SARA PETRY (1) and KATRIN BEYER (2)
(1) PhD student, Earthquake Engineering and Structural Dynamics Laboratory (EESD), School of Architecture, Civil and Environmental Engineering (ENAC), École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, firstname.lastname@example.org
(2) Assistant Professor, Earthquake Engineering and Structural Dynamics Laboratory (EESD), School of Architecture, Civil and Environmental Engineering (ENAC), École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, email@example.com
In regions with low to moderate seismicity, unreinforced masonry (URM) is commonly used for the construction of low to mid-rise buildings consequently a large stock of residential buildings contain modern URM walls as structural elements. When these structures are subjected to seismic loading, the stiff URM walls attract a considerable part of the lateral forces and need therefore to be considered in seismic design and assessment. The response of URM walls subjected to lateral in-plane loading, however, is not yet fully understood and estimates for some of the crucial design and assessment parameters are unsatisfactory for example displacement capacity and the effective stiffness.
The work presented herein will contribute to improving the design and assessment methods for URM wall structures built with modern hollow core clay bricks when subject to lateral in-plane loads. First an experimental programme which examines the lateral non-linear in-plane response of URM walls is introduced. Second, a new mechanical model is developed which describes the full force-displacement response of URM walls as they show significant flexural behavior when approaching collapse.
Six URM wall tests are presented. The walls were built at full-scale using a typical modern Swiss hollow clay brick and a commercially available standard cement-based mortar. During the tests, the walls were subjected to quasi-static load cycles with increasing drift demands, while controlling the boundary conditions (axial load and moment restraint at the top of the walls) to simulate the typical loading of a ground floor wall in a URM building. Throughout the quasi-cyclic tests of all URM walls, the deformations of the walls were recorded using a digital photogrammetric measurement system tracking the displacement field of the walls. This measurement was synchronized with the force measurement such that global and local engineering demand parameters of the walls could be linked. This point was crucial for the proposition of a new set of limit states (LSs) that link, local damage states, to characteristic points of the global force-displacement curve of URM walls.
A new mechanical model is proposed which describes the nonlinear force-displacement response of flexurally dominated URM walls approaching collapse. The model is developed in two steps. First, an analytical part is derived based on the plane section hypothesis in conjunction with a non-tension material with a linear-elastic constitutive material law in compression. The model assumes that only the compressed part of the wall contributes to the wall resistance and accounts for a softening in stiffness due to the reduction of the effective area. In a second step, new criteria are developed which predict the occurrence of the previous proposed local LSs, which are then incorporated in the analytical model. The new model is validated using test results reported both in this paper and from others’ [references]. Good agreement between test results and analytical predictions for the effective stiffness and displacement capacity of in-plane loaded URM walls was achieved.
Key words: Unreinforced masonry; Seismic design; Displacement capacity; Effective stiffness; Quasi-static cyclic testing