RMS Titanic - Construction - Expansion Joints

Source: ANATOMY OF THE TITANIC - Tom McCluskie

An additional design feature was the incorporation of expansion joints, inserted in the superstructure above the Bridge deck, one forward and one aft, resulting in the whole superstructure being completely severed at these points. On a structure as large and complex as Titanic, provision had to be made to allow the structure to flex. On Titanic,as on modern vessels, this flexing was a design feature. If such a rigid structure were not allowed to flex, it would quite literally snap like a dry twig.

Today, this flexing movement is compensated for in the type of steel used and the internal bracing and structure of the vessel. In the case of Titanic, however, modern high quality steels were not available. Consequently, incorporated into the vessel's superstructure was a pair of what were quite literally hinges - expansion joints designed to allow the ship to flex up and down in time with the motion of the sea. One joint was approximately on third of the length of the ship from the bow and the other was approximately one third from the stern, in effect splitting Titanic into three equal sections. The joints themselves were made of leather, steel and iron riveted to the deck plates.

Any movement in the vessel would be imperceptible to the passengers, but the maximum amount of deflection in the hull of Titanic would be as much as two feet from the horizontal. The major stresses on the hull that the expansion joints were designed to counter were when the ship hogged or sagged.

Hogging is when the hull of a vessel is supported in the middle on a large wave, leaving the bow and stern unsupported. Gravity then takes effect and pulls both areas downward causing the vessel to assume the shape of a banana.

Sagging is the opposite effect and occurs when the bow and stern are supported on waves and the middle part of the ship is unsupported. This drops downward, or sags, into the trough of the wave.

Such conditions are frequently experienced when the vessel is travelling through heavy seas or extremely stormy conditions.