DISCRETE ELEMENT MODEL FOR IN-PLANE LOADED VISCOELASTIC MASONRY

A viscoelastic constitutive model is proposed to evaluate the evolution in time of historical masonry behavior. Masonry structures may be subject, over time, to damage due to creep phenomena, accompanied by a consequent redistribution of stresses and strains. Two models are presented and compared. A discrete element model and a continuous model based on analytical homogenization procedures. Both models are based on the following assumptions: (i) the structure is composed of rigid blocks; (ii) the time dependence of masonry behavior is concentrated in mortar joints, modelled as viscoelastic interfaces. The rigid block hypothesis is particularly suitable for historical masonry, in which stone blocks may be assumed as rigid bodies; the hypothesis of viscoelastic mortar is based on the observation that nonlinear phenomena may be concentrated in mortar joints. The continuum homogenized model provides, in an analytical form, constitutive equivalent viscous functions; the discrete model describes masonry as a rigid skeleton such as to evaluate both its global and local behavior. A parametric analysis is carried out to investigate the effect of (i) mortar-to-brick thickness ratio; (ii) masonry texture (running versus header bond); and (iii) size of heterogeneity (block dimensions) with respect to panel dimensions. Elementary cases are proposed to directly compare constitutive functions of continuum and discrete models. In addition, a meaningful case is proposed: a masonry panel in which the principal stresses are both of compression and the no-tension assumption may therefore be discounted. A further investigation pointed out the sensitivity to heterogeneity size such as to verify model reliability and applicability field.