ARCHES AND AKCHED RIBS 169 



Haunches : the parts of the arch next to the abutments. 



Springing line : the line AB joining the ends of the intrados. 



Rise: the distance from the springing line to the highest point of the 

 intrados. 



Spandrel : the space above the extrados. In the case of an arch supporting 

 a roadway, the filling deposited in this space is called the spandrel filing. 



Voussoir : any one of the successive stones in the arch ring of a masonry arch. 



Keystone : the central voussoir. 



In constructing an arch the material is supported while being put 

 in place by a wooden structure called a center, the outer surface of 

 which has the exact form of the soffit of the required arch. The 

 center is constructed by making a number of frames or ribs having 

 the form of the intrados of the required arch, and then placing these 

 ribs at equal intervals along the axis of the arch and covering them 

 with narrow wooden planks, called lagging, running parallel to the 

 axis of the arch. When the arch is completed, or, in case of a con- 

 crete arch, when the material has hardened sufficiently to resist the 

 stress due to its weight, the centers are removed, thus leaving the 

 arch self-supporting. 



138. Load line. Since the filling above an arch has the same form 

 as the arch itself, it must be partly self-supporting. In designing an 

 arch, however, no advantage is taken of this fact, and it is assumed 

 that any portion of the extrados supports the entire weight of the 

 material vertically above it. The only exception to this is in the 

 construction of tunnel walls, in which case it would be obviously 

 unnecessary as well as impracticable to construct an arch sufficiently 

 strong to support the entire weight of the material above it. 



If the filling above an arch is not of the same material as the arch 

 rin.u r , subsequent calculations are greatly simplified by constructing a 

 load line which shall represent at any point the height which a filling 

 of the same material as the arch itself must have in order to produce 

 the same load as that actually resting on the arch. The vertical 

 intercept between the intrados and the load line will then represent 

 the load at any given point of the arch. 



In case of a live load the load line will have a different form for 

 each position of the moving load. 



Problem 161. A circular arch of 20 ft. span and 6 ft. rise, with an arch ring 

 3 ft. thick, is composed of concrete weighing 140 Ib./ftA Construct the load line 



