| Summary: | The vulnerability of unreinforced masonry (URM) buildings to out-of-plane damage and collapse has been clearly demonstrated in past earthquakes. Given sufficient anchorage to the diaphragms (a minimum-level retrofit), a URM wall subjected to out-of-plane inertial forces will likely develop a horizontal crack at an intermediate height. This crack will cause the wall to behave as two semi-rigid bodies, which rock in the out-of-plane direction. Past studies have demonstrated that the out-of-plane stability of a URM wall connected to the diaphragms can be related to the height to thickness ratio (h/t) and the spectral acceleration at 1 s. However, treatment of the effects of diaphragm flexibility and ground motion variability on out-of-plane wall stability in studies to date has been limited.
This dissertation presents an experimental and analytical study examining the out-of-plane stability under seismic loading of URM walls connected to flexible diaphragms. In the experimental phase, five full-scale unreinforced solid clay brick wall specimens spanning one storey were subjected to earthquake ground motions using a shake table. The top and bottom of the walls were connected to the shake table through coil springs, simulating the flexibility of the diaphragms. The apparatus allowed the wall supports to undergo large absolute displacements, as well as out-of-phase top and bottom displacements, consistent with the expected performance of URM buildings with timber diaphragms. Variables examined experimentally included diaphragm stiffness and wall height.
An analytical rigid body model was validated against the experimental results, and it was demonstrated that the model was able to reproduce the observed rocking behaviour with reasonable accuracy. The validated model was used to undertake a parametric study investigating the effects of numerous parameters on out-of-plane wall stability. Ground motion variability was accounted for by using a large suite of motions. Based on the results of the modelling, an updated out-of-plane assessment procedure was proposed. The procedure, which could be incorporated into ASCE 41, provides reference curves of h/t vs. spectral acceleration at 1 s, along with correction factors for axial load, wall thickness, ground-level walls, and exposure.
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