Old Masonry under Seismic Loads: Stiffness Identification and Degradation

The behaviour of masonry under seismic loads has recently become of great interest. New methods for design and analysis, as well as models for resistance, have been developed. The main focus has been on experimental and analytical research for the development of new brick and mortar material whilst the characteristics of old brick material and mortar have been neglected. With the introduction of Eurocode 8 (EC), the behaviour of old masonry structures under seismic load became an interesting topic for architects and engineers. In particular, the available resistance capacity of existing masonry structures is highly relevant to the retrofit and/or improvement of existing buildings. Deformation energy, or ductile response, is a source for energy dissipation of structures. Since normal levels of internal damping represent only a small portion of energy dissipation, large material deformations such as those required in building components to perform a ductile behaviour, are often associated with cracking and degradation of its stiffness, particularly in masonry structures. The stiffness identification of masonry walls in an uncracked state is examined by laboratory tests. Further, stiffness degradation due to an imposed pseudo dynamic horizontal load pattern is evaluated. In particular, the wall specimens are distinguished by different mechanical characteristics like old and new brick units, low and high strength bricks and two common brick layouts. Beyond the effect of stiffness degradation in masonry on the inelastic response characteristic parameters for energy dissipation are compared within the varying masonry specimens. Due to damage propagation, caused by imposed shear load, typical failure states are determined. The results of a series of laboratory tests are analysed. Resistance and deformation characteristics of masonry under shear are compared to specifications according EC 6.