Repairing floor joist problem

I gutted a room in my 100 year old brick row house. The room is 18x15. I also removed an old narrow staircase that had been covered up for decades. The staircase ran to the 2nd floor along the left-hand side of the house. The stairs ran between the brick wall (left side) and a non-structural wall of 1 x 4 pine.

I removed the stairs and the 1 by 4 wall, then brought down the lath and plaster ceiling. The joists run the 15' width and are buried into the brick on both sides. Where the staircase was, the floor joists are attached to a 10' long beam made of two 2x10s ( marked as L below). All the beams are cut so that they lock together, though they did use nails to secure them as well.

My problem is that the previous owner ran ductwork vertically up through the old staircase, and then horizontally through the beam - and right in the middle of the beam. There is a 4x10" hole in this beam has really weakened it and there is a noticeable sag in the floor. I used a floor jack to secure it temporarily.

I need to find a way to fix the beam, and to cover the old staircase. I could remove the beam, replace with two new 2x10s and attach them, and the floor joists using joists hangers. Then create two more beams for where the staircase was, and run joists between them. I could attach one side right into the brick wall on the left.

I am not sure if the floor joists and hangers will support the floor. The room above is a living room should I have a small party.... I would want the floor to come crashing down!!

Any ideas would be appreciated.

I apologize ahead for the lousy visual below!



Brick floor brick wall
wall joists
II--------------------------------------------II
II--------------------------------------------II
II========================II <- double joist 2x10
II LL---------------------II 18
II steps LL---------------------II
II were LL---------------------II feet
II here duct -> LL---------------------II
II LL---------------------II long
II LL---------------------II
II========================II <- double joist 2x10
II--------------------------------------------II
II--------------------------------------------II
II--------------------------------------------II

15 foot wide