PRESENT STATUS OF THE CONCRETE SHIP. 187 



been made of the transverse strength. This is not commonly done in steel ship design, 

 for the size of the transverse frames is fixed by the classification societies, being 

 based upon data accumulated from many years' experience. The methods employed 

 in our calculations are fully described in the paper referred to above and will not be 

 repeated here. 



Specialists have been employed to develop the various elements of the struc- 

 tural design. In reinforced concrete design it is essential that we know the distribu- 

 tion, character and magnitude of the strains throughout a structure in order to 

 properly locate and proportion the steel reinforcement. 



There has been considerable discussion relative to the combined torsion and 

 shear strains in a ship. A very thorough analytical investigation of this subject has 

 been made by H. M. Westergaard, one of our engineers. The results of these studies 

 will, no doubt, interest you and they are given below : — 



Torsional stresses are produced when the ship passes over the waves making an 

 acute angle with the waves. In order to find what increase of stresses can be ex- 

 pected due to this effect, an analysis was made of the torsional moments and the 

 stresses produced by them. To find approximately the wave condition giving the 

 maximum effect, a rectangular water-line section was first assumed, width one- 

 eighth of the length. The usual assumption was made that the hydrostatic water 

 pressure varies directly with the depth under the wave surface. For the sake of 

 simplicity a sine wave was assumed instead of a trochoid,, correction for the differ- 

 ence being made afterwards. As usual the wave height was taken one-twentieth the 

 length. The maximum torsion midship was then found to occur when the wave crest 

 makes an angle of 19 degrees with the ship, when the line of the wave crest passes 

 through the center of the ship and when the length of the wave is 0.475 times the 

 length of the rectangular water-line section. Using the notation — 



T = total torsional moment. 



/ = moment of inertia of water-line section. 



the maximum static torsion midship was found to be expressed by the formula : — 



7 = 3.20 Ib.-feet' X / 



(/ measured in feet* gives T in Ib.-feet.) 



This formula must now be revised from three viewpoints. First, the wave is a 

 trochoidal wave, not a sine wave. Second, the water-line section is a ship-shape 

 section, not a rectangle. Third, the dynamic action should be considered. Assum- 

 ing the wave length and angle as indicated above, the first two causes for revision 

 appear to have equal and opposite effect, the trochoidal wave shape increasing the 

 effect by 6.5 per cent beyond what is found for the sine wave, while the ship-shape 

 water-line has a similar decreasing influence on the constant in formula (i) (the 28' 

 water-line, design 41, was used in the comparative computation). The third cause 

 for revision, the dynamic action during rolling, appears not to change the value of 

 the maximum torsion midship, provided the mass is fairly uniformly distributed. 



