66 AN ANALYSIS OF THE ISHERWOOD SYSTEM 



the center of strength) as the distance to the extreme fiber, the polar section modu- 

 lus equals 735 ^- 13.13 = 55-98 inches^ 



The moment of inertia of the side frame, deducting for the rivet holes, equals 

 186.86 inches*, and the section modulus equals 25.5 inches^ 



The Second Deck Connection. — Four possible modes of failure present them- 

 selves at the juncture of the second deck beam to the frame. The beam bracket 

 may be ruptured, as shown in Fig. i , Plate 36 ; the frame may fail at the toe of the 

 bracket; the riveting of the bracket to the frame or to the beam may fail; or, 

 finally, tJie beam may fail at the inboard toe of the bracket. It is very evident, 

 however, that the bracket plate is considerably stronger than the rivets connect- 

 ing it to the beam or to the frame, so that the first mentioned possibility may at 

 once be discarded. Furthermore, the section modulus of the side frame, without 

 correcting for the riveting or for the strip of shell plating contributing to the 

 strength of the frame, is 21.9 inches^ as compared to the section modulus of 12.7 

 inches^ for the deck beam channel, so that the possibility of failure of the side frame 

 at the toe of the beam bracket may also be abandoned. 



For the strength of the bracket riveting, considering the rivets in one arm of 

 the bracket only, we may divide the sum of the second moments of the rivets 

 about the center of the rivet nearest the heel by the distance to the toe rivet. The 

 section modulus thus obtained is 26.02 inches'. Calculating the strength of the 

 deck beam at the toe of the bracket, including again a strip of plating, we find 

 that the section modulus is 16.1 inches^ 



The Upper Deck Connection. — Applying the same reasoning to the upper deck 

 connection, we find the section moduli for the riveting and for the deck beam at 

 the toe of the bracket to be 18.61 inches' and 13.2 inches' respectively. 



The Combined Strength. — As a representation of the ability of the hull to re- 

 sist forces tending to cause diagonal distortion, we may, then, take the sum of the 

 least section moduli at each connection, or, excluding the upper deck, (25.5 -f 16.1), 

 X 2 ^ 83.2 inches', and including the upper deck, 83.2 -}- 2 X i3-2 = 109.6 inches', 

 the factor of two being introduced by the fact that failure cannot occur at one side 

 of the vessel alone. 



The corresponding values per foot length of the vessel are 38.4 inches' and 

 50.5 inches', the frame spacing being 2 feet 2 inches between peak bulkheads. 



Diagonal Strength of the Isherwood Ship. — The corresponding probable points 

 of failure in the Isherwood vessel are: — 



At the bilge connection : At longitudinal No. 14. 



At the second deck: At the second longitudinal from the 



side of the vessel. 

 At the upper deck: At the second longitudinal from the 



side of the vessel. 



These points of failure, while they are taken at the points of least section mod- 

 uli, are somewhat further removed from the theoretical points of failure than were 



