I have a 20' span I need to install an 8" X 8" X 20'3" beam in between. I don't enough room left on the beam for a proper mortis and tennon joint. I do have enough room for a shoulder on each side. It is on the end of the building with no siding and I have full access to the outside and front of the joint. I am considering cutting a shoulder on each side. I was then considering counter sinking a steel plate with holes for a (3) 1/2" thick lag and pulling the shoulder into the beam using a the lag bolts.

I think of it as using a big lag bolt and washer to pull the oak beam into the shoulder. Has anyone succeeded with a repair like this? I was wondering if the building moving with the wind would be enough to loosen the lag bolt?

Clairification:

The rear wall is in the construction process. I have added and pinned the beam to the far left edge. The right side has not been installed yet. Below is the beam I want to use. It already has the center drilled and mortised for the center support (not shown). The real problem is the ends (see below).

Both ends have "mortise" where I need "tenon". I want to keep the orientation of beam so I am somewhat limited. I was thinking of cutting it almost flush and cutting a into the side of the building's beam about 1/4" deep and 8" X 8" to form a indented shelf (I am sure there is word for it). I would then raise the 20' beam into position and pull the beam into the shelf with lag bolts. The lag bolts would be installed parallel to the grain. I have some 1/2" by 16" lag bolts. I would install a plate to spread the load out on the existing beam. Any thoughts about the lag bolts loosening up or pulling out? I have never seen this done and I wonder if there is a reason why.

Note: Please excuse the post quality, I am still learning the software.