, and foundation. Is there a better alternative or a less expensive way to shore up the floor. The floor is already sagging and inch or so in the attic and there is a sag in the top of the knee wall that the rafters sit on the side of the overhead door. Currently it looks like a lot of roof weight to put on the 2x12 floor system and the 18ft lam-beam above the overhead door. I live in north Iowa and there are snow loads to consider also.