Theory of Limit Design

This paper shows that the capacity load which a redundant structure can carry is not limited to the load that stresses one member to the elastic limit. In fact, the capacity load of a structure is reached only after as many members of a structure, corresponding in number to the structure's redundants, have all reached their elastic or buckling limit strength. As a load is gradually applied to a structure, its redundant members, in general, are successively stressed, one after another, until they reach their elastic or buckling limit strength. The remaining members may be designated as the primary system which still behaves elastically, and which can carry a still greater load, safely, until one of its members is likewise stressed to the elastic or buckling limit strength. This final load condition is defined as the “capacity load” upon which the design of the structure is based. Thus, if a structure is to have a factor of safety of two, the specified loading is doubled and the designer would find the “limit design” that would permit the redundant members in the structure to reach their elastic limit, or buckling strength.