Ductility of ultra-high strength concrete flexural elements subjected to seismic shear
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AbstractThe seismic behaviour of flexural elements constructed with concrete strengths up to 150 MPa is investigated experimentally and analytically. The experimental program examined the behaviour of ten ductile beam-exterior column sub-assemblages. Each specimen consists of a 250x350x1000 mm beam cast monolithically with a 300x450x1050 mm column stub. The specimens were mounted in a position such that the column stub was connected to the strong floor and the beam-end connected to the actuator to displace horizontally. The test variables included the concrete compressive strength (30, 70, and 150 MPa), the compression-tension reinforcement ratio (0.33 and 1.0), the web reinforcement ratio (0.8 and 1.6%), and the shear span to depth ratio (2.0 and 3.0). Specimens were subjected to a quasi-static displacement-controlled system of sets of cyclic displacements of increasing amplitude. In general, the increase in concrete strength to 70 MPa improves the seismic behaviour of the beams. On the other hand, the increase in concrete strength to 150 MPa also improves the seismic behaviour but only under certain limitations. The influence of the increase in concrete strength depends on the other variables and definite conclusions could not be reached. The impact of the studied variables on the hysteretic behaviour is investigated. The available curvature ductility, displacement ductility, dissipated energy, viscous damping, and deformation capacity in the beam plastic hinges were analyzed. Also, the relationship between the displacement ductility and the aforementioned seismic characteristics are analyzed. The effect of shear demand on the length of potential plastic hinges is analyzed. The behaviour of shear-resisting mechanisms is examined in depth. A guideline to design the web reinforcement based on the ratio of shear strength-shear demand and the potential shear demand to obtain a targeted ductility, is presented. The difference between experimental and analytical load-deformation relationships is investigated. A sensitivity analysis is carried out to explore factors affecting beam curvature ductility. Predicting beam ductility and shear strength considering uncertainty is investigated. A Fuzzy-logic based approach is suggested to estimate the beam displacement ductility under unidirectional loading and displacement ductility and shear strength under cyclic loading.
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