M.  Tomaževič¹, M. Gams¹ and S. Lu²

1. Slovenian National Building and Civil Engineering Institute, miha.tomazevic@zag.si, matija.gams@zag.si
2. Wienerberger A.G., Suikai.Lu@wienerberger.com

ABSTRACT

Shear failure mechanism, characterised by the formation of diagonal cracks, most frequently represents the critical failure mode of unreinforced and confined masonry walls when subjected to lateral in-plane seismic loads. Although other mechanisms are also possible, seismic resistance of a masonry structure depends predominantly on the shear resistance of the walls. The results of cyclic lateral resistance tests of nine unreinforced walls with height/length aspect ratio 0.7, constructed with hollow clay blocks laid in thin layer mortar with vertical joints of mechanical interlocking type, and subjected to three different levels of vertical precompression, have been used to compare the experimentally obtained resistance values with the results of calculations. It has been found that the shear friction failure (step-formed cracks, following the bed and head joints) determines the resistance at precompression less than 10 % of compressive strength and diagonal tension failure (diagonally oriented cracks passing the units) at higher precompression levels. It has been shown that typical equations, used for the calculation of the shear resistance of walls, do not have general validity, because they reflect the type of the shear failure, for which they had been developed. The resistance of the walls has been also assessed by a simple diagonal strut mechanism, where the resistance depends on the vertical load and inclination of struts, determined by dimensions of masonry units and masonry bond. Although surprisingly good correlation between the experimental and calculated values has been obtained at all precompression levels, further development of such model is needed before practical use.

KEYWORDS: unreinforced masonry walls, shear failure mechanism, shear friction failure, diagonal tension failure, modelling, compressive strut, lateral resistance.

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