It is shown that when heavily curved circular beams of constant cross section are subjected to a compressive load along the “open” diameter, the load for the start of significant deformation as indicated by the reduction in diameter along the line of action of the load, may be estimated by the use of rectangular plastic stress blocks based on the yield stress of the material. Comparison of the experimental values obtained on specimens of duralumin and mild steel with those predicted theoretically are, on the whole, only slightly underestimated.