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This paper presents the results of cyclic tests investigating the in-plane behavior of URM walls as the main lateral resistant members in school buildings. Generally, URM walls of existing building in the country contain different material properties with different strengths. Therefore, considering the effect of changing in material properties on their failure modes is required. A hypothetical two-story URM building was defined as the prototype structure of the presented cyclic in-plane research study. This prototype structure was selected to be a proper representative of existing URM school buildings in Iran. First story walls of this building with a loading bay equal to five meter were selected as the test specimens. Some properties of specimens were assumed and considered stationary to make the global response of the specimen match with the real condition of existing URM buildings in Iran. For this purpose, the thickness and the height of the walls were considered 0.35 and 3 m, respectively. The experimental study was carried out on two full-scale specimens. A special height-to-length aspect ratio of the wall was selected to generate enough lateral strength to ensure the bed-joint sliding failure of the URM wall during in-plane cyclic experiment. Lateral strength, Q, of considering URM walls was considered the lesser of the lateral strength based on bed-joint sliding, rocking, diagonal tension or toe crushing strengths. Changing in material properties of walls was considered as the major parameters in the investigation. The specimens were constructed in the structural laboratory of Building and Housing Research Center (B.H.R.C) and subjected to a sequence of cyclic loads. In the construction of walls, similar brick with different mortar and then different strengths were used. According to the results, the behavior of specimens revealed that they could be able to maintain a significant amount of their resistance after cracking without considerable strength degradation especially for the wall with proper mortar strength. Therefore, URM walls can resist lateral load after formation of the first crack with little lose of strength. According to this ductile behavior of walls and partnership of other elements in lateral load capacity of URM building, the estimation of system strength could be more than limited strength by the first crack of the wall. Changing in material properties of wall affected the failure mode, lateral resistant, displacement capacity, and energy dissipation. The specimen with proper mortar strength experienced bed-joint sliding failure mode, while the specimen with weaker mortar strength experienced mixed sliding-diagonal failure mode. The relationships presented in the standards can present relatively proper prediction for shear sliding capacity of the specimens, while the predicted capacity of them for diagonal capacity is overestimated. Test results showed that the improving the material properties of URM wall improved the lateral strengths of specimens. The wall constructed with the mortar with more appropriate properties revealed more lateral resistant and could experience more deformations. This wall could dissipate much more energy. The experimental m-factors were higher than current code values for both specimens and improving the material properties resulted in a considerable increment in m-factor.

Keywords: Unreinforced Masonry Walls, cyclic test, failure mode, mortar properties, school building

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