The paper presents the numerical implementation of a new proposed interface model for modeling
the mechanical response of mortar joints in masonry walls. The interface laws are formulated
in the framework of elasto-plasticity for non standard materials with softening which
occurs in mortar joints due to applied shear and tensile stresses. Its theoretical framework is
fully based on the plasticity theory. The finite element formulation is based on eight noded
isoparametric quadrilateral elements and six noded contact elements. The Von Mises criterion
is assumed to simulate the behavior of the units. The interface laws for contact elements are
formulated to simulate the softening behavior of mortar joints under tensile stress; a normal
linear cap model is also used to limit compressive stress. The capabilities of the interface
model and the effectiveness of the computational procedure are investigated by making use of
numerical examples that simulate the response of a masonry wall tested under shear in the
presence of an initial pre-compression load. Experimental results are provided in the literature
to compare with numerical analysis results. The computer predictions correlate very well
with the test data. The predicted ultimate load of the masonry wall is estimated to be about
93.25% of the ultimate load according to the test data. The width of the compressive diagonal
is about 40% of the length of the diagonal. The proposed model is applied to simulate a stone
masonry shear wall. The predicted load-displacement curve is in accordance with the observed
data, and diagonal failure is predicted with the use of the distribution of stresses in the stone
masonry wall. The width of the compressive diagonal for stone masonry shear wall is about
30% of the length of the diagonal. In addition, a closed form solution was proposed based on
the solid mechanics and the compressive effective width of the wall. The closed form solution
is better than ATC and FEMA-307 in which predicts the ultimate lateral load of unreinforced
masonry walls relatively well. Then it can be concluded that the proposed close form solution
can be used satisfactorily to analyze masonry structures similar to those considered herein.