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Several experimental studies were performed with reinforced concrete (RC) beam-column sub-assemblages to investigate the progressive collapse resistance of frame structures under column loss in the past decade. Most of the experiments suggested that the catenary action could be used as the final defensive mechanism against collapse. Also, analytical and numerical studies have indicated that the pseudo-static response obtained from the nonlinear static load-deflection curve may be used to predict the maximum dynamic response under column loss. It was learned that development of the catenary action was dependent on the beam-end rotational capacity. From most of the experimental and numerical studies, it was observed that there is a strength-decreased transition phase after the peak arch resistance. This transition region may imply an unstable snap-through behavior under a real dynamic column loss scenario. Dynamic propagation of local snap-through behavior could even result in structural collapse. Hence, analytical resolution of the rotational demands for effective catenary action of RC beams is proposed in this study. The general nonlinear static load-deflection curves RC beam-column sub-assemblages under gravitational monotonic loadings were idealized as four piecewise linear phases at first, based on the yield strength, peak arch resistance, leveled-off strength, and peak catenary resistance. Thereafter, the corresponding pseudo-static load response was analytically derived for each phase. A criterion for the effective catenary action was defined to determine the associated chord rotation demand. Based on the analytical formulation, numerical investigation was performed to understand the variation of the chord rotation demand with the key parameters in the derivation. The study results indicated that chord rotation demand for the effective catenary action increased with the peak arch rotation demand. It was increased moderately with decreased stiffness ratio in the strength-decreased region. This implied that RC members with a deep section require larger rotation capacity at the beam-ends to develop the catenary action. One way to reduce the beam-end rotation demand of deep sections is to increase the stiffness ratio in the catenary phase. This may be done by increasing the tensile reinforcement of the RC members. Since RC members with a deep section are usually responsible of large shear and/or moment, it is suggested that their peak arch strength can be used as the collapse resistance for the sake of safety. For RC members with an ordinary section, the rotation demand of 0.20 radians, which is recommended by the UFC guidelines, is suitable for the effective catenary action.

progressive collapse, effective catenary action, pseudo-static response, chord rotation