Behavior of masonry-infilled nonductile reinforced concrete frames

Failure of masonry panels under in-plane loading can be attributed to three simple modes: slipping of mortar joints, cracking of clay bricks and splitting of mortar joints, and middle plane spalling. In this paper a suitable strength criterion is connected to each collapse mode. In more detail, a frictional law is associated with the slipping, which accounts for the shear strength depending nonlinearly on normal stress (modified Mohr-Coulomb criterion of intrinsic curve). Splitting can be expected by the maximum tensile strain criterion (Saint Venant), orthotropic nonsymmetric elasticity being assumed for the material. Eventually panels exhibit spalling when the maximum compressive stress (Navier criterion) is attained under biaxial loading. Strength parameters are then identified on the basis of experimental results and a comparison with the reliable criteria found in the literature is carried out. The validity of the proposed failure criteria to predict the experimental failure modes in a nondimensional stress space, normalized with respect to the normal stress, has been tested in a qualitative manner for the three fundamental failure modes. A quantitative comparison between experimental and analytical results has been carried out for the cases where significant scatters are concerned. The proposed failure criteria seem to be in good agreement with experimental results, within the limits of: small-size panels, single withes, solid units, regular mortar joints, and in-plane loads. Further, these criteria can be used together with a suitable two-dimensional finite-element model, and then directly used to carry out the limit analysis of masonry walls, modeled by a discrete number of panels of finite size. The potential application of the proposed criteria to actual cases is also illustrated. In fact, a specific example is worked out to show how to apply these criteria to predict the failure load and failure mode of a particular masonry panel.