I've noticed on several lugged steel frames that the seat stays appear to be welded to the outside of the lug at the seattube/toptube junction. E.g.,
That suggests to me that the only structural support for the seatstays is the bead of...uh, solder (molten metal, brazing material, whatever the heck one calls the goo that adheres two pieces in a welded frame).
Am I right? And, am I wrong to think that this would be a weak spot in the structure?
Some seat stays are pinned to the seat lug, but this provides no real structural support. Otherwise, you are right. It is as it appears. But this is not a high stress joint in the frame. In the main triangle it is the least stressed joint. As the the question of weakness, something is strong when it is strong enough for the application. This is a proven design that works and is strong enough for the application, ergo, not a weak spot.
I've seen a couple different frames where one of the seat stays came loose. Not sure if the brazing was suspect or what. Hard to argue though with the fact that tons of frames have been build that way without issue.
Becareful buying/selling bike parts on-line. I learned the hard way. :(