Use of Soft Layers for Seismic Response Modification of Structural Masonry Walls

Abstract

The development of the sliding-based modification method for the seismic in-plane behavior of structural unreinforced masonry (URM) walls is the objective of the present research project. This objective is achieved by using engineered deformable (soft) layers, which are already implemented in URM walls, placed at the bottom and/or the top of the wall, however, for the sake of providing a moisture barrier in the form of a damp-proof course membrane, ensuring sound insulation or accommodating the short-term or long-term differential movements between the masonry walls and the floor and ceiling construction. After a comprehensive review of work on response of masonry walls that develop sliding, with a special attention paid to studies on the shear behavior of URM with incorporated soft layers, and after selecting the soft layers from those available on the Swiss market, an experimental investigation was conducted. The first part of the experimental investigation was aimed at providing the information on the in-plane compressive and shear behavior of masonry with a so called multi-layer bed joint, i.e. with a (core) soft layer protected by two layers of elastomer before placed in the middle of the mortar bed joint. The main reason for protecting the core soft layers was to reduce the potential cyclic shear loading-caused damage, i.e. to insure durability of the soft layer bed joint. The second part of the experimental investigation comprised a series of static-cyclic tests on full-scale structural masonry walls with a multi-layer bottom bed joint that were conducted in two phases. The preliminary phase was aimed at determining the most suitable type of core soft layer for the main testing phase. Within the main phase, the influences of the pre-compression level, the aspect ratio and the size effect on the behavior of URM walls with a multi-layer bed joint were investigated. The results obtained indicate that the load-bearing URM walls with a multi-layer bed joint, in spite of the prevailing sliding response, could exhibit a significant shear capacity, which depends on the type of core soft layer material, the applied level of pre-compression and on the loading speed. As compared to the walls without a multi-layer bed joint, URM walls with a multi-layer bed joint have a smaller initial stiffness. Importantly, the multi-layer bed joints provide a significantly large ultimate displacement capacity to the URM walls, thus modifying and improving their seismic response. The ultimate displacement capacity is, however, strongly influenced by the extent of shear cracks that develop in the wall, the occurrence of tensile cracks in the head joints at the bottom block course, and reduction of the effective area of the wall. In general, it can be concluded that multi-layer bed joints in URM walls act to modify the seismic response of URM structures and improve their seismic performance. To support design of URM walls with multi-layer bed joints, a method to construct an idealization of the horizontal force-displacement response envelope for the tested URM walls with multi-layer bed joints is proposed. The parameters of this envelope are defined to capture the strength, stiffness and ultimate displacement capacity of the walls, as well as to model sudden drops in wall horizontal force resistance caused by the reduction of the wall effective area. Further, a mechanics-based analytical model of the loading speed-dependent in-plane shear behavior of the masonry multi-layer bed joint with a rubber granulate core soft layer is developed. The model is developed by assuming the elastic-perfectly viscoplastic behavior of the multi-layer bed joint. This model is further extended to describe the horizontal force-displacement behavior of URM walls with a rubber granulate core soft layer in the multi-layer bottom bed joint and validated against the experimental results. A supplementary investigation on the interaction between the in-plane and transverse URM walls with soft layers that can be regarded as a common element of the URM structures is presented in the last part of the thesis. Two series of static-cyclic shear tests on I-shaped (flanged) URM wallettes with a rubber granulate soft layer in the bottom bed joint were performed. The flanges considerably increased the horizontal force resistance and the initial stiffness of URM wallettes, however, they restrained the motion of the web of the wallettes to some extent, which was detrimental to realizing the intended seismic response modification purpose of the soft layer in bottom bed joints.