The collapse vulnerability of reinforced concrete building frames constructed before the
introduction of modern seismic codes has been well documented by earthquake reconnaissance, but the mechanisms that lead to collapse are not yet well understood. The collapse of a structure can occur only if the structure loses its ability to support gravity loads. Among other causes, the loss of gravity load capacity can result from column buckling, unseating of the supported beam, P-δ instability, or degradation of axial capacity due to column shear failure. This last cause and the effect of the axial load failure on the rest of the building frame are the focus of the study presented in this report.
An empirical model, based on the evaluation of results from an experimental database, is developed to estimate the drift at shear failure of existing reinforced concrete building columns. A shear-friction model is also developed to represent the general observation from experimental tests that the drift at axial failure of a shear-damaged column is directly proportional to the amount of transverse reinforcement and is inversely proportional to the magnitude of the axial load. The two drift capacity models are incorporated in a nonlinear uniaxial constitutive model implemented in a structural analysis platform to allow for the evaluation of the influence of shear and axial load column failures on the response of a building.
Shake table tests were designed to observe the process of dynamic shear and axial load failures in reinforced concrete columns when an alternative load path is provided for load redistribution. The results from these tests provide data on the dynamic shear strength and the hysteretic behavior of columns failing in shear, the loss of axial load capacity after shear failure, the redistribution of loads in a frame after shear and axial failures of a single column, and the influence of axial load on each of the above-mentioned variables. An analytical model of the shake table specimens, incorporating the proposed drift capacity models to capture the observed shear and axial load failures, provides a good estimate of the measured response of the specimens.
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