Experimental and analytical dynamic collapse study of a reinforced concrete frame with light transverse reinforcement
Post-earthquake investigations have shown that the primary cause of collapse in cast-in-place beam-column frames is failure of columns, beam-column joints, or both. As axial failure of one or more member in a frame structure does not necessarily constitute the collapse of that structure, understanding frame-system behavior and frame member interactions leading to collapse is essential in assessing the seismic collapse vulnerability of this type of structure.
This study investigates, both experimentally and analytically, the seismic collapse behavior of non-seismically detailed reinforced concrete frames. To accomplish this task, a 2D, three-bay, three-floor, third-scale reinforced concrete frame is built and dynamically tested to collapse. The test frame contains non-seismically detailed columns whose proportions and reinforcement details allow them to yield in flexure prior to initiating shear strength degradation and ultimately reaching axial collapse (hereafter referred to as flexure-shear critical columns). Experimental data is provided on the dynamic behavior of flexure-shear critical columns sustaining shear degradation and loss of axial load capacity and on load redistribution in a frame system after shear and axial failure of columns.
Analytical modeling of the test frame is undertaken up to collapse. Good agreement between analysis and experiment is achieved up to shear failure in columns. A new zero-length fiber-section implementation of bar-slip rotational effects is introduced. A new shear failure model is introduced that determines column rotations at which shear strength degradation in flexure-shear critical columns is initiated.
An analytical model of the test frame is subjected to several near-fault ground motions recorded during the 1994 Northridge earthquake. Variability of ground motions from site to site (so called intra-event variability) and directivity effects are found to play an important role in analytical prediction of structural collapse. A new ground motion intensity measure that relates well to frame damage is proposed.