92 AN ANALYSIS OF TESTS OF WATER-TIGHT BULKHEADS. 



these stiff eners is in fact only 45 pounds as against 77 pounds per foot run of 

 the double stiffeners, and their moment of inertia is little more than two- 

 thirds the moment of inertia of these latter. Whether bracketed at both 

 ends or at the foot only, a close correspondence is generally found between 

 the calculated and the observed curve. On the whole these tests, as well 

 as others not here recorded, seem to warrant the statement, that stiffeners 

 of the type and spacing here used and bracketed at both top and bottom 

 may be expected to behave as fixed at the ends under the assumptions 

 here made and within the theoretical stresses to which they have been 

 exposed in these tests. 



The cause of the greater efficiency of the single stiffeners must be 

 sought in their more rational construction. These stiffeners are constructed 

 of single I-bars of great depth with a face plate on the free flange, and are 

 connected by two lines of rivets to the bulkliead plating, whereby this 

 plating is enabled to work with the stiffener. (See Fig. 4, b.) Having no 

 riveted connections, the I-bar itself presents no weakness, and the flanges 

 are connected by a solid web. 



By the double stiffeners the connection of the Z-bars along the neutral 

 axis is effected by a single line of rather open-spaced rivets, passing through 

 four thicknesses (two flanges, one liner and one bulkhead plate) . (See Fig. 

 4, a.) This connection is ill adapted to resist the great lengthwise shearing 

 forces which exist along the neutral axis near the apex of the brackets. 

 A small sliding is liable to take place in the plane of the neutral axis, and 

 the two stift'eners, which together make up a double stiffener, will then 

 to some extent behave as if they were independent of each other. The 

 points of inflexion will move nearer to the brackets, and it is not difficult 

 to understand that the stiffener will now behave approximately as if free 

 to turn about the apex of the brackets. 



This weakness of the double stiffeners was observed already by Wood- 

 ward in his tests of the center-line bulkhead of the Illinois. (See Plate 28, 

 test No. I.) It was then found, that although the brackets did not show 

 any sign of yielding, the permanent set of the stiffeners as well as their 

 elastic deflection were excessive, indicating a weakness in the stiffeners 

 themselves independent of the bracketing. 



It seems likely that all double stiffeners here analyzed suffer from 

 the same weakness as the lUinois's stiffeners, a too small effective moment 

 of inertia, even although a small height of bracket may be in some cases 

 responsible in some measure for the great deflection. It is of course 

 possible to avoid the weakness here referred to by connecting tlie bars on 

 opposite sides more intimately with each other, for instance by means of 

 two lines of closely spaced rivets, but in such case the flanges meeting on 



