230 



SPECIAL TYPES OF BRIDGE TRUSSES. 



Art. 124. 



ments must be capable of resisting a horizontal thrust or else 

 the feet of the arch must be held by a tie between them (Fig. 

 150 (a)). 



Figs. 173 and 174 show three-hinged arches; there is a 

 hinge at each support and one at the crown. The arch of Fig. 

 173 is called a spandrel-braced arch', the bottom chord may be 

 conceived to be an arch which is braced by the trussing. If 



BRACED ARCH 

 Fig. 173. 



the arch is parabolic there will be no stress in the top chord 

 or diagonal members, provided there is a full and symmetrical 

 load; the trussing is stressed under partial load. In Fig. 174, 

 the arch consists of two trusses set up against each other, the 

 loads being applied, through posts, at the upper joints. Arches 



THREE HINGED ARCH 

 Fig. 174. 



of this kind are of many different styles, and plate girders are 

 often used in place of the trusses. 



Arches are also built without hinges and with only two 

 hinges, but for these the reactions are statically indeterminate. 

 Stresses in arches with hinges are calculated upon the assumption 

 that there is no friction at the hinges. The reactions and stresses 

 for a three-hinged arch, being statically determinate, their deter- 

 mination offers no difficulties to one familiar with the applica- 

 tion of the laws of equilibrium (46). 



