The aim of the thesis is to find the ultimate load of a frame structure under the assumption of perfect rigid plasticity. This is known as the limit state of the structure and forms the basis of modern design procedures and codes of practice. Nowadays, all modern codes of design of structures are based on limit state analysis. Two well kwnown theorems govern this type of analysis, namely the lower bound theorem, for which the problem variables are generalized stresses and the upper bound theorem, for which the unknowns are displacements and permits a similar treatment than traditional elastic analysis.