Late change from slab on grade to full basement.

[Naison]

Hello all,

I am building my second frame but after doing the site excavation, I was surprised that I can get a full basement instead of being slab on grade (ledge) which was the original design. 

My question is what options do i have to support the center posts of the middle 2 bents of the 4 bent frame. The original plan brought them to grade with a footing. The posts are already cut (18' 8x12 red oak) so ideally I don't want to change them (make 8' longer to make additional height of basement) so looking for ideas. 

The basement will be all mechanicals/storage so not looking for the expense of another timber framed joist system if it can be avoided. 

My thoughts for discussion are:
1) run a tie beam under the center posts spanning the width of the bent (foundation wall side to side)and then add an additional posts to make the basement height joining with the tie, use 2x framing to complete the joist system
2) steel i-beam spanning width of bent (foundation wall side to side), set the post on the i-beam sill board and support with steel columns, 2x joists.
3) ?

Any experience or ideas will be appreciated.

[Ljohnsaw]

3) Go full length:


 

[Naison]

One carrying beam spanning the 4 bents, i like that idea, perpendicular running 36' instead of parallel to the bents but would be 2x 32' runs. thanks.