A girder built in to supports at one or at both ends and forming an arc of a circle in plan, is subjected, at each section, to both bending and twisting moments. At the supports, fixing moments of both kinds are called into play, and until these are known the resultant moment tending to cause rupture at any section is indeterminate. The following investigation is devoted to a consideration of the general state of elastic equilibrium of such a girder under various systems of loading.