This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
All of the previous demonstrations in arches have been made on the basis that the arch is made up of voussoirs, which are acted on only by compressive forces. The demonstration would still remain the same, even if the arches were monolithic rather than composed of voussoirs; but in the case of an arch composed of voussoirs, it is essential that the line of pressure shall pass within the middle third of each joint, in order to avoid a tendency for the joint to open. If the line of pressure passes very far outside of the middle third of the joint, the arch will certainly collapse. An elastic arch is one which is capable of withstanding tension, which practically means that the line of pressure may pass outside of the middle third and even outside of the arch rib itself. In such a case, transverse stresses will be developed in the arch at such a section, and the stability of the arch will depend upon the ability of the arch rib to withstand the transverse stresses developed at that section. A voussoir arch is, of course, incapable of withstanding any such stresses. A monolithic arch of plain concrete could withstand a considerable variation of the line of pressure from the middle third of the arch rib; but since its tensile strength is comparatively low, this variation is very small compared with the variation that would be possible with a steel arch rib. A reinforced-concrete arch rib can be designed to stand a very considerable variation of the line of pressure from the center of the arch rib.