PRESENT STATUS OF THE CONCRETE SHIP. 189 



.V ^ maximum unit shear due to regular vertical shear action. 

 t = maximum unit torsional shear. 



s' = maximum combined unit shear occurring at some intermediate wave condi- 

 tion. 



The formulas are: — 



As conclusions may be stated: — The torsional stresses are not negligible by 

 themselves. In arranging the cross-section of the ship it should be held in mind that 

 these stresses exist and that they are resisted most effectively by complete closed 

 shells, formed by deck, sides, and bottom, or by decks, sides, bottom, and longitudinal 

 bulkheads. At points where the section changes — for instance, at the ends of hatch 

 openings — the local stresses due to torsion may reach high values unless special rein- 

 forcement is provided and unless the details are carefully worked out. However, 

 when the design is such that the torsional stresses in the shell are kept reasonably 

 small, say not more than one-quarter or one-third of the maximum shear due to the 

 regular vertical shear action, then the increase of the combined shear beyond the 

 value of the regular shear is very small and can be considered as negligible. 



In addition to the analytical studies it has been necessary to obtain data .on the 

 physical properties of structures of unique design and concrete mixtures which had 

 seldom before been used in structural work. These structural investigations are 

 made under the direction of W. A. Slater at the Bureau of Standards' laboratories 

 at Washington and Pittsburgh, the laboratory of Lehigh University at Bethlehem, 

 Pa., and the Office of Public Roads, Arlington, Va. 



The investigations are made to establish safe working stresses and standards of 

 design. Tests already made demonstrate the safety of the designs first made and in- 

 dicate how weights may be further reduced without sacrificing strength. 



One of the first serious difficulties in the design of concrete ships was that of 

 getting sufficient strength to prevent cracks forming diagonally in the vertical sides 

 of a ship of practicable weight. Using the methods ordinarily employed in rein- 

 forced concrete design, a shell thickness of at least 15 inches would have been re- 

 quired instead of the 4 inches used in our 3,500-ton cargo ship. 



Tests of large reinforced concrete beams were started to make certain that no 

 mistake was being made in using a 4-inch shell. These first tests were made on : — 

 (a) Beams 4 feet 4 inches deep and 18 feet 6 inches long; (b) one beam 10 feet deep 

 and 22 feet long; and (c) specimen ship frames of full-size cross-section and 20 feet 

 span. The frames were cut off at a point corresponding to the point of inflection or 

 4 feet 6 inches above the top of the keel. The tests were made in the 10,000,000- 

 pound testing machine at the Bureau of Standards, Pittsburgh. 



