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Composite Beam Retrofit

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A Contractor has demolished a reinforced wall 41 feet long leaving an
8"x14" R.C. beam with 2No.4 bottom bars and No.3 bars at12" c/c spanning
41 ft unsupported. I can introduce 6"x6" m.s.tube columns at 12' c/c.
The load above is a lecture hall and the factored bending moments for a
12' is 51 kip ft over supports and 36 kipft at mid span.

The existing beam is therefore inadequate. W shapes are not readily
available but I can get 8" x8"x5/16" tubing which I can rip into to 8x4
channels. Head room is a problem.

If i use the channels flat, cupping the underside of the beam fixed with
epoxy anchors,can I assume that the beam fails over the supports and
behaves as a simply supported beam, then design as a composite beam for
mid span moments only.

If yes the section is still inadequate if you ignore the 2 bottom bars
in the existing beam. Can someone advise me how can i analyze the entire
composite section inclusive the rebar,channel and concrete.
Thank you
mark Francois



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