On the application of the yield-line method to masonry infills subjected to combined in-plane and out-of-plane loads

The influence of infills on the seismic response of frame structures has been long recognised. On the one hand, the presence of infills may be beneficial, due to their contribution to dissipate energy. On the other hand, irregular infill distributions in plan or elevation can lead to concentration of the displacement demand in localised parts of the building. It is noted that the lack of one or more panels may depend on the original building layout or may be generated by the infill collapse during a seismic event. It is therefore of interest the assessment of their capacity to resist out-of-plane loads. In this paper, the use of the yield-line theory for the estimation of the out-of-plane infill strength is investigated. The method is described in detail and an example of derivation of the related equations is presented. Afterward, a modification of such equations is suggested to account for different contact conditions at the infill-frame interface. Moreover, specific attention is paid to the assessment of the masonry flexural strength, which is a basic parameter for the application of the method. Finally, a reduction factor is calibrated to extend the method to those cases in which previous in-plane damage is present. Experimental tests available in the literature are used to verify and calibrate the proposed equations and coefficients.