Assessment of SHCC overlay retrofitting of unreinforced load bearing masonry for seismic resistance
Thesis (MEng)--Stellenbosch University, 2018.
ENGLISH ABSTRACT: A significant stock of multi-story unreinforced load bearing masonry (ULM) structures is located in the Cape Town region (South Africa). The region is categorised as a light to moderate seismic region. Retrofitting techniques of these buildings have been proposed at Stellenbosch University by a group of researchers. The research is motivated by the large number of ULM structures constructed prior to seismic activity becoming a requirement in the South African National Standard in 1989. These buildings will undergo poor shear behaviour during seismic events due to their brittle in-plane shear response. A Strain Hardening Cement-based Composite (SHCC), which can be applied by spraying, was developed as a retrofitting overlay material to improve shear behaviour during seismic events. Triplet tests were performed on the SHCC for characterizing the bonding strength of the masonry/SHCC interface and the shear behaviour while the SHCC form fine multiple cracks. Drying shrinkage of free and restrained specimens was measured. The characterized response of the SHCC enabled the design of favourable overlay thickness and obtaining test specifications on walls loaded by an in-plane shear force. This research also reports the nonlinear finite element analysis (FEA) of the large-scale tests performed previously at Stellenbosch University. The computational model is calibrated and used for extrapolation to other cases. A debonded strip interface strategy is analysed and subsequently validated by laboratory tests to obtain an improved ductility performance. Eight walls with varying wall and overlay thickness (single and double leaf walls, and 15 mm and 30 mm overlay) were previously tested in the Stellenbosch University structures laboratory. The calibrated data is used to perform nonlinear analyses, using the DIANA FEA software, on masonry walls retrofitted with an SHCC overlay and subjected to a pull-over force, while restraint is provided vertically to simulate the effect of multiple stories. The performed analysis and material parameters on shear walls with and without SHCC overlay are used as calibration. However, the primary aim was the calibration of the double leaf wall retrofitted with 15 mm overlay. An anisotropic continuum model, allowing for compression hardening along with compression and tension softening, was used to model the masonry wall. A rotating smeared cracking principal stress limit function is used for the modelling of the SHCC, simulating elastic-perfect plastic tensile behaviour. Coulomb-friction plane interface is used to model the masonry/SHCC interface. After computational calibration, alternate debonded strip interfaces are applied and modelled. The analysis reveals that the debonded interfaces provide an enlarged crack distribution and enhanced ductility. The results of tests performed on four double leaf walls containing debonded strip interfaces and 15 mm overlays, and one without the strips are presented. The tests demonstrate an enlarged cracking region and improved ductility of walls containing debonded strips, compared to the reference wall without debonded strips. The enhanced wall deformation capacity brought about by the strip debonded overlay improves the potential of exceeding the earthquake displacement demand expected in the particular region and founding soil type. It can be concluded that the sprayed SHCC retrofitting of shear walls with debonded strip interfaces provides significant in-plane strength and ductility enhancements when subjected to seismic loading regimes. Further development and testing on full scale ULM buildings are recommended.
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