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Beam Reinforcement using Bolts

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I have a existing floor system consisting of M12x11.8 beams at 2'-0"
o.c. that span 18 feet with a cantilever of 9 feet.  The floor consists
of 2" concrete with a 1" terrazzo floor (5" total thickness).  This
building was built in 1949 and it is desired to add an elevator that
will require 3 beams be cut in the center span toward the cantilever.
This will require the beams adjacent to the opening to now support the
additional load (the new moment in the cantilever is 57 k*ft versus the
original moment of 18 k*ft).  I am looking at reinforcing the existing
beams and have thought of using channels within the depth of the beam.
I am thinking of bolting the channels to the existing beam because with
a preliminary check I will need to use C10's and there will not be room
to weld due to the existing floor slab.  I am thinking of using slip
critical bolts and shoring the existing while the bolts are installed.
Is there any problems with this?  Any other recommendations on how I
could reinforce these?  Also, the existing floor has a plaster ceiling
on metal lathe, would anyone consider this to adequately brace the
bottom flange of the cantilever (curious because the original design
would require that the plaster adequately brace the bottom flange)?

Any comments are appreciated.