Scott--The H&W blueprints of Titanic's expansion joint that I have seen are apparently the same as the ones you refer to. They show the joint does affect the hull girder because the shell plating was effectively extended above the strength deck. Intentional or not, this was the result of attaching the bulwark to the outer edge of the strength deck.
On modern ships, this problem is avoided by allowing a small "slot" between the bulwark and the strength deck. Sometimes this slot is mistaken for a waterway, but its real purpose is isolation of two very different parts of the ship's structure.
So, the original Olympic class expansion joint was effectively a "notch" in an upward flange of the hull girder. That made it an unacceptable stress riser. However, things got worse. The end of the joint was simply given a radius equal to the width of the joint. These days, good naval architecture is to have significantly greater radius on openings in critical areas.
If the bulb-shaped endings of the joints discovered on Britannic are any indication, H&W realized the need to increase the radius of the joint ends. The learning curve was apparently alive and working quite well in the drawing office of Titanic's shipbuilder.
Expansion joints were needed to allow lightweight superstructure to be built on top of the hull girder. H&W engineers knew that as the ship moved through a seaway, the hull would bend and flex. They also knew that if the lightweight superstructure could be isolated from this flexing, it could be built more like buildings on shore with square corners to windows and doors. The two expansion joints were an attempt to accomplish this feat.
However, although the superstructure above the strength deck was not part of the hull, its existence did influence the flexing of the hull girder. In effect, each part of the superstructure acted as a doubler on top of the strength to help it resist bending. But, in way of the expansion joints there was suddenly no increased support for the strength deck.
This is why Dr. Foecke said, "I've been saying for 10 years in scientific and engineering talks on the Titanic that the expansion joint turned the ship into the worlds largest hollow shell three-point bend specimen."
Pulling the stern up out of the water caused a huge bending force to be applied to the hull girder. But, the expansion joints forced most of that bending into the after expansion joint. At some point, about 11 to 15 degrees of bow-down tilt, the strain became stronger than the strength of materials.
The role of the expansion joint came to light in studying the two pieces of double bottom that were pulled out as the ship broke apart. The two pieces apparently split in a direct vertical line below the aft expansion joint.
Wandering into my own speculation, I see a "girdling crack" similar to the one that caused the loss of the Great Lakes bulk freighter Argus in November, 1913. This ship simply broke up and sank while being observed by another ship, the Crawford. Captain Iler said, "The Argus seemed to crumple like an eggshell, then she was gone."
Waves in the 1913 storm on the Great Lakes exceeded 35 feet. This was at or beyond the design specifications for ships of that era. More than one suffered cracking as a result. Those cracked ships that survived seemed to have done so because their double bottoms held, while the single-thickness vertical sides and decks were breached.
As an aside, the steel in 1913-era Great Lakes ships was equivalent in quality to that in Titanic. I have found no evidence that any ships sank because of "brittle steel." However, many ships experienced rivet failures caused by flexing and bending of their hulls.
--David G. Brown