Abstract-Commercial and residential buildings are usually constructed of steel or reinforced concrete frames with unreinforced masonry infill panels. Many codes of practice ignore the stiffness during design process for ease of design. Many designer engineers also do not know how to incorporate this additional stiffness from the unreinforced masonry infill panels into the design process. This leads to an incorrect assessment of the building's robustness when subjected to lateral loading and especially when subjected to seismic ground motion. The natural frequencies, corresponding mode shapes and the strength of the building is therefore incorrectly assessed leading to an overdesign. When these buildings are modelled using Finite Element Analysis, it is imperative that the strength of the unreinforced masonry infill panels are correctly accounted for, to prevent over strengthening the columns to yield the same experimental test results. The paper therefore focuses on describing the possible failure modes caused by unreinforced masonry infill panels on reinforced concrete framed structures and the FE modelling techniques required to obtain a computationally efficient model. The investigation shows the significant effect of the unreinforced masonry infill panels on the robustness of the global model. Keywords-stiffness of unreinforced masonry panels, FE analysis, equivalent strut methodI.

Low income unreinforced masonry structures in the southwestern region of the Western Cape Province of South Africa present a safety and economic threat if subjected to moderate intensity seismic activity. Previous experience and investigations conducted have shown the vulnerability to damage and/or collapse when these structures are subjected to moderate seismicity. Most unreinforced masonry buildings in South Africa were constructed prior to the first codified provision for seismicity contained in SABS 0160 of 1989. Numerous other residential unreinforced buildings were constructed since then. To date these types of structures have not been evaluated for South African conditions, and concerns have been expressed whether these structures conform to the new codified guidelines contained in SANS 10160-4 of 2011, which is almost a verbatim repetition of the European code for seismic design. Before an analysis of unreinforced masonry buildings can be conducted, it is imperative to determine the effect which opening have on unreinforced masonry walls. This was achieved by conducting experimental tests on four unreinforced masonry walls with various size openings, to determine its effect on the in-plane stiffness, failure frequency and failure displacement. Experimental results show that there is a relationship between the opening sizes versus the wall stiffness, failure frequency and failure displacement.