Progress,  page 105
Concrete mantel beam ...
OK now, here's where pure dumb luck came into play.  The upper opening of the damper
assembly was 24 inches wide and 4 inches across.  The code requires 12 inches for the diameter
of a round chimney to support the size firebox I had built.  So I needed soemthing that would
make that transition, and it just so happened that I had an excess piece of HVAC ductwork that
did just
exactly that.  What are the odds of that happening?  I decided to use that ductwork as a
form (supported by those weird triangular pieces), pour concrete around it, and then cut the
ductwork out after the concrete cured.  I hate to admit it, but I mulled over this scheme and
about a half dozen alternatives for about three days before I decided to go ahead with it.
Here's the ductwork form in place. Keep in mind that once I poured concrete around this
thing I was committed to it, and if it didn't work I'd be chiseling concrete forever to fix it.