Experimental Investigation of Masonry Infilled R/C Frames

Although it is well documented that infills significantly affect the dynamic characteristics and the seismic re- sponse of the bare basic structural system, codes are reluctant to encourage consideration of infills as main structural ele- ments, mainly due to structural uncertainties and computational complexities. Part of the uncertainties are due to the very many parameters affecting the behavior of the system, such as infill materials, reinforcing of infills, connection to the sur- rounding frame, geometry, relative stiffness and strength, local construction techniques etc. In the present paper three suc- cessive experimental programs, conducted at the R/C and Masonry Structures Laboratory of the Aristotle University, are described, commented and discussed. All of them refer to single-storey one-bay 1:3 scale R/C moment resisting frames. The first program consisted of 18 specimens, 2 bare and 16 unreinforced masonry (URM) infilled. The second program was an extension and a supplement of the previous one. It consisted of 20 more specimens, 6 bare and 14 URM infilled. The third program was directed towards the investigation of quick and low cost strengthening methods of R/C frames damaged by earthquakes, using several infill techniques. For this purpose 10 of the damaged specimens of the first pro- gram were repaired and strengthened. The significant change of the static and dynamic charac- teristics of the bare basic structural system by the incorpora- tion of infills is a fact stated by many authors and experi- enced in every day praxis. Older versions of codes (1, 2) provide specific instructions for the design and construction of infilled structures, recommending two alternatives: either an effective isolation of the infills from the surrounding frames, so that their structural effects can correctly be ne- glected, or a tight placing of the infills so that their interac- tion with the frames should be properly considered in the design, detailing and construction, especially for seismic excitations. In the first case of the isolation of the infills, the struc- tural system is clear and relatively reliable but, as separation at the bottom of the infill and adequate resistance to out of plane seismic loads are difficult to achieve, this can be haz- ardous (3). Furthermore, the infills add a significant undesir- able mass. In the second case of the tight placing of the in- fills, the increased strength, stiffness and energy dissipation capacity seem to improve the seismic behavior of the regular buildings although base shear may be increased because of the increase of stiffness (4). Recent codes (5) give special attention to the consequences of irregularities in plan and in elevation produced by the infills, which should be taken into account.