The Rankine-Merchant Load and its Application

Abstract

Relationships between the elastic critical loads, non-linear elastic load-deflection behaviour, rigid-plastic behaviour and elastic-plastic failure loads of frame structures are discussed. The concept of deteriorated critical loads is explained, and its value in obtaining an understanding of the criteria for failure of structures in the elastic-plastic range demonstrated by reference to examples. The use in the Rankine-Merchant formula of the two idealised load factors for elastic buckling and rigid-plastic collapse to obtain an approximation to the true elastic failure load is explained, and it is shown what idealisations have to be made to arrive at a formal proof. This enables conclusions to be reached over the situations in which the Rankine-Merchant load tends to give either a high or a low estimate of the true failure load. Correlations between the Rankine-Merchant loads and the true collapse loads obtained in both theoretical and experimental investigations are presented. Finally proposals are made to enable the Rankine-Merchant load, either in its original or in a modified form, to be used reliably for design purposes.