The logs above those first replacements were not as thoroughly degraded, however, and could be merely refaced (the process of removing approximately the outer half of the log and grafting on a new exterior carefully sized to emulate the original) on the exterior which had two major benefits.
First, as these logs were continuous through the fireplace masonry the structural requirements for lateral design could remain relatively minor. Should they have needed complete replacement, the connections at the fireplace and the corner would have been compromised. Diagonal cross bracing was all that was needed to satisfy the project's engineer.
The second benefit to refacing was that at least the interior of the historical site could remain original (despite the completely replaced logs where required). After staining, the ability to distinguish between new and old was nearly impossible.