I was lucky enough to read some earlier posts about fixing exposed beams. When I bought my house a year and a half ago, I though we were going to have to replace it. Upon further inspection, I found that the rot really wasnt too bad and it was "fixable." Below is how I did it...pretty easy actually.
First, I got some magic expoxy from a place called the Rot Doctor. http://www.rotdoctor.com/ This stuff is kind of nasty, but it does wonders. You essentially clear out all of the old rotted wood with a stiff brush, or whatever. You mix both the a-b parts and pour it onto the beam to saturate it. It has the consistency of mineral spirit. It soaks right into the wood, where over the next few days, it hardens, and kills off all of the fungus contained in the wood.
Rot doctor has some filler material that you can add to the a-b mixture to fill up the voids, but my situation didn't require it, as it was just on top of the beam.
After making this repair, I got a metal beam cap that comes down about 2" on each side. That goes right over the top. Glue it down, and paint it up and you are done.
I ended up getting 2x the amount needed, so i have a 2 quarts - an A, and a B, brand new unopened. I also have a length of beam cap to more than cover your atrium.
The expoxy was $75, I'll sell that for $40. The beam cap I'll sell for $50. Imagine, fixing your beam for under $100!
Please send me a message if you are interested. I am in Lucas Valley in San Rafael. Happy Eichlering!