128 



LECTURE 



Fig. 55. 



Should the firch have the simple form of Figure 46, especially if 

 the depth of the arch stones is small compared with the span, there 



is danger that the weight near the 

 crown, acting with more effect in the 

 radial direction than that near the 

 haunches, will rupture the arch, as 

 in Figure 54, the crown sinking and 

 the joint there opening on the inside, 

 and the haunches rising. Conversely, 

 if an arch is loaded too heavily on 

 the haunches, as it may be by the 

 weight of masonry at F, in Figure 

 53, or by soft earth in the same 

 position, it will fail by the pushing up of the crown. Judgment is, 



therefore, required in planning such 

 works, in order to avoid these accidents. 

 The cusped arch. Figure 55, much 

 used in certain stjdes of architecture, 

 was originally designed to prevent the 

 failure shown in Figure 54. It is evi- 

 dent that the inner points or cusps at 

 the haunches would resist a change of 

 form, as in Figure 54, by increasing the 

 moment of resistance of the particles 

 near the outside. This arch should properly be made of only four 

 stones, as in the figure, and is therefore adapted to small structures, 

 such as the heads of windows. 



The form is a very ornamental and pleasing one apart from its 

 utility, and admits of elaborate decoration, which is hardly the case 

 with a simple circular arch. 



We must not permit the curved form to become too intimately 

 associated with our idea of the arch. We have seen in Figures 47, 

 52 that an arch need not strictly have any curved surface at all, and, 

 on the other hand, a curved beam is not by any means an arch merely 

 from its form. The cast-iron structure in Figure 50 ceases to be an 

 arch when we remove the tie rod which prevents its ends from 

 spreading, and unless we prevent such spreading by fixed abutments, 

 a comparatively small weight will cause it to fail, by giving way to 

 the strain of tension along its lower side, just as any other beam 

 would do. 



In a stone arch the wedge-shaped pieces or arch stones continually 

 tend to press or pack more tightly together,, and thus to keep up the 

 stiffness of the structure. 



Technicall}^, the inner surface of an arch is called the intrados, the 

 outer the extrados. 



Frequently, as in Figure 49, the curves of these surfaces are simi- 

 lar and parallel ; sometimes they differ, this being to a certain extent 

 a matter of taste or of the fancy of the architect. 



Curves other than circles may be employed for the intrados, ac- 

 cording to certain necessities of construction. The ellipse, as in 



