182 



STRENGTH OF MATERIALS 



corresponding joint of the projected arch by laying off the horizontal 

 and vertical components of the latter to two different scales ; in other 

 words, by projecting the pressure back again onto the original arch. 



III. ARCHED RIBS 



*149. Stress in arched ribs. The arch is frequently used in im-taL 

 constructions, especially in such structures as roofs and bridges, in 

 the form of a curved beam composed either of a solid web and flanges 

 or built up like a truss. Sucli a metal arch is called an arched rib. 



FIG. i:w 



The fundamental difference between a concrete or masonry arch 

 and an arched rib is that the latter, being composed of metal, is 

 capable of resisting bending moment. For an arched rib, therefore, 

 it is not essential that the equilibrium polygon shall lie within the 

 boundaries of the arch; it may, in fact, either cross the arch or lie 

 entirely on either side, the only condition for stability being that the 

 arched rib must be sufficiently strong to resist the bending moment 

 thus produced. 



*For a brief course the remainder of this chapter may be omitted. 



