Sources of Lateral Instability and Deformation Limits of Boundary Elements of Special Structural Walls

Abstract eng:
In certain common cases, boundaries at the critical section of multistory reinforced concrete shear wall buildings are expected to undergo large compressive strains that can cause flexural-compression failure. This is especially true for crosssection geometries of L-, C- and T-shaped walls, which, under certain loading conditions, trigger a large tensile force on the flexural-tension side where longitudinal reinforcement is concentrated. This large tensile force, along with the vertical loading, has to be equilibrated by a compression force that can only develop on a narrow stem by further increasing the neutral axis depth. These actions could result in large compressive strains and require a stable plastic response of the narrow stem to be developed. To test the compressive strain limits of code-compliant wall boundaries, a set of 6 full-scale prismatic reinforced concrete rectangular prisms, representative of boundary elements of structural walls with special detailing (per ACI-318), were tested in the laboratory under monotonic incremental axial load until failure. A constraint imposed to the longitudinal and transverse reinforcement layout is that the tested special boundary elements had to be constructible in practice, with no special considerations. This limited the minimum spacing between adjacent layers of transverse reinforcement to 4 in. [100 mm], resulting in tie-spacing-to-longitudinal-bar-diameter ratio (s/db) between 3.2 and 4.5. In some specimens, all internal bars were restrained by ties with 135-degree seismic hooks, while in other cases, approximately half the bars were not tied and only restrained by the long leg of the perimeter hoop. Instrumentation was setup to measure axial strains in the cover and core concrete, axial strains along the length of tied and non-tied bars, as well as transverse strain in ties. The out-of-plane displacement and average shortening of the specimens was also recorded. Analysis of the experimental results includes the evaluation of the impact of asymmetric concrete cover spalling and longitudinal bar buckling in the out-of-plane stability of the specimens. Comparison of localized strains measurements with average shortening of the specimens are studied to specify practical deformation limits over various gage lengths along the specimen height. Results suggest that a practical limit in the longitudinal and transverse reinforcement layouts that can be provided might have been reached. The tested specimens showed limited average compressive strain ductility, because after the onset of concrete cover crushing, out-of-plane bending action and bar buckling are triggered. Plastic deformation concentrates over short lengths of approximately two to three wall thicknesses.

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