The Titanic‘s keel was laid down on March 16, 1909. Designated Hull No. 401 (The Olympic had been Hull No. 400), she soon had more than 3,000 workmen swarming over her growing shape. She would have a total of nine decks, starting with the uppermost, the Boat Deck, then lettered A through G, with a lower deck below that for the ship’s machinery, the Tank Top deck. (A second lower deck, the Orlop Deck, was only a partial deck. Another name for A Deck was the Promenade Deck, and the two names were used interchangeably.)
The design of the Olympic-class ships was based on that of their immediate predecessor, the Adriatic, the last and largest of the White Star Line’s “Big Four.” “Based on” does not mean, however, despite what some uninformed commentators have said, that Harland and Wolff simply took the design of the Adriatic, lengthened it, made it wider, and threw a couple of extra decks on the superstructure, along with two additional funnels to accommodate the added boilers. The Harland and Wolff designers already had decades of experience with the stresses–hogging, sagging, racking, and so on–to which ships’ hulls were subjected, and while the Olympic-class liners were pushing the upper limits of hull size that could still be built to traditional proportions, the keels, scantlings, and strength decks of these new liners were all strengthened and, where needed, reworked to assure that the Olympic, Titanic, and Gigantic would be able to endure the stresses of service on the North Atlantic.
The Titanic‘s frames were set 36″ center to center, running her entire length, with her plating laid over her frame and her internal structure; in areas of the hull were expected to be subjected to particular stress, the shell plating was doubled. These plates, some of which measured eight feet high and thirty-two feet long, were typically an inch thick. B-deck served as the upper strength deck, the whole hull structure from keel to B-Deck becoming what engineers call a “box girder,” a particularly strong structural shape that allows for the sort of flexibility that ships require in a seaway. The frames were made entirely of mild steel, which was also used for the huge shell plates that formed the hull, and was typical of shipbuilding in the early 20th Century.
By the end of October 1910, the Titanic‘s shell plating was completed, and shortly thereafter the launching date was set for May 31, 1911.