Not good. Or sufficient to support a second floor and snow load. You need a structural engineer to design footers to accept the point loads that will be created. And you will need specific engineered beams or steel. And you really need a good GC to build the temporary support walls to carry the load while you fiddle around with removal and replacement of the existing support.

Or just hire s good GC at the start.

"Good plan or not? "

Maybe,then maybe not. A center support wall doesn't tell us what and how much it supports. Only inspection from roof down can tell what wall is carrying. Your explaination sound's like you are combineing existing 36" and 48" openings with a new 69" opening for a total of 153" because you say " So basically the support wall will end up having a 13' span ,supported by bla bla. You may need some onsite help before getting too far along.

It might work, but will require some engineering to verify the structure. Rather than use 2x12s, I would used engineered LVL beams. The LVL manufacturers will provide a table that shows beam dimensions and span maximum, based on the assumption of one story above. However, you may have to go beyond that basic calculation if you have something non-standard in the house above.

Then you need to look at how the beams are supported from below. If the posts or jack studs supporting the beam are directly on your footings (you said there is a footing in the middle of your crawl space?), or on a poured cement pad of the right thickness, it should work.

I replaced about 14 feet of the beam running down the middle of my house, as part of a 1st floor renovation. We had to use two LVL beams side by side to get the required strength without making the beam too high. My jack posts rest on pads on the 1st floor that are right on top of a steel beam in the basement, and that beam is supported by steel posts that run down to cement pads in the basement floor. It cost me about $500 to have a structural engineer come out and verify all the structure and weight transfer, so that the city would approve my renovation plan. My GC did all the calculations (and got them right), but the city wanted it signed off by a PE.

Bruce

Maybe, maybe not. Depends on what's under it.

You're not spanning 13' with a 2x12. I like the LVL idea.

A good framer familiar with your local inspection department may get you out of this safely without an engineer.