Effective Stiffness and drift capacity of modern unreinforced masonry walls

Code design of unreinforced masonry (URM) buildings is based on elastic analysis, which requires as input parameter the effective stiffness and the ultimate drift capacity of the in-plane force-displacement response of URM walls. Eurocode 8, for example, estimates the effective stiffness as 50% of the gross sectional elastic stiffness. Code provisions for the drift capacities are usually empirical equations relating a base drift value to the failure mode and a slenderness ratio. However, comparisons with experimental results have shown that neither approach yields accurate predictions. In this paper, 61 full-scale shear-compression tests of modern URM walls of different masonry typologies from the literature are investigated. It shows that both the initial and the effective stiffness increase with increasing axial load ratio and that the effective-to-initial stiffness ratios are approximately 75% rather than the used 50%. An empirical model for the computation of the effective-to-initial stiffness ratio is suggested based on a recently developed analytical approach, which attributes the loss in stiffness to diagonal cracking and brick crushing. As for the drift capacity, the experimental evidence supports the fact that it reduces with increasing axial load and increases with increasing shear span ratio. A recently developed analytical model for the prediction of the ultimate drift capacity for both shear and flexure controlled URM walls is presented. It considers the effect of kinematic and static boundary conditions on the drift capacity. A comparison to test results shows that the proposed models yield better estimates than current code provisions.