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.
A voussoir arch is an arch composed of separate stones, called voussoirs, which are so shaped and designed that the line of pressures between the stones is approximately perpendicular to the joints between the stones. So far as it affects the mechanics of the problem, it is assumed that the mortar in the joints between the voussoirs acts merely as a cushion, and that the mortar has no tensile strength whatever, even if the pressure at any joint should be such as to develop tensile action. It is this feature which constitutes the distinction between a voussoir arch and an elastic arch, which is assumed to be an arch of such material that tensile or transverse stresses may be developed.