Strength domains and return algorithm for the lumped plasticity equivalent frame model of masonry structures

The equivalent frame model for the assessment of in-plane mechanisms of masonry walls can be suitably based on a 2-node force-based finite element (FE) consisting of a central linear elastic element, two flexural hinges and a shear link with elastic-perfectly plastic behavior. The hinge state is determined by a predictor-corrector method. The predictor calculates the element stress field using the tangent flexibility matrix. The corrector consists of a return algorithm based on the Haar-Kármán principle and the gradient projection method, both leading to good convergence properties and to optimality characteristics of the solution.