Non-proportional loading for 3-D stress situations in Sequentially Linear Analysis

This article presents a new non-proportional loading strategy for Sequentially Linear Analysis (SLA), which is a robust secant stiffness based procedure for nonlinear finite element analysis of quasi-brittle materials, like concrete and masonry. The strategy is based on finding the principal planes for a total strain based fixed cracking model, by searching for the critical plane where the normal stresses due to the scaled combination of two non-proportional loads is equal to the allowable strength. For a plane stress situation (2D), the scaling factor λ is ex-pressed as a function of θ, the inclination of an arbitrary plane to the reference coordinate system, and a one dimensional (θ) optimization of λ is done to determine the principal plane and the resulting fixed crack coordinate system. This approach has been illustrated to match up to the closed form solution, obtained previously based on the principal stress theory, using single element tests and a quasi-static test pushover test on a masonry shear wall. Finally, the concept for the 3-D stress situation is presented, where the optimization problem becomes two-dimensional, with respect to l and m (two-directional cosines).