*C.BAGGIOand **P.TROVALUSCI * Terza Universita degli Studi di Roma ** Universita di Roma La Sapienza

Block masonry made of stiff and strong elements dry assembled together, or connected by poor mortar, behaves essentially as a system of interacting rigid blocks. Due to the lack of coherence and to the presence of friction along the joints the governing equations are those of a discrete system of rigid blocks with unilateral and frictional constraints. Turning into account the similarities with the problem of limit analysis for rigid/plastic systems with non-associative flow rules, a procedure of nonlinear programming is implemented which provides the ultimate load and the collapse mechanism for masonry made of blocks with various dimensions and dispositions. The solution of the problem of nonlinear programming is tackled using the results of a linear programming obtained by replacing friction with dilatancy. Some examples are given for two- and three-dimensional systems with many degrees of freedom pointing out the strong influence of the size and the texture of the units on the ultimate strength of the masonry.