14 LANDER, Stresses in the Main Spars of Monoplanes. 



moment. If, on the other hand, the outer wire be attached 

 at a point nearer to the wing tip, the bending moment at 

 B is lessened, but the direct force in the spar is increased, 

 and it may well happen that this compression may be 

 increased to an inconvenient degree. 



In determining the strength of a spar it is necessary 

 to consider the stresses which may be induced under 

 abnormal conditions, in addition to those which obtain 

 when everything is perfectly adjusted. These abnormal 

 conditions may be brought about by the elastic or acci- 

 dental stretching of a wire, or even by the breakage of 

 one or more wires. In the latter case, the above method 

 of computing the bending moments may be used directly 

 and it may be found possible to arrange for factors of 

 safety high enough to obviate accident even under these 

 abnormal conditions. The most likely event, however, 

 to occur, is the stretching or misadjustment of a wire and 

 the consequent elevation of one of the points of attach- 

 ment above the line of the others. Claxton Fidler has 

 shown how this may be dealt with for the simple con- 

 tinuous beam by modifying the heights of the character- 

 istic points above the free ended base line. These heights 

 should be increased or decreased by a given distance 

 according as the prop to which they refer has risen above 

 or sunk beneath the line of the other props. A modifica- 

 tion of the characteristic points for the props lying on 

 either side of the sunken prop is also necessary, this 

 modification being in the opposite direction and equal to 

 half that of the sunken prop. This, however, is discussed 

 at length in Claxton Fidler's " Treatise on Bridge Con- 

 struction," to which the reader is referred. The extension 

 suggested above for cases where the wires are attached at 

 points lying away from the neutral axis can however be 

 applied equally well to the case of the sunken prop, and 



