In reference 1 a general method of solution was developed for the plastic design of rigid frame structures. The particular parts of that report that had to do with single and multiple spans, pinned-base gable frames were presented in a separate report which also included design curves and examples. This paper presents the same type of information for pinned-base "lean-to" type structures.

The assumptions that will be made are the same as those listed in the earlier papers. That is,

(1) As moment approaches its fully plastic value, Mp, curvature increases indefinitely;
(2) Equilibrium can be formulated in the undeformed position;
(3) No instability occurs prior to the attainment of the fully plastic load;
(4) The influence of shear and thrust is neglected;
(5) There is a known amount of moment that can be transmitted through the connections;
(6) All loads are increased proportionally, and
(7) Failure corresponds to that condition where the structure is reduced to a mechanism through the development of yield (or plastic) hinges.

These assumptions correspond to those made in "simple plastic theory."

As shown in references 3 and 4 the necessary and sufficient conditions for a plastic solution, according to the simple plastic theory, are:

(1) The structure must be in equilibrium;
(2) The moment at any section must be less than or equal to the fully plastic moment value (i.e., |M|≤ Mp); and
(3) A mechanism must be formed.

Several different approaches or methods of solution may be used which will ensure that these three conditions are met. The one that will be followed in this report is the Mechanism Method. In essence, this type of solution assumes that all possible failure configurations are examined and that the load corresponding to each is determined. The maximum load that the structure can sustain is then the one having the lowest critical load value.