"[B]uilding stones and concrete exhibit the properties of elasticity, although not so perfectly as to permit a constant ratio between stress and strain... For compression within the limits of safe stress, however, such proportionality may be assumed without any considerable error so that a constant coefficient of elasticity may be used... The theory of the elastic arch may therefore be applied to arches of all classes of masonry, including monolithic arches of concrete, provided no tensile stresses or only very small tensile stresses are allowed to occur. The same theory also forms the basis for the design of reinforced concrete arches..."